Published: 19 May 2022 Updated: 19 October 2023
Table of contents

Environment

Are carp a symptom or a cause of environmental damage?

Non-scientific

The answer is both. Australian rivers experience many environmental pressures – carp are just one. Separating carp impacts from other sources of environmental stress is difficult for two main reasons. Firstly, carp thrive in rivers that are already degraded, and tend to intensify the impacts of other environmental pressures. Secondly, the footprint carp leave on the environment extends over large areas, and can result in sudden shifts between different ecosystem states (e.g. clear vs. muddy water) - rather than varying predictably in direct relation to carp numbers.

Research on carp impacts through the 1990s provided evidence that carp really do damage river ecosystems. Carp muddy waters, increase nutrient levels (promoting blue-green algae blooms), and reduce abundance of water plants (macrophytes), invertebrates (e.g. aquatic insects and crustaceans), and some fish. Carp also increased water turbidity (muddiness) in 91% of studies, reduced invertebrates in 94%, and reduced water plants in 96% of studies.

Carp impacts also tend to be interlinked and cumulative. Their bottom-feeding behaviour reduces water clarity, limiting sunlight to water plants. This, in turn, reduces habitat and/or food for invertebrates, native fish and waterbirds. The overall effect of these impacts is to shift ecosystems from a predominantly clear-water state to a murky, nutrient-rich state (‘eutrophication’). Once an ecosystem shifts in this direction, it can be difficult to reverse, meaning that the river will remain muddy for some time, even as carp numbers fluctuate in locations within the system.

Find out more about The Carp Problem.

Scientific

Carp are both a cause and a symptom of environmental damage in Australian waterways. However, separating impacts caused by carp from other stressors is difficult. Carp impacts occur across a range of spatial and temporal scales, interact with other stressors and have cumulative, emergent properties. Understanding carp impacts and quantifying them scientifically is difficult, requiring well-planned, multi-year experiments in different types of systems.

Nonetheless, increased research on carp impacts through the 1990s provided evidence that carp clearly do damage river ecosystems. This research included systematic reviews and meta-analyses, which combine and analyse data from numerous studies on a particular topic, as well as large-scale experiments. These studies prove that carp muddy waters, increase nutrient levels (thereby promoting blue-green algae blooms), and reduce abundance of water plants (macrophytes), invertebrates (e.g. aquatic insects and crustaceans), and some fish species (Vilizzi et al., 2014, 2015; Weber and Brown, 2009). For example, Weber and Brown (2009) found that carp increased water turbidity (muddiness) in 91% of surveyed studies, reduced invertebrates in 94%, and reduced macrophytes in 96% of surveyed studies. A more recent meta-analysis supported these results, finding strong evidence for carp impacts on all the same ecosystem components (Vilizzi et al., 2015, Table 1).

 

Table 1. Summary of the effects of Carp of freshwater ecosystems
(Results of casual criteria analysis for hypotheses - taken from Vilizzi et al. 2015)
Component
Trajectory
Strength of evidence
Water quality    
  • Turbidity
Increase High
  • Nitrogen
Increase Very high
  • Phosphorous
Increase Very high
Vegetation    
  • Phytoplankton / cholophyll a
Increase Moderate
  • Aquatic macrophytes
Decrease Very high
Invertebrates    
  • Zooplankton
Change Inconsistent evidence
  • Benthic invertebrates
Decrease High
Vertebrates    
  • Fish
Decrease High
  • Amphibians
Decrease Moderate
  • Waterfowl
Decrease Insufficient evidence

 

Carp impacts also tend to be interlinked. Adult carp feed by sucking sediments from the river bed, filtering out food items and puffing the remaining mud into the water column. This feeding style reduces water clarity, which limits the sunlight penetrating down to macrophytes on the river bed. Fewer macrophytes mean less habitat for invertebrates and native fish. Smaller carp compete with native fish for planktonic food sources. The cumulative effect of these impacts is to shift ecosystems from a predominantly clear-water state (‘oligotrophic’), to a murky, nutrient-rich state (‘eutrophic’). Shifts from one state to another are often termed ‘phase shifts’ in ecology. Once an ecosystem has shifted to a new phase, reversing the change is usually difficult, meaning that the river will remain muddy for some time, even as carp densities fluctuate in various locations within the system.

Australian studies have also demonstrated carp impacts in Australian waterways. King et al. (1997) examined effects of carp density on turbidity, phytoplankton (microscopic algae), and nutrients in two billabongs. The study found clear evidence of carp impacts, with the authors reporting that, "in these natural billabongs, high standing stocks of carp caused increases in turbidity and more intense algal blooms."

Finally, a recent study on the lower Murray River used an experimental design with considerable power to detect carp effects, demonstrating that carp can drive phase shifts from clear to dirty water states, with the latter characterised by poor populations of macrophytes and aquatic invertebrates (Vilizzi et al. 2014). Importantly, this study also indicated that carp may cause environmental damage at lower densities than previously considered.

In summary, acknowledging that Australian rivers face degradation from many sources should not preclude carp control. Nor should action to reduce carp impacts diminish efforts to restore waterway health via other means. Rather, an integrated carp control program could support broader river rehabilitation programs, including habitat restoration and water-quality remediation.

References

  • King, A. J., Robertson, A. I. and Healey, M. R. (1997). Experimental manipulations of the biomass of introduced carp (Cyprinus carpio) in billabongs. I. Impacts on water-column properties. Marine and Freshwater Research 48, 435 – 443.
  • Vilizzi, L., Tarkan, A. S. and Copp, G. H. (2015). Experimental evidence from causal criteria analysis for the effects of common carp Cyprinus carpio on freshwater ecosystems: a global perspective. Reviews in Fisheries Science and Aquaculture 23, 253 – 290.
  • Vilizzi, L., Thwaites, L. A., Smith, B. B., Nicol, J. M. and Madden, C. P. (2014). Ecological effects of common carp (Cyprinus carpio) in a semi-arid floodplain wetland. Marine and Freshwater Research 65, 802 – 817.
  • Weber, M. J. and Brown, M. L. (2009). Effects of common carp on aquatic ecosystems 80 years after “Carp as a Dominant”. Reviews in Fisheries Science 17, 524 – 537.

 

How did carp get here?

Non-scientific

Slow beginnings

Carp had a slow start in Australia, which is surprising given their wide distribution and high numbers today. During the mid-1800s, attempts were made to introduce carp in Victoria, New South Wales, and Tasmania. None of these early introductions appear to have resulted in large, self-sustaining populations. Similarly, two attempted introductions in Victoria during the 1870s failed to become established. Localised populations of carp became established in New South Wales around 1907-1910, following two introductions comprising a total of about 15 carp into an inlet pond above Prospect Reservoir. This strain (genetic variant) of carp, known as the ‘Prospect Strain’ probably maintains a locally-restricted distribution in the area.

The Boolarra strain appears

Early introductions were followed by other releases, some involving up to 50,000 fish, through the 1930s-1950s. However, carp numbers seem to have remained relatively low through to the early 1950s. This pattern of limited geographic spread and relatively low abundance changed when carp (produced by Boolarra Fish Farms Pty. Ltd. in Gippsland during the late 1950s) were introduced into a reservoir at Morwell, Victoria in the 1960s. Rapid spread of these ‘Boolarra Strain’ carp within Victoria followed, and by 1962 a Victorian state government inquiry had determined that carp should be eradicated.

We’ve got a problem: expansion to the present day

‘Boolarra Strain’ carp had gained access to the Murray River by the mid-late 1960s, despite eradication attempts using poisons by the Victorian Department of Fisheries and Wildlife. Extensive flooding in 1974-75, and again during the early 1990s, facilitated the species’ spread. People also aided the spread of carp, through deliberate translocation, undetected presence of carp among stocked native fish, and the use of small carp as live bait for predatory fish. The latter is thought to be the primary mechanism explaining the presence of carp in several Tasmanian lakes, and in NSW coastal river systems. Ornamental carp (also known as Koi) continue to be released by the public.

Scientific

See the non-scientific answer to this question.

 

Why are carp such a problem in Australia?

Non-scientific

There are two main reasons why carp have become a dominant pest in Australia. The first relates to their biology: carp can tolerate a wide variety of environmental conditions, have a broad diet, grow rapidly, mature early, can produce large numbers of eggs, are strong swimmers, good jumpers, and do well in ecosystems that are modified by humans. Carp also spawn earlier than many Australian native species, which means that their juveniles have access to food and other resources before many native fish species.

Environmental conditions at the time carp began dominating Australian waterways is also an important factor. The initial explosion of carp numbers in Australia in the 60‘s-70’s occurred during a ‘perfect storm’ of sorts. Many native fish species had experienced significant declines in numbers due to historically high commercial fishing pressure, widespread reduction in habitat, extensive construction of dams, weirs and other barriers to their migration, and declines in water quality due to widespread poor land use and urbanisation. These elements combined to provide the ideal conditions for a successful invader such as carp to flourish.

 

Scientific

Koehn (2012) provides a detailed summary of the factors that are likely to have led to the success of carp in Australia. To summarise, carp possess many of the characteristics of a successful invader. Specifically, they can tolerate a wide variety of environmental conditions, have a broad diet, grow rapidly, mature early, are highly fecund, are highly dispersive, and do well in systems that are modified by humans (Gehrke 1997) (see Table 1 below from Koehn 2004).

Attribute Details
Invasion history, wide distribution and abundance Introduced and successfully established throughout Europe, Asia, AFrica, North America, South and Central America, Australia, New Zealand, Papua New Guinea and some islands of Oceana
Wide environmental tolerances High environmental tolerances with: temperature tolerance 2-40.6c, salinity up to about 14 parts per thousand (0.4 seawater salinity) and pHs 5-10.5, oxygen levels as low as 7% saturation and generally occure in most types of freshwater habitat
High genetic variability Three genetic strains in Australia

Early sexual maturity

Males at 1 year, females at 2 years
Short generation time 2-4 years
Rapid growth Hatching of eggs is rapid (two days at 25c) and newly hatched carp grow very rapidly
High reproductive capacity They are highly fecund braodcast spawners with egg counts as high as 2 million per female
Broad diet Omnivore / detritivore
Gregariousness Schooling species
Possessing natural mechanisms of dispersal Mobile species with fish moving between schools. Dispersal can also occur with the downstream drift of larvae. Rates of transfer can be affected by conditions such as flooding
Commensal with human activity Bred as an ornamental and aquaculture species, used as bait and sought by some anglers

 

The environmental conditions that were prevalent during the 1960’s-1970’s when carp numbers dramatically increased in Australia were also relevant. There was likely to be particularly low predatory pressure on carp during this period as a result of high commercial fishing pressure, widespread reduction in habitat, prolific construction of dams, weirs and other barriers to their migration (Koehn 2001), and water quality was generally poor due to widespread inappropriate landuse practices (Koehn 2004). Coupled with the rapid growth rate of carp and large size when mature, this is likely to have enabled a large cohort to overwhelm predatory pressure and rapidly attain a size that precluded predation.

References

  • Gehrke PC (1997). Differences in composition and structure of fish communities associated with flow regulation in New South Wales. In: Harris JH and Gehrke PC (Eds), Fish and Rivers in Stress: the NSW Rivers Survey. NSW Fisheries Office of Conservation and Cooperative Research Centre for Freshwater Ecology, Cronulla and Canberra. Pp 169–200.
  • Koehn J (2001). The impacts of weirs on fish. In: The Proceedings of The Way Forward on Weirs. Presented on 18-19th August 2000, at the Centenary Lecture Theatre, Royal North Shore Hospital, St Leonards, NSW. Inland Rivers Network, Sydney. Pp 59–66.
  • Koehn J (2004). The biology, ecology and vulnerabilities of carp, IN Fulton W and Hall K (eds) (2014). Forum proceedings: Carp management in Australia — state of knowledge. 19–20 June 2012, Melbourne. PestSmart Toolkit publication, Invasive Animals Cooperative Research Centre, Canberra, Australia.

 

 

How many carp are there in Australia?

Non-scientific

Fisheries scientists and managers tend to talk about how much (biomass or density) rather than how many (abundance). This is in part because biomass and density - which estimate total weight of a given organism in a certain area at a given time - is generally more useful as it relates to the amount of energy available. For a pest fish species such as carp, which quickly reach a size too large for most predators, and estimate of biomass can help reflect the amount of energy locked up in carp populations, and therefore unavailable to other species in the food web.

Carp have become the dominant freshwater fish in south-east Australia, comprising up to 80% of the fish biomass in many areas, resulting in biomasses as high as 3144 kg/ha and densities of up to 1000 individuals/ha in some parts of the Murray-Darling Basin. In consecutive excessively wet years, carp biomass could take the volume up to one million tonnes, conversely, in dry periods biomass reduces.

To date, estimates of carp biomass/density in Australia have largely been at local or regional scales, and can vary widely. Research being conducted as part of the National Carp Control Plan will provide the most accurate and comprehensive estimate of carp biomass in Australia to date. This estimate is vital to informing clean up strategies and the modelling of possible release scenarios to deliver optimal carp control outcomes through biocontrol.

 

Scientific

Fisheries scientists and managers tend to talk about how much (biomass or density) rather than how many (abundance). This is in part because absolute abundance is hard to measure in fish, but also because biomass and density, which estimate total weight of a given organism in a certain area at a given time is generally more useful as it relates to the amount of energy available to the next trophic level. Further, for a pest fish species such as carp that quickly reach a size too large for most predators (Koehn, 2004), estimation of biomass can help reflect the amount of energy locked up and unavailable to other species.

Carp have become the dominant freshwater fish in south-east Australia (Koehn, 2004), comprising a significant proportion of the fish biomass in many areas, resulting in biomasses as high as 3144 kg/ha and densities of up to 1000 individuals/ha in some parts of the MDB (Harris and Gehrke, 1997).

To date, estimates of carp biomass/density in Australia have largely been at local or regional scales, and vary from 190kg/ha in Moira Lake (Brown et al., 2003); to between 150-690kg/ha in a range of billabongs (Hume et al., 1983); 690 kg/ha in the Bogan River (Reid and Harris, 1997); 619 kg/ha in the Campaspe irrigation channels (Brown et al., 2003), and 10-40 kg/ha in the Logan/Albert system (Norris et al 2011).

Estimation of biomass or density at larger spatial scales bring greater uncertainty. However, estimates of carp biomass and density will be important for the NCCP, to inform mathematical modelling of virus spread, and development of strategies to clean up fish after possible virus release.

For this reason, it is proposed to improve estimates of carp biomass and density as a deliverable of the NCCP research program.

References

  • Brown P, Sivakumaran KP, Stoessel D, Giles A, Green C and Walker T (2003). Carp Population Biology in Victoria. Marine and Freshwater Resources Institute, Department of Primary Industries, Snobs Creek.
  • Harris J.H and Gehrke P.C. (1997). Fish and Rivers in Stress. The NSW Rivers Survey. NSW Fisheries Research Institute & the Cooperative Research Centre for Freshwater Ecology. Sydney.
  • Hume DJ, Fletcher AR and Morison AK (1983). Carp Program. Final Report. Arthur Rylah Institute for Environmental Research, Fisheries and Wildlife Division, Ministry for Conservation, Victoria.
  • Koehn, J. (2004). Carp (Cyprinus carpio) as a powerful invader in Australian waterways. Freshwater Biology 49, 882–894.
  • Norris A, Chilcott K, Hutchison M and Stewart D (2011). Carp Surveys of the Logan and Albert Rivers Catchment 2006-2009. PestSmart Toolkit publication. Invasive Animals Cooperative Research Centre, Canberra, Australia.
  • Reid, D. D. and Harris, J. H. (1997). ‘Estimation of total abundance: the calibration experiments’. In: Harris, J.H. and Gehrke, P.C. (Eds) Fish and Rivers in Stress – The NSW Rivers Survey, pp. 63–760. NSW Fisheries Office of Conservation, Cronulla, NSW; and the Cooperative Research Centre for Freshwater Ecology, Canberra, ACT.

 

 

What carp control measures have been undertaken and why haven’t they worked?

Non-scientific

Commercial carp fishing fills niche markets for human consumption, fish leather, aquaculture feedstock, bait and fertiliser. Local consumer demand for carp is limited to 50-60 tonnes per year at present. Demand from these niche markets is not enough to have any significant reduction in the current carp population.

Manual carp removal, including trapping and controlling access to breeding grounds, has seen some success in Tasmania's Lake Crescent and Lake Sorell. Lake Crescent was declared free of carp in 2007 after 12 years of manual removal work. Carp removal work is continuing in Lake Sorell. The cost of the program has been about $11 million.

The NCCP research identified a gentic biocontrol technology that showed promise for application to carp in Australia. However, considerable technical and logistical challenges would need to be overcome before this was ready to use in Australia.

Scientific

Various methods have been trialled to control carp, or reduce their impacts in Australia over the years. Primarily these have involved physical removal (e.g. netting, angling, trapping) or poisoning. All of these methods have advantages and disadvantages relating to their effectiveness, ease of use, size specificity (some remove only adult carp), impacts on non-target organisms, and cost. Methods employed for controlling carp and their impacts are summarised within Table 1, and are further discussed below.

Table 1. Carp-control or impact reduction methods used, trialled or under investigation in Australia (modified from NSW DPI, 2010).

 

Effectiveness

(Impact on carp population)

 

 Size specificity

 Impacts on
non-target organisms

 Costs/resources

 Feasibility/limitations on use

Community carp fishing competitions/musters/fish-outs (angling)

Potential to remove large numbers of carp from localised area.

Little long-term effect on populations.

Size specific - generally targets larger fish, not juveniles.

Non-target species can be released, although not all survive.

Human resource intensive; however, events are often independently organised by community groups.

High interest from community groups in conducting these types of events.
Increases community awareness.
Ineffective in reducing population numbers or removing residual carp from waterways.

Commercial harvesting (hauling/netting/trapping)

 

 

Potential to remove large quantities of carp quickly in specific locations.

Market forces limit long-term effect on population.

Dependent on mesh size.

Some bycatch of non-target species.

Self-funding, but only if carp populations and market price allow for a viable, self-sustaining industry. Otherwise fee-for-service.

Requires consistent supply of large quantities for market to remain viable.
Market returns justify effort only under specific conditions (high carp biomass, proximity to markets, minimal obstructions such as snags).
Currently low viability because of low market price and high costs of fishing.
Not viable for removal of residual carp populations in connected waterways.
Judas-carp technique (where males are radio-tagged and act as 'tracker fish') may enhance effectiveness and efficiency of commercial harvest by targeting spawning or winter aggregations; this would require commitment to a long-term control program.

Rotenone

 

 

 

Potential to kill discrete populations of carp quickly.

Not size specific.

Broad-scale application kills virtually all non-target species as well as carp.
Rotenone baits have been trialled but tended to be rejected by carp; the rotenone may also leach out and thus affect non-target species.

Human resource intensive (planning, application and removal/disposal of large quantities of carp).
Moderate costs.

Illegal to use except under and in accordance with Australia Pesticides and Veterinary Medicines Authority (APVMA) permit.
May be feasible to eradicate small, discrete populations under specific circumstances (e.g. new populations) where benefits clearly outweigh harm to native species.
Not suitable for broad-scale use because of impacts on non-target organisms.
Use of baits is not currently feasible without further improvement.

Fishway carp separation cages

 

 

 

Potential to remove large quantities of carp and, in some circumstances, eliminate carp from stretches upstream of cages.
Effectiveness depends on the proportion of the carp population that is static vs. migrating. This is unknown for many sites.

Size specific - generally capture larger carp (>250mm).

Minimal - designed to release native fish and vertebrates.

Installation cost ranges from $15,000 to $45,000 (or more) per fishway.
Ongoing maintenance is human-resource intensive; requires regular checking and disposal of captured carp.

Highly feasible where suited to existing fishways, or where new fishways are being designed.
Lack of carp-disposal options may limit feasibility at some sites.
Fish composting technology may be an effective utilisation and disposal method (where feasible and based on resources available from local agencies/groups).

Carp exclusion devices (e.g. mesh screens, 'finger traps') fitted to wetland regulators

Carp exclusion devices prevent access of mature carp to wetlands or other spawning grounds.
Potential to substantially reduce carp populations if breeding hotspots are targeted.

Size specific - generally exclude only larger carp

May affect native fish recruitment by also excluding native species from spawning grounds.

Relatively inexpensive. Requires supporting infrastructure and ongoing maintenance.

Already installed at many sites in the Murray-Darling Basin.
For maximum effectiveness, requires ecological research to identify recruitment areas for carp and native species.
'Finger traps' may be more effective but are still at prototype stage.

Sex biasing technology

 

 

 

In theory could eventually provide total eradication, but technically difficult, and effects may not be seen for up to 100 years due to long generation time of carp.

Effectiveness would depend on many factors, including: heritability; fitness of modified fish; size of carp population at time of release; and number of modified fish released.

Not size specific.

Not size specific.

Very expensive technology, still under development, total costs unknown.

Unclear. Still many technical hurdles to overcome before ready for laboratory or field trials.
Would initially require stocking of large numbers of fish carrying sex biasing construct.
Would require integrated implementation with other initiatives.
Risk of public non-acceptance of intentional release of modified pests into natural environment. Requires extensive public consultation.

Cyprinid herpesvirus-3 (CyHV-3)

 

 

 

Causes mass mortalities of carp - potential to substantially reduce populations (at least until resistance develops).

Not size specific, although juveniles believed to be more susceptible.

Species-specific (McColl, In Prep), but mass carp mortalities post release could have water quality impacts detrimental to native species.

Unknown; still under investigation.

 

Strong temperature relationship may impact effectiveness at low or high temperatures. Hybridisation between goldfish and carp could reduce effectiveness.
Risk of public non-acceptance of intentional release of biocontrol agent.

Risk of resistance from carp dependant industried. Requires extensive public consultation.

 

Further analysis...

Community carp fishing competitions.

Recreational fishing events are popular within the Australian community, and such events can enable quantities of carp biomass to be removed from an area, although research suggests that this does not result in a lasting reduction in carp numbers (Gehrke, 2010). For example, the mean estimated population reduction by anglers in the Goondiwindi Carp Cull was reported to be 0.5% compared to 13.4% for electrofishing (Norris et al., 2013). Similarly in 2008, anglers in the Goondiwindi Carp Cull removed 40 carp from lagoon habitats in south-eastern Queensland, equivalent to 1.9% of the estimated population, and much lower than the catches provided by other methods (Gehrke, 2010).

Brown and Walker (2004) demonstrate that unless carp populations can be a reduced by a large percentage, physical removal is unlikely to offer an effective method for carp control. Similarly, Gehrke (2010) suggests that low-cost carp angling events provide an effective method for promoting community awareness of issues surrounding carp in the Murray-Darling Basin, but their effectiveness in reducing carp populations and environmental impacts is low (Norris et al., 2013).

Commercial harvesting

Graham et al. (2005) summarise predominant commercial fishing methods used for carp, which include electrofishing, hauling, trapping, mesh netting and angling. The limited acceptance of carp for human consumption in Australia limits its market value, with most being sold to produce low-value fishmeal, fish oil, pet food, fertiliser and stock feed. Wilson (1998), cited in Graham et al. (2005), suggested that at the time of writing, fishers needed to catch 5 – 6 tonnes of carp per week (at 80 cents/kg) to make an economic return. This means that the fishery is generally only viable under conditions that allow the removal of large volumes of carp at minimal cost. Another factor limiting the effectiveness of commercial fishing in controlling carp abundance is the increasing cost of production as biomass is reduced.

Electrofishing

Whilst electro-fishing is effective for carp removal in areas of high density, it is less effective in deep water, high turbidity or flow, and can be expensive both in terms of capital and labour costs. Whilst non-target species are also stunned during electrofishing, they generally quickly recover, and mortality levels are normally low under appropriate operating circumstances. Electrofishing is generally not widely used as a commercial fishing method for carp in Australia, but Graham et al. (2005) report that supplies of carp to a processing factory at Sale are regularly supplemented with electro-fished carp from tributaries to the Gippsland Lakes, particularly during drought conditions when carp retreat into the rivers as the lakes become more saline.

Seining

A seine net is a large sock-shaped net with a pair of long hauling lines attached to each side of the open end. The net (including ropes) is ‘shot’ from the boat around a concentration of fish and then hauled back to the boat or shore by drawing on the lines. In this way, the fish are progressively corralled into the back of the net, or cod end. Seining is one of the more effective methods for catching large quantities of carp (Bajer et al., 2011). Catch records from 2001/02 show that approximately 15 t were caught by drag net from Lake Brewster, after carp were attracted to the hauling area with berley, and up to 1000 t of carp are harvested annually from Lake Wellington, mostly by seine (Graham et al., 2005). Application of this method is limited to shallow lakes or dams where the substrate is clear of obstructions or where the bottom is relatively smooth, firm, and clear of snags (Graham et al., 2005). Most natural waterways are unsuited to seining as lakes and riverbeds are normally littered with woody debris and other snags (Graham et al., 2005).

Trapping

Unbaited drum nets were widely used to target native fish prior to discontinuation of commercial fishing in the Murray-Darling Basin in 2003. Larger baited rectangular traps have also been shown to be effective for carp but require easy access to the water. Baited traps are most effective when set downstream in flowing waterways, and when fitted with netting wings to one or both banks to guide carp into the trap. Non-target impacts of trapping can be significant, particularly for air-breathing vertebrates, if not fitted with an escape device or accessible air space.

Mesh-netting

Mesh-netting was historically the dominant method used for harvesting native fish, in the Murray-Darling. Captured fish can be damaged through scale loss and meshing injuries, and air breathing vertebrates can also become entangled. However, Graham et al. (2005) report that setting mesh nets in shallow water and frightening fish towards the net can effectively enable carp to be targeted whilst having minimal impact on non-target species.

Judas carp

The Judas carp technique (wherein males are radio-tagged to then enable the school of fish they associate with to be targeted as they re-integrate) may enhance effectiveness and efficiency of commercial harvest by targeting spawning or winter aggregations which contain populations of sexually mature carp (Gilligan et al., 2010). This method has been trialled in Lake Cargelligo in the lower Lachlan catchment and in Tasmania with some success, however is most useful in areas of low carp abundance (Bajer et al., 2011).

Rotenone

There are no fish poisons, or piscicides available that are specific to common carp, and no chemicals are fully registered as piscicides in Australia. Rotenone is the only chemical currently legal to use in Australia to control any pest fish, and it is occasionally used for this purpose (Rayner and Creese, 2006). Rotenone interrupts cellular respiration in gill-breathing animals by blocking the transfer of electrons in the mitochondria. Acute exposure to toxic levels reduces cellular uptake of blood oxygen, resulting in increased cellular anaerobic metabolism and associated production of lactic acid causes blood acidosis (Fajt and Grizzle, 1998).

Historically, Australian states and territories have applied for a ‘minor use’ permit to be able to use chemicals such as rotenone for a specified time and under permit conditions, including that there:is a high probability of successfully eradicating the pest fish, with a low chance of immigration or recolonization; has been a review of environmental factors that identified benefits outweigh impacts on native species; is no risk to the health of humans, stock or domestic animals through direct contact or contaminated drinking water; and, is generally strong public and political support for the operation.

Researchers have attempted to develop a carp-specific targeting method using rotenone, through integrating it into floating pellet baits. This method was found to be unsuccessful due to low buoyancy and palatability (Gehrke, 2003). Further development and testing would be required (and a separate APVMA permit approved) before rotenone baits could be utilised to target carp populations. Although it is a potential option to eradicate discrete new populations of carp in some limited circumstances, rotenone is not appropriate for use to control carp on a large scale.

Fishway carp separation cages

Carp separation traps that exploit the species’ jumping behaviour have been implemented at numerous locations throughout the Murray-Darling Basin. Trials with these devices on the Murray River revealed that these traps were effective in catching carp and passing native fish, with 88.8% of carp caught, 99.9% of native species passed, and catches of up to 5 t per day in some instances. It is clear that their effectiveness can be variable (Table 2), with catch per unit effort across seven installations reported to be 14.7kg per week, or 1.4 carp per day (pers. comm. M. Gordos). For this reason the NSW Department of Primary Industries no longer recommends the installation of carp separation cages at remote, un-manned locations.

 

Table 2. Carp separator trap effectivenss at sevarl sites within the Murray-Darling Basin (Gordos, M. unpublished data).
Site
Sampling days
Captured Carp
Total Mass (kg)
Kg wk1
Bulgeraga Creek Culvert

69

295

326.9

33.2

Yallakool Creek Weir

105

213

124.7

8.3

Coligen Creek Weir

128

133

129.6

7.1

Stevens Weir

102

205

224.0

15.4

Island Creek Weir

153

54

182.0

8.3

Bumbuggan Weir

53

49

143.0

18.9

Booligal Weir

373

419

625.6

11.7

TOTAL

989

1368

1755.6

14.7

 

Exclusion devices

Carp exclusion devices can prevent access of mature carp to wetlands or other spawning grounds, having potential to substantially reduce carp populations at a localised scale if breeding hotspots are targeted. However such methods are size specific, generally excluding only larger carp, and may affect native fish recruitment by also excluding native species from spawning grounds.

Exclusion devices are relatively inexpensive and are already installed at many sites in the Murray-Darling Basin, however require supporting infrastructure and ongoing maintenance. For maximum effectiveness exclusion devices require ecological research to identify recruitment areas for carp and native species. This information will assist in determining appropriate deployment locations. 'Finger traps' may be a more effective technique, though are still at prototype stage.

Sex biasing approaches, including daughterless carp technology

In theory sex biasing approaches including the species specific daughterless carp technology could eventually provide total eradication of common carp, however these methods have never been proven in carp, and many have never been proven in any fish species.

The effectiveness of any genetic sex biasing approach including daughterless carp technology would depend on many factors, including: heritability; fitness of modified fish; size of carp population at time of release; and number of modified fish released.

There are still many technical hurdles to overcome before sex biasing approaches such as daughterless carp technology would ready for laboratory or field trials. Daughterless carp would initially require stocking of large numbers of genetically modified fish and would require integrated implementation with other initiatives. There are risks of public non-acceptance of intentional release of genetically modified pests into the natural environment and would therefore requires extensive public consultation.

If able to be used, effects on carp populations would be unlikely to be seen for up to 100 years due to the long generation time of carp. These pest control methods are still under development, and so feasibility and associated costs for delivery are difficult to estimate.

Cyprinid herpesvirus-3 (CyHV-3)

Cyprinid herpesvirus 3 (or CyHV-3, hereon referred to as the carp virus) can cause mass mortalities in carp under the right conditions with potential to substantially reduce wild populations, at least until resistance develops. The virus has been shown to be species-specific, and is not size specific, although juveniles are believed to be more susceptible. The strong relationship between temperature and the virus is understood to reduce in effectiveness at low or high temperatures; furthermore hybridisation between goldfish and carp could reduce the virus effectiveness. There is also the risk of public non-acceptance of the intentional release of a biocontrol agent.

As is the case with daughterless carp technology, CyHV-3 requires extensive public consultation, and further quantification of costs. Challenges may also arise as a result of resistance from stakeholders who value carp, including the ornamental koi carp industry, koi enthusiasts and commercial fishers. Furthermore, mass carp mortalities as a result of the virus release could have water quality impacts detrimental to native species. The NCCP has engaged researchers to explore these issues.

References

  • Bajer, P., Chizinski, C. and Sorensen, P. (2011). Using the Judas technique to locate and remove wintertime aggregations of invasive common carp. Fisheries Management and Ecology, 18, 497-505.
  • Brown, P. and Walker, T. I. (2004). CARPSIM: stochastic simulation modelling of wild carp (Cyprinus carpio ) population dynamics, with applications to pest control. Ecological Modelling, 176, 83-97.
  • Fajt, J. R. and Grizzle, J. M. (1998). Blood respiratory changes in common carp exposed to a lethal concentration of rotenone. Transactions of the American Fisheries Society, 127, 512-516.
  • Gehrke, P. C., St Pierre, S., Matveev, V. and Clarke, M. (2010). Ecosystem responses to carp population reduction in the Murray-Darling Basin. Canberra, Australia: Murray-Darling Basin Authority.
  • Gehrke, P. C. (2003). Preliminary assessment of oral rotenone baits for carp control in New South Wales. Managing invasive freshwater fish in New Zealand. Wellington New Zealand: Department of Conservation.
  • Graham, K. J., Lowry, M. B., Walford, T. R. and Wales, N. S. (2005). Carp in NSW: Assessment of distribution, fishery and fishing methods, NSW Department of Primary Industries, Cronulla Fisheries Centre.
  • McColl, K. A., Sunarto, A., Slater, J., Bell, K., Asmus, M., Fulton, W., Hall, K., Brown, P., Gilligan, D., Hoad, J., Williams, N. and Crane, M. (2016). Cyprinid herpesvirus 3 as a potential biological control agent for carp (Cyprinus carpio) in Australia: non-target species testing. Journal of Fish Diseases, 40, 1141-1153.
  • Norris, A., Chilcott, K. and Hutchison, M. (2013). The Role of Fishing Competitions in Pest Fish Management. In: PestSmart Toolkit. Invasive Animals Cooperative Research Centre, Canberra.
  • Rayner, T. S. and Creese, R. G. (2006). A review of rotenone use for the control of non-indigenous fish in Australian fresh waters, and an attempted eradication of the noxious fish, Phalloceros caudimaculatus. New Zealand Journal of Marine and Freshwater Research, 40, 477-486.

 

 

Is a virus the most effective way to control carp?

Non-scientific

Since carp numbers exploded in Australia in the 1970’s, a variety of measures have been used to try and control carp. However, all have been unsuccessful in reducing carp impacts on a large scale. Biological control (a virus) offers some key advantages over other control approaches as it can be species-specific and highly effective when used correctly. It is also relatively cost effective.

Research indicates when conditions are right (ideal temperature, high density of carp and optimal virus concentration) the virus can result in significant carp mortality. Specificity to the target organism is also a fundamental pre-requisite for a biological control agent. While considerable evidence indicates that the virus can only infect carp some questions remain here.

Research under the NCCP has investigated key traits of the virus, the target species (carp) and their role in Australian ecosystems. This research has been used to develop potential strategies for release of the virus to deliver optimal results for carp control.

 

Scientific

Since carp numbers in Australia dramatically increased in the 1970’s a number of other methods have been tried to control carp in recent decades, without widespread success. Biocontrol agents offer a number of key advantages over other pest control methods:

They can be quite species-specific when selected carefully (Suckling and Sforza 2014).

They are often capable of sustaining themselves using the population of the intended target pest species. This means although it may cost a bit to introduce a new biocontrol agent to an environment, it's a tactic that may only need to be applied once due to the self-perpetuating and self dispersing nature of biocontrol agents (Van Lenteren et al., 2003). This also means that biological control can also remain in place and effective for a much longer time than other methods of pest control. These attributes mean that biological control can be quite cost effective long terms (Saunders et al. 2010).

Most important of all, it can be highly effective when implemented on the basis of good science (Fenner and Ratcliffe 1965, Cook & Fenner 2002, Jean-Yves and Fourdrigniez 2011, Van Rensberg et al. 1987), however incomplete knowledge can lead to sub-optimal outcomes (Suckling 2013).

Carp biological control is now one of the most well researched vertebrate pest biocontrol examples worldwide. Research led by the CSIRO indicates the carp herpesvirus can effectively reduce carp populations under the right conditions. Virus concentration, temperature and carp density appear to be critical factors in delivering a successful outcome.

It is important to note that the carp virus alone will not completely eradicate carp - some carp will inevitably survive, and over time populations would rebuild. Therefore, continuing investigation of synergistic control measures, such as those that aim to alter carp reproduction biology, is important to ensure that we maximise the success of any control carp impacts.

References

  • Cooke, B.D., Fenner, F., 2002. Rabbit haemorrhagic disease and biological control of rabbits. Wildlife Research 29, 689–706.
  • Fenner, F., Ratcliffe, F.N., 1965. Myxomatosis. Cambridge University Press, London.
  • Jean-Yves M and Fourdrigniez M (2011). Conservation benefits of biological control: The recovery of a threatened plant subsequent to the introduction of a pathogen to contain an invasive tree species. Biological conservation, 144: 1, P 106-113
  • McColl K, Cooke B, Sunarto A (2014) Viral biocontrol of invasive vertebrates : lessons from the past applied to cyprinid herpesvirus-3 and carp (Cyprinus carpio) control in Australia. Biological control, 72, pp 19-117.
  • Saunders G, Cooke B, McColl, K Shine R, and Peacock T (2010). Modern approaches for the biological control of vertebrate pests: An Australian perspective. Biological control 52 288-295.
  • Suckling D.W. (2013) Benefits from biological control of weeks from New Zealand range from negligible to massive: A retrospective analysis. Biological control 66 27-32
  • Suckling DM, Sforza RFH (2014) What Magnitude Are Observed Non-Target Impacts from Weed Biocontrol? PLoS ONE 9(1): e84847. doi:10.1371/journal.pone.0084847
  • van Lenteren JC, Babendreier D, Bigler F, Burgio G, Hokkanen HMT, Kuske S., Loomans AJM, Menzler-Hokkanen I., Van Rijn PCJ, Thomas MB, Tommasini MG and Zeng Q-Q (2003). Environmental risk assessment of exotic natural enemies used in inundative biological control. Biocontrol Sci Technol 48: 3–38. doi: 10.1023/A:1021262931608.
  • van Rensburg, P.J.J., Skinner, J.D., van Arde, R.J., 1987. Effects of feline panleucopaenia on the population characteristics of feral cats on Marion Island. Journal Applied Ecology 24, 63–73.
  • Van Wilgen, B.W., De Wit, M.P., Anderson, H.J., Le Maitre, D.C., Kotze, I.M., Ndala, S., Brown, B. and Rapholo, M.B (2004). Costs and benefits of biological control of invasive alien plants: case studies from South Africa: working for water. South African Journal of Science 100, 113-122

 

 

What about sex-biasing control approaches, like daughterless carp and Trojan Y?

Non-scientific

Sex biasing constructs such as the daughterless gene and Trojan Y can provide novel management options and even total eradication in theory, but there are many technical and societal hurdles still to be overcome before any sex biasing approach would be ready for laboratory or field trials.

Historical work on sex biasing constructs has been well supported in Australia. Effectiveness depends on many factors, including: the type of gene construct; heritability; fitness of fish carrying the sex biasing construct; size of the target species population at time of release; and number of modified fish released.

Researchers have considered genetic and sex biasing approaches that may complement possible use of the carp herpesvirus. 

While the technical and logistical challenges involved in deploying genetic biocontrol measures for carp remain considerable, these approaches do offer some potential and could be part of a future management approach using multiple methods.

 

Scientific

Trojan Y is the easiest approach from a regulatory perspective because it is not a genetically modified organism (GMO), however the target of only producing male fish may be hard to achieve based on existing evidence of success in other species. Then, given normal Mendelian inheritance drive population change, its application requires the release of a high proportion of individuals carrying the gene-construct relative the whole population so effects may only be seen in the order of 100 years unless the carp population can first be driven to very low levels by other approaches e.g. the carp virus (Thresher et al. 2014b) due to the long generation time of carp (Thresher et al. 2014a). There would therefore be a need to stock large numbers of modified carp into natural ecosystems for many years to 'swamp' natural populations with modified carp, but this may not be as hard as it sounds given the high fertility of female carp (Thresher et al. 2014a).

The GM daughterless approach developed by CSIRO through the 1990s to 2000s may be more effective theoretically, because normal fertility females with the construct are less likely to have fertility issues, however the same numbers of modified carp would need to be developed for release. Stocking rivers with large numbers of genetically modified fish, may not be publicly acceptable as this would be an intentional release of genetically modified pests into natural environments.

Recent developments in gene-technology have also led to possibility of the development of synthetic gene-drive sex biasing carp constructs (Thresher et al. 2014a). This could be another form of GM daughterless, but where the all progeny of a mating between a modified carp and a wild type carp would carry the active modifier construct. Inheritance of the construct is therefore likely to be much higher than 50:50. This could in theory drive carp populations to extinction without having to swamp the feral population with carp carrying a GM construct. This approach however is still very controversial and may take many years before it could even be considered as an acceptable approach if ever (Borel, 2017). 

References

  • Borel, B. (2017). How Genetically Modified Mice Could One Day Save Island Birds - CRISPR, a new gene-editing technology, has the potential to help scientists combat invasive predators. But is tinkering with nature worth the risk? Audubon Magazine Summer 2017 (http://www.audubon.org/magazine/summer-2017/how-genetically-modified-mice-could-one-day-save )
  • Thresher, R., Hayes, K., Bax, N., Teem, J., Benfey, T. and Gould, F. (2014a). Genetic control of invasive fish: technological options and its role in integrated pest management. Biological Invasions, 16, 1201-1216.
  • Thresher, R., Van de Kamp, J., Campbell, G., Grewe, P., Canning, M., Barney, M., Bax, N., Dunham, R., Su, B. and Fulton, W. (2014b). Sex-ratio-biasing constructs for the control of invasive lower vertebrates. Nat Biotech, 32, 424-427.

 

 

Can't you release a predator to eat the carp?

Non-scientific

While predators like the Murray Cod and some birds (e.g. egrets and pelicans) eat carp, they are not able to control carp impacts. Introducing a non-native predator or significantly increasing numbers of a native predator to control carp would not be wise. Unlike virus/host relationships, which can be quite species-specific, predator/prey relationships can be quite elastic. So, there is a high risk of non-carp species being preyed on by introduced predators.

 

Scientific

It is true that predators such as Murray cod eat carp. A dietary study found 35% of Murray cod sampled (all cod >500mm total length) contained carp (Ebner, 2006). Similarly, Baumgartner (2005) found cyprinids (carp and goldfish) constituted up to 25% of prey occurrence in Murray cod sampled from the Murrumbidgee River. Though predators undoubtedly exert pressure on carp populations, they are unable to reduce carp numbers to below thresholds known to cause environmental impacts.

A number of studies have suggested that threshold densities above which carp cause ecological damage are around 100–174 kg/ha (Haas et al., 2007; Bajer et al., 2009; Matsuzaki et al., 2009), substantially lower than the historic threshold estimate of 450 kg/ha (Fletcher et al., 1985), which has formed the basis of much carp impacts research (Vilizzi et al., 2014).

Some researchers have suggested threshold densities to be even lower: Brown and Gilligan (2014) modelled that an integrated pest control approach would be required to reduce carp densities in the Lachlan River Catchment below the estimated threshold density (88kg/ha).

Introducing a novel predator to reduce carp numbers below threshold densities would not be wise. 

References

  • Bajer, P.G., Sullivan, G. and Sorensen, P.W. (2009). Effects of a rapidly increasing population of common carp on vegetative cover and waterfowl in a recently restored Midwestern shallow lake. Hydrobiologia 632, 235–245
  • Baumgartner, L. J. (2005). Effects of weirs on fish movements in the Murray-Darling Basin, University of Canberra.
  • Brown, P. and Gilligan, D. (2014). Optimising an integrated pest-management strategy for a spatially structured population of common carp (Cyprinus carpio) using meta-population modelling. Marine and Freshwater Research 65, 538–550.
  • Ebener, B. (2006). Murray cod an apex predator in the Murray River, Australia. Ecology of Freshwater Fish, 15, 510-520.
  • Fletcher, R.F., Morison, A.K. and Hume, D.J. (1985). Effects of carp (Cyprinus carpio) on communities of aquatic vegetation and turbidity of waterbodies in the lower Goulburn River basin. Australian Journal of Marine and Freshwater Research 36, 311–327.
  • Haas, K., Köhler, U., Diehl, S., Köhler, P., Dietrich, S., Holler, S., Jensch, A., Niedermaier, M. and Vilsmeier, J. (2007). Influence of fish on habitat choice of water birds: a whole-system experiment. Ecology 88, 2915–2925.
  • Matsuzaki, S., Usio, N., Takamura, N. and Washitani, I. (2009). Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis. Oecologia 158, 673–686.
  • Vilizzi, L., Thwaites, L.A., Smith, B.B., Nicol, J.M. and Madden, C.P. (2014). Ecological effects of common carp (Cyprinus carpio) in a semi-arid floodplain wetland. Marine and Freshwater Research 65, 802 – 817.

 

 

Where did the virus come from?

Non-scientific

The carp virus, Cyprinid herpesvirus 3, is a naturally-occurring organism first observed to kill large numbers of carp in 1997 in Germany, and then in Israel and the USA in 1998. The virus has since been detected in over 33 countries globally. The exact origin of the carp virus has not been determined, but the virus may have circulated among wild carp populations before appearing in aquaculture.

 

Scientific

The carp virus is a naturally-occurring organism first observed in association with a mass carp mortality event in 1997 in Germany (Bretzinger et al., 1999).

Similar events soon followed in Israel and the USA in 1998 (Hedrick et al., 2000). The virus has since been detected in over 33 countries globally (Haenen et al., 2004; Pokorova et al., 2005; Haenen et al., 2009; OIE, 2015), with transhipment of carp by koi owners/breeders and aquaculture operators considered the most likely mechanism explaining viral spread (Gilad et al., 2003). Other vectors, such as waterbirds, cannot be ruled out, but are considered a low probability (Taylor et al., 2010, Taylor et al., 2011).

The virus is now reported from at least 33 countries worldwide (Haenen et al., 2004, 2009; OIE, 2015) including:

Europe: Belgium, Denmark, France, Austria, Switzerland, Poland, Luxembourg, Italy, Germany (Bretzinger et al., 1999), the Netherlands, Slovenia (Toplak et al., 2011), Sweden, Czech Republic, Romania, Spain (OIE, 2015) and Britain (Denham, 2003; Taylor et al., 2010b).

Asia: Philippines, Thailand, Malaysia, Taiwan (Tu et al., 2004; Cheng et al., 2011), Japan (Sano, 2004; Matsui et al., 2008; Uchii et al., 2009), Indonesia, China (Dong et al., 2011; OIE, 2015), Korea (Oh et al., 2001), Singapore (reported on AquaVetMed, 27 September 2005).

Africa: South Africa. North America: USA and Canada (Garver et al., 2010).

Exact origin of the carp virus has not been determined, but scientists suggest that it may have circulated among wild carp populations before appearing in aquaculture (Uchii et al., 2014).

 

References

  • Bretzinger, A., Fischer-Scherl, T., Oumouna, M., Hoffman, R. and Truyen, U. (1999). Mass mortalities in koi carp, Cyprinus carpio, associated with gill and skin disease. Bulletin of the Eurasian Association of Fish Pathologists 19, 182 – 185.
  • Cheng, L., Chen, C. Y., Tsai, M. A., Wang, P. C., Hsu, J. P., Chern, R. S. and Chen, S. C. (2011). Koi herpesvirus epizootic in cultured carp and koi, Cyprinus carpio, in Taiwan. Journal of Fish Diseases, 34, 547-554.
  • Denham, K. (2003). Koi herpesvirus in wild fish. The Veterinary Record, 507.
  • Dong, C., Weng, S., Li, W., Li, X., Yi, Y, Liang, Q. and He, J. (2011). Characterization of a new cell line from caudal fin of koi, Cyprinus carpio koi, and first isolation of cyprinid herpesvirus 3 in China. Virus Research, 161, 140-149.
  • Garver, K. A., Al-Hussinee, L., Hawley, L. M., Schroeder, T., Edes, S., Lepage, V., Contador, E., Russell, S., Lord, S. and Sevenson, R. M. W. (2010). Mass mortality associated with koi herpesvirus in wild common carp in Canada. Journal of Wildlife Diseases, 46, 1242-1251.
  • Gilad, O., Yun, S., Adkison, M. A., Way, K., Willits, N. H., Bercovier, H. and Hedrick, R. P. (2003). Molecular comparison of isolates of an emerging fish pathogen, koi herpesvirus, and the effect of water temperature on mortality of experimentally infected koi. Journal of General Virology, 84, 2661-2668.
  • Haenen, O. and many others (2009). Results of global koi herpesvirus questionnaire 2009. Central Veterinary Institute, Wageningen; Epizone; National Veterinary Institute, Technical University of Denmark. http://orbit.dtu.dk/fedora/objects/orbit:106499/datastreams/file_6475577/content
  • Haenen, O., Way, K., Bergmann, S. M. and Ariel, E. (2004). The emergence of koi herpesvirus and its significance to European aquaculture. Bulletin of the Eurasian Association of Fish Pathologists 24, 293 – 307.
  • Hedrick, R. P., Gilad, O., Yun. S., Spangenberg, J. V., Marty, G. D., Nordhausen, R. W., Kebus, M. J., Bercovier, H. and Eldar, A. (2000). A herpesvirus associated with mass mortality of juvenile and adult koi, a strain of common carp. Journal of Aquatic Animal Health 12, 44 – 57.
  • Matsui, K., Honjo, M. I. E., Kohmatsu, Y., Uchii, K., Yonekura, R. and Kawabata, Z. I. (2008). Detection and significance of koi herpesvirus (KHV) in freshwater environments. Freshwater Biology, 53, 1262-1272.
  • Oh, M., Jung, S., Choi, T., Kim, H., Rajendran, K., Kim, Y., Park, M. and Chun, S. (2001). A viral disease occurring in cultured carp Cyprinus carpio in Korea. Fish Pathology (Japan).
  • Pokorova, D., Vesely, T., Piackova, V., Reschova, S. and Hulova, J. (2005). Current knowledge on koi herpesvirus (KHV): a review. Veterinary Medicine, 50, 139 – 147.
  • Sano, M., Ito, T., Kurita, J., Yuasa, K., Miwa, S. and Iida, T. (2004). Experience on common carp mass mortality in Japan. Proceedings of the meeting on current status of transboundary fish diseases in Southeast Asia: occurrence, surveillance, research, and training. Manila, Philippines, 23 – 24 June 2004 (pp. 13 – 19). Tigbauan, Iloilo, Philippines: SEAFDEC Aquaculture Department.
  • Toplak, I., Fajfar, A. G., Hostnik, P. and Jencic, V. (2011). The detection and molecular characterization of koi herpesvirus (KHV) in Slovenia. Bulletin Of The European Association of Fish Pathologists, 31, 219-226.
  • Taylor, N. G., Dixon, P. F., Jeffery, K. R., Peeler, E. J., Denham, K. L. and Way, K. (2010). Koi herpesvirus: distribution and prospects for control in England and Wales. Journal of Fish Diseases, 33, 221-230.
  • Taylor, N. G., Norman, R. A., Way, K. and Peeler, E. J. (2011). Modelling the koi herpesvirus (KHV) epidemic highlights the importance of active surveillance within a national control policy. Journal of Applied Ecology, 48, 348-355.
  • Tu, C., Weng, M., Shiau, J. and Lin, S. (2004). Detection of koi herpesvirus in koi Cyprinus carpio in Taiwan. Fish Pathology, 39, 109-110.
  • Uchii, K., Matsui, K., Iida, T. and Kawabata, Z. (2009). Distribution of the introduced cyprinid herpesvirus 3 in a wild population of common carp, Cyprinus carpio Journal of Fish Diseases, 32, 857-864.
  • Uchii, K., Minamoto, T., Honjo, M.N. & Kawabata, Z. (2014). Seasonal reactivation enables Cyprinid herpesvirus 3 to persist in a wild host population. FEMS Microbiology and Ecology 87, 536–542
  • World Organisation for Animal Health (OIE; 2015). Koi herpesvirus disease. Chapter 2.3.7. http://www.oie.int/fileadmin/Home/eng/Health_standards/aahm/current/chapitre_koi_herpesvirus.pdf
  • World Organisation for Animal Health (OIE; 2015). World Animal Health Information Database (WAHID). http://www.oie.int/wahis_2/public/wahid.php

 

 

Is the virus present in Australian waterways?

Non-scientific

There is no indication that the virus exists in Australian carp populations, but a comprehensive survey has not been conducted. The two most closely related viruses to CyHV-3 are reported to be present in Australian waterways. They are CyHV-1 (carp pox virus) which is carp specific, but not particularly contagious and only lethal to small juvenile fish, and CyHV-2 (goldfish hematopoietic necrosis herpesvirus) that only affects goldfish (Carassius auratus). 

 

Scientific

PCR surveys of 849 carp from eastern Australia failed to detect any evidence of CyHV-3 being present in Australian carp populations (McColl and Crane (2013). This preliminary work also did not detect the presence of the two most closely related viruses to CyHV-3 : CyHV-1 (carp pox virus) which is carp specific, but not particularly virulent and only lethal to small juvenile fish, and CyHV-2 (goldfish hematopoietic necrosis herpesvirus) that only affects goldfish (Carassius auratus). However, both CyHV-1 and CyHV-2 are generally recognised as being present in Australia.

 

References

  • McColl KA and Crane MStJ (2013). Cyprinid herpesvirus 3, CyHV-3: its potential as a biological control agent for carp in Australia. PestSmart Toolkit publication, Invasive Animals Cooperative Research Centre, Canberra, Australia.
  • Stephens FJ, Raidal SR and Jones B (2004) Haematopoietic necrosis in a goldfish (Carassius auratus) associated with an agent morphologically similar to herpesvirus. Aust Vet J 82:167-169.

 

 

How does the virus work?

Non-scientific

The carp virus is highly contagious for carp and is mostly transferred through carp-to-carp contact.

Physical contact between infected and non-infected carp provides the most effective transmission route. Carp can also become infected simply by swimming in the same waterbody as infected individuals, but extremely high viral concentrations are required for this to occur, and in general this transmission pathway is likely to be much less effective than direct physical contact between an infected and a susceptible carp. The carp virus also infects and kills carp most effectively within a certain temperature range (approximately 16-28°C).

 

Scientific

The primary source of entry for CyHV-3 in carp is via the skin and gills (Hedrick et al., 2000; Costes et al., 2009), through direct fish to fish transmission (Costes et al., 2009), or indirectly through contact with contaminated fish faeces (Dishon et al., 2005) or other fomites (i.e. objects to which viral particles adhere) (Minamoto et al., 2011; Minamoto et al., 2012).

The virus then rapidly spreads to the kidney, spleen, fins, intestine, and brain (Gilad et al., 2004). Within the optimal temperature range, the course of infection in carp is that fish cease feeding within 3 days post exposure (dpe) and become lethargic. They then either lie at the bottom of the tank, or gather close to the water inlet or sides of the pond and gasp at the surface of the water.

Evidence of gill necrosis coupled with increased mucous secretion can present at approximately 3 dpe (Rakus et al., 2013), and these are the most consistent gross clinical signs of disease. Uncoordinated movements, erratic swimming, and twitching may occasionally be seen in very small fish (4 – 10 cm). Death occurs within 3 – 4 days after the onset of clinical signs of disease (i.e. from about 7 dpe), with most mortality occurring between 8 – 12 dpe. Loss of function in the skin, gills, kidney and gut probably account for death of the fish, but secondary bacterial, parasitic or fungal infections are also common among infected fish and often contribute to mortalities.

While transmission via water is one means through which the virus enters a carp's body, primarily via the skin and gills (Hedrick et al., 2000), direct fish to fish transmission is also possible via the skin (Costes et al., 2009). The latter transmission pathway is likely most effective (Tolo et al.,2021; Kirkland & Hick, 2022). Indirect transmission due to the persistence of CyHV-3 in fish faeces (Dishon et al., 2005), plankton (Minamoto et al., 2011), freshwater mussels and crustaceans (Kielpinski et al., 2010) has also been reported.  The virus may also enter via oral mucosa when fish feed on CyHV-3 infected tissue (Fournier et al., 2012).

Breeding sites and aggregations are postulated to be the primary location and time of transmission of CyHV-3 within populations (Uchii et al., 2011; Raj et al., 2011; McColl et al., 2014). Within Australia there is a steadily improving understanding of carp ecology (Brown et al., 2005; Gilligan and Asmus, 2012). In particular, the identification of discrete hotspots of carp recruitment throughout the Murray-Darling Basin offers opportunities for the targeted control of carp populations. Epidemiological modelling has been developed that takes account of viral, host and environmental factors to inform development of a release and clean up strategy.

References

  • Brown, P., Sivakumaran, K. P., Stoessel, D. and Giles, A. (2005). Population biology of carp (Cyprinus carpio) in the mid-Murray River and Barmah Forest Wetlands, Australia. Marine and Freshwater Research 56, 1151-1164.
  • Costes, B., Raj, V. S., Michel, B., Fournier, G., Thirion, M., Gillet, L., Mast, J., Lieffrig, F., Bremont, M. and Vanderplasschen, A. (2009). The major portal of entry of koi herpesvirus in Cyprinus carpio is the skin. Journal of virology, 83, 2819-2830.
  • Dishon, A., Perelberg, A., Bishara-Shieban, J., Ilouze, M., Davidovich, M., Werker, S. and Kotler, M. (2005). Detection of carp interstitial nephritis and gill necrosis virus in fish droppings. Applied and Environmental Microbiology, 71, 7285–7291.
  • Fournier, G., Boutier, M., Stalin Raj, V., Mast, J., Parmentier, E., Vanderwalle, P., Peeters, D., Lieffrig, F., Farnir, F. and Gillet, L. (2012). Feeding Cyprinus carpio with infectious materials mediates cyprinid herpesvirus 3 entry through infection of pharyngeal periodontal mucosa. Vet Res, 43.
  • Gilad, O., Yun, S., Zagmutt-Vergara, F. J., Leutenegger, C. M., Bercovier, H. and Hedrick, R. P. (2004). Concentrations of a koi herpesvirus (KHV) in tissues of experimentally infected Cyprinus carpio koi as assessed by real-time TaqMan PCR. Diseases of Aquatic Organisms, 60, 179-187.
  • Gilligan, D. and Asmus, M. (2012). ‘Identifying significant hotspots of carp recruitment offers opportunities for the control of carp populations’. In: Fulton, W. and Hall, K., eds. Forum Proceedings: Carp management in Australia — state of knowledge, 2012 Melbourne. Pestsmart.
  • Hedrick, R., Gilad, O., Yun, S., Spangenberg, J., Marty, G., Nordhausen, R., Kebus, M., Bercovier, H. and Eldar, A. (2000). A herpesvirus associated with mass mortality of juvenile and adult koi, a strain of common carp. Journal of Aquatic Animal Health, 12, 44-57.
  • Kielpinski, M., Kempter, J., Panicz, R., Sadowski, J., Schütze, H., Ohlemeyer, S. and Bergmann, S. M. (2010). Detection of KHV in freshwater mussels and crustaceans from ponds with KHV history in common carp (Cyprinus carpio). The Israeli Journal of Aquaculture, 62(1), 28-37.
  • Kirkland, P. & Hick, P. (2022). Evaluating the role of fish-to-fish transmission of Koi herpesvirus. Final Report to FRDC, Canberra, February 2022, 35 pp, CC BY 3.0. FRDC project number 2020-104, NCCP research project 6.
  • McColl, K., Cooke, B. and Sunarto, A. (2014). Viral biocontrol of invasive vertebrates: Lessons from the past applied to cyprinid herpesvirus-3 and carp (Cyprinus carpio) control in Australia. Biological Control, 72, 109-117.
  • Minamoto, T., Honjo, M. N., Yamanaka, H., Tanaka, N., Itayama, T. and Kawabata, Z. I. (2011). Detection of cyprinid herpesvirus-3 DNA in lake plankton. Research in Veterinary Science, 90, 530-532.
  • Minamoto, T., Honjo, M. N., Yamanaka, H., Uchii, K. and Kawabata, Z. I. (2012). Nationwide cyprinid herpesvirus 3 contamination in natural rivers of Japan. Research in Veterinary Science, 93, 508-514.
  • Raj, S. V., Fournier, G., Rakus, K., Ronsmans, M., Ouyang, P., Michel, B., Delforges, C., Costes, B., Farnir, F. and Leroy, B. (2011). Skin mucus of Cyprinus carpio inhibits cyprinid herpesvirus 3 binding to epidermal cells. Veterinary research, 42, 1-10.
  • Rakus, K., Ouyang, P., Boutier, M., Ronsmans, M., Reschner, A., Vancsok, C., Jazowiecka-Rakus, J. and Vanderplasschen, A. (2013). Cyprinid herpesvirus 3: an interesting virus for applied and fundamental research. Vet Res, 44, 85.
  • Tolo, I.E., Bajer, P.G., Wolf, T.M., Mor, S.K. & Phelps, N.B.D. (2021) Investigation of Cyprinid herpesvirus 3 (CyHV-3) disease periods and factors influencing CyHV-3 transmission in a low stocking density trial. Animals 12, DOI: https://doi.org/10.3390/ani12010002
  • Uchii, K., Telschow, A., Minamoto, T., Yamanaka, H., Honjo, M. N., Matsui, K. and Kawabata, Z. I. (2011). Transmission dynamics of an emerging infectious disease in wildlife through host reproductive cycles. ISME, 5, 244-251.

 

 

Can other species transmit the carp virus?

Non-scientific

Non-carp species maybe able to carry and transmit the virus without being infected. After being “shed” by infected carp, the carp virus can survive in the water for around 3 days. However, if the virus does not infect another carp within that time, the virus will die.

While in the water, the virus may also stick to non-carp fish, sediment, plankton or other organisms/items, and may infect carp that come in contact with it. The virus does not infect these non-carp species or items - they simply carry the virus - much the same way as your dog could carry the human common cold virus on its fur if you sneezed into your hand and then patted it. 

 

Scientific

Transmission is most effective via direct carp to carp transmission (Costes et al., 2009). However transmission can also occur indirectly through contact with contaminated fish faeces (Dishon et al., 2005) or other fomites (i.e. objects to which viral particles adhere) (Minamoto et al., 2011; Minamoto et al., 2012).

References

  • Costes, B., Raj, V. S., Michel, B., Fournier, G., Thirion, M., Gillet, L., Mast, J., Liefrig, F., Bremont, M. & Vanderplasschen, A. 2009. The major portal of entry of koi herpesvirus in Cyprinus carpio is the skin. Journal of virology, 83, 2819-2830.
  • Dishon, A., Perelberg, A., Bishara-Shieban, J., Ilouze, M., Davidovich, M., Werker, S. & Kotler, M. 2005. Detection of Carp Interstitial Nephritis and Gill Necrosis Virus in Fish Droppings. Applied and Environmental Microbiology, 71, 7285–7291.
  • Minamoto, T., Honjo, M. N., Yamanaka, H., Tanaka, N., Itayama, T. & Kawabata, Z. I. 2011. Detection of cyprinid herpesvirus-3 DNA in lake plankton. Research in Veterinary Science, 90, 530-532.
  • Minamoto, T., Honjo, M. N., Yamanaka, H., Uchii, K. & Kawabata, Z. I. 2012. Nationwide Cyprinid herpesvirus 3 contamination in natural rivers of Japan. Research in Veterinary Science, 93, 508-514.

 

 

Is the aim of the NCCP to eradicate carp?

Non-scientific

No. It is important to note that the carp virus alone will not eradicate all carp from Australia. Australia’s experience with two other viruses that were introduced to control rabbits has reinforced that lesson. Neither the myxomatosis virus or the rabbit calicivirus could eradicate rabbits.

What viruses can do is cause a substantial drop in the numbers of their target species and reduce the ecological impacts caused by that species. Earlier studies have suggested carp start impacting on ecosystem health at densities of 100–174 kg/ha. Studies have shown that carp density is currently much higher in some areas. Biological control aims to reduce carp density below levels known to cause environmental harm. It is possible that the carp virus could be used in conjunction with control methods.

 

Scientific

If it proceeds, carp biocontrol would aim to reduce the environmental damage caused by carp by reducing carp density below threshold levels known to cause environmental harm. Managing an invasive species below a density threshold, above which impacts to environmental values are unacceptable, is a key component of Integrated Pest Management (Braysher and Saunders, 2003).

Previous research has demonstrated that carp begin to impact on water turbidity (muddiness) when density exceeds 50–75 kg/ha (Zambrano and Hinojosa, 1999; Vilizzi et al., 2014), and that noticeable shifts from a clear to a turbid water state occurs at 200–300 kg/ha (Williams et al., 2002; Parkos et al., 2003; Haas et al., 2007; Matsuzaki et al., 2009).

Declines in aquatic vegetation cover and detrimental effects on aquatic macrophytes have been reported at carp densities ranging from 68 to 450 kg/ha (Hume et al., 1983; Fletcher et al., 1985; Osborne et al., 2005; Pinto et al., 2005; Bajer et al., 2009; Vilizzi et al., 2014) and a decline in native waterfowl use was reported when carp densities reached ~100 kg/ha (Bajer et al., 2009). These impacts stem largely from the carp’s benthic feeding behaviour (Sibbing et al., 1986) and are most commonly reported in shallow off-stream habitats (Parkos et al., 2003) where carp congregate (Smith and Walker 2004a; Stuart and Jones 2006a, 2006b).

A number of studies have suggested that threshold densities for carp to be 100–174 kg/ha (Haas et al. 2007; Bajer et al. 2009; Matsuzaki et al. 2009), which are much lower than historic estimates of 450 kg/ha (Fletcher et al., 1985). Some researchers have suggested threshold densities to be even lower; Brown and Gilligan (2014) modelled that an integrated pest control approach would be required to reduce carp densities in the Lachlan River Catchment below the estimated threshold density (88kg/ha).

There are many examples where MDB carp populations have exceeded these threshold levels—Moira Lake: 190 kg/ha (Brown et al., 2003); a range of billabongs: 150–690 kg/ha (Hume et al., 1983); Bogan River: 690 kg/ha (Reid and Harris, 1997); and the Campaspe irrigation channels: up to 619 kg/ha (Brown, et al. 2003).

The NCCP research program has developed the first comprehensive carp biomass estimate for Australia. This information, combined with epidemiological modelling, indicates that the carp virus could reduce carp populations to varying degrees, particularly when combined with other control methods.

References

  • Bajer, P. G., Sullivan, G. and Sorensen, P. W. (2009). Effects of a rapidly increasing population of common Carp on vegetative cover and waterfowl in a recently restored Midwestern shallow lake. Hydrobiologia, 632, 235–245.
  • Braysher, M. and Saunders, G. (2003). PESTPLAN – a guide to setting priorities and developing a management plan for pest animals. Bureau of Rural Sciences and the National Heritage Trust, Canberra, ACT.
  • Brown, P. and Gilligan, D. (2014). Optimising an integrated pest-management strategy for a spatially structured population of common carp (Cyprinus carpio) using meta-population modelling. Marine and Freshwater Research, 65, 538–550.
  • Brown, P., Sivakumaran, K. P., Stoessel, D., Giles, A., Green, C. and Walker, T. (2003). Carp population biology in Victoria. Marine and Freshwater Resources Institute, Department of Primary Industries, Snobs Creek, Victoria.
  • Fletcher, R. F., Morison, A. K. and Hume, D. J. (1985). Effects of Carp (Cyprinus carpio) on communities of aquatic vegetation and turbidity of waterbodies in the lower Goulburn River basin. Australian Journal of Marine and Freshwater Research, 36, 311–327.
  • Haas, K., Köhler, U., Diehl, S., Köhler, P., Dietrich, S., Holler, S., Jensch, A., Niedermaier, M. and Vilsmeier, J. (2007). Influence of fish on habitat choice of water birds: a whole-system experiment. Ecology 88, 2915–2925.
  • Hume, D. J., Fletcher, A. R. and Morison, A. K. (1983). Carp Program Report No. 10. Final Report. Arthur Rylah Institute for Environmental Research, Fisheries and Wildlife Division, Ministry for Conservation, Melbourne, Victoria.
  • Matsuzaki, S., Usio, N., Takamura, N. and Washitani, I. (2009). Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis. Oecologia, 158, 673–686.
  • Osborne, M., Ling, N. and Hicks, B. (2005). ‘Abundance and movement of koi carp (Cyprinus carpio haematopterus) in the lower Waikato River system’. In: Proceedings of the 13th Australasian Vertebrate Pest Conference, Te Papa, Wellington, New Zealand, 2–6 May 2005, pp. 56.
  • Parkos, J. J. III, Santucci, V. J. Jr, and Wahl, D. W. (2003). Effects of adult common Carp (Cyprinus carpio) on multiple trophic levels in shallow mesocosms. Canadian Journal of Fisheries and Aquatic Sciences, 60, 182–192.
  • Pinto, L., Chandrasena, N., Pera, J., Hawkins, P., Eccles, D. and Sim, R. (2005). Managing invasive Carp (Cyprinus carpio) for habitat enhancement at Botany Wetlands, Australia. Aquatic Conservation: Marine and Freshwater Ecosystems, 15, 447–462.
  • Reid, D. D. and Harris, J. H. (1997). ‘Estimation of total abundance: the calibration experiments’. In: Harris, J.H. and Gehrke, P.C. (Eds) Fish and Rivers in Stress – The NSW Rivers Survey, pp. 63–760. NSW Fisheries Office of Conservation, Cronulla, NSW; and the Cooperative Research Centre for Freshwater Ecology, Canberra, ACT.
  • Sibbing, F. A., Osse, J. W. M. and Terlouw, A. (1986). Food handling in the Carp (Cyprinus carpio): its movement patterns, mechanisms and limitations. Journal of the Zoological Society of London, 210, 161–203.
  • Smith, B. B., and Walker, K. F. (2004a). Spawning dynamics of common carp in the River Murray, South Australia, shown by macroscopic and histological staging of gonads. Journal of Fish Biology, 64, 336–354.
  • Stuart, I. G. and Jones, M. J. (2006a). Movement of common Carp, Cyprinus carpio, in a regulated lowland Australian river: implications for management. Fisheries Management and Ecology, 13, 213–219.
  • Stuart, I. G. and Jones, M. J. (2006b). Large, regulated forest floodplain is an ideal recruitment zone for non-native common carp (Cyprinus carpio). Marine and Freshwater Research, 57, 333–347.
  • Vilizzi, L. Thwaites, L., Smith, B., Nicol, J. and Madden, C. (2014). Ecological effects of common carp (Cyprinus carpio) in a semi-arid floodplain wetland. Marine and Freshwater Research, 65, 802–817.
  • Williams, A. E., Moss, B. and Eaton, J. (2002). Fish induced macrophyte loss in shallow lakes: top-down and bottom-up processes in mesocosm experiments. Freshwater Biology, 47, 2216–2232.
  • Zambrano, L. and Hinojosa, D. (1999). Direct and indirect effects of carp (Cyprinus carpio) on macrophytes and benthic communities in experimental shallow ponds in central Mexico. Hydrobiologia 408/409, 131–138.

 

 

Won't this be like rabbits?

Non-scientific

It's tempting to ask if viral biocontrol was used on rabbits in Australia, yet there are still rabbits around, might carp biocontrol be the same?

Rabbit biocontrol in Australia has actually been highly successful. In fact, the combination of myxoma virus and calici virus still limits rabbit numbers to about 15% of their potential numbers, and without them the cost to agriculture alone would be in excess of $2 billion per year.

Just like rabbits, carp biocontrol is unlikely to ever eradicate every carp. However, it is possible the carp virus may help to reduce carp density below levels known to cause environmental harm, which is the aim of the NCCP. Just like farmers are enjoying the benefits of reduced rabbit numbers, so too will communities benefit from healthier and cleaner waterways.

Earlier studies have suggested that carp start impacting on ecosystem health at densities of 100–174 kg/ha. Numerous studies have shown that in places, carp density is currently much higher.

The research under the NCCP has provided the first comprehensive carp biomass estimate for Australia. Carp biomass fluctuates dramatically with rain fall and river flows but in general ranges from about 200,000 tonnes up to 1 millions tonnes after consecutive years of heavy rain fall. In many places, carp density (kg/ha) exceeds levels likely to cause environmental damage.

 

Scientific

Rabbit biocontrol is perhaps the most successful example of vertebrate pest biocontrol worldwide. The combination of myxoma virus and rabbit haemorrhagic disease virus still limits rabbit numbers to about 15% of their potential numbers, and without them the cost for agriculture alone would be in excess of $2 billion per year.

The cumulative environmental benefits of the release of myxoma virus (MV) in 1950 and rabbit haemorrhagic disease virus (RHDV) in 1995 includes landscape scale native vegetation regeneration, increased abundance of native plants and animals, continued persistence of many native threatened species, large scale carbon biosequestration, and improved landscape and ecosystem resilience. The cumulative economic benefits for agriculture alone from MV and RHDV over 60 years are estimated at $70 billion, or an average of $1.17 billion per year.

Just like rabbits, carp biocontrol is unlikely to ever kill the last carp. However, it is possible that the carp virus may help to reduce carp density below levels known to cause environmental harm, which is the actual aim of the NCCP.

Earlier studies have suggested that carp start impacting on ecosystem health at densities of 100–174 kg/ha. Numerous studies have shown that in places, carp density is currently much higher.

The research under the NCCP has provided the first comprehensive carp biomass estimate for Australia. Carp biomass fluctuates dramatically with rain fall and river flows but in general ranges from about 200,000 tonnes up to 1 millions tonnes after consecutive years of heavy rain fall. In many places, carp density (kg/ha) exceeds levels likely to cause environmental damage.

Find out more about Rabbit control: http://www.pestsmart.org.au/wp-content/uploads/2014/03/RabbitBiocontrol.pdf

 

 

Won't this be like cane toads?

Non-scientific

The introduction of cane toads (Rhinella marinus) to Australia in the 1930s is one of the most notable examples of poor early biological control practice.

Introduced before Australia had mature biosecurity legislation or environmental risk assessment based regulations for importing exotic organisms, the cane toad was released based on overseas commentary and without any direct assessment of its likely effectiveness as a biocontrol agent on the targeted cane beetle.

After release, it quickly became apparent that the toxic toads did not effectively prey on the cane beetle, but were devastatingly efficient in preying Australian native species. Free from natural predators, and with abundant food supplies, toad numbers and distribution quickly exploded, creating the ecological disaster we see today. Today, Australia’s biosecurity regulatory environment is world class and nothing like this could legally happen again.

Under these regulations, if carp biocontrol is used, it will be informed by rigorous planning and world-class risk assessment processes based on robust evidence to indicate carp biocontrol can be done safely and effectively. 

 

Scientific

The introduction of cane toads (Rhinella marinus) to Australia in the 1930s is one of the most notable examples of early poor biological control practice.

Introduced before Australia had any mature biosecurity legislation, or environmental risk assessment based regulations for importing exotic organisms, it was released based on overseas commentary and without direct assessment of its likely effectiveness as a biocontrol agent on the targeted cane beetle.

After release it quickly came to light that the toxic toads did not effectively prey on the cane beetle, though were devastatingly efficient in preying on a diversity of Australian native species. Free from natural predators, and with abundant food supplies, toad numbers and distribution quickly exploded, creating the ecological disaster we see today. The Australian biosecurity regulatory environment is now world class and nothing similar could legally happen again.

Under these regulations, if carp biocontrol is used it will be informed by robust planning, and careful risk assessment processes based on robust evidence to indicate carp biocontrol can be done safely and effectively.

Australia's National Carp Control Plan is operating in a completely different era in which the efficacy and host specificity of the carp virus on common carp been extensively studied and peer reviewed. Research conducted to date demonstrates that common carp present in Australia are highly vulnerable to the carp virus, that the virus only causes disease in European carp (also known as common carp), and that all other species are not susceptible.

Effectiveness on European carp

Research proposed under the National Carp Control Plan will further build our understanding of possible risks and benefits of carp biocontrol. A thorough, systematic quantitative assessment of social, economic and ecological risks is proposed, in addition to a robust and transparent process for quantifying expected benefits and costs of carp biocontrol in Australia.

The disease caused by the carp virus is highly contagious and extremely virulent with mortality that can reach 80-100% in naïve carp populations (Hedrick et al., 2000; Gilad et al., 2003; Sunarto and Cameron, 2005; Matsui et al., 2008; OIE, 2015).

Safety for non-target species

Research conducted to date demonstrates that the carp virus is specific to common carp and its ornamental variety, koi carp (Hedrick et al., 2000), although susceptibility varies across carp strains (OIE, 2014). A broad range of other fish species has been tested with varying degrees of rigour; none developed the disease (Bretzinger et al., 1999; Perelberg et al., 2003;  Haenen et al., 2004; Haenen and Hedrick, 2006; Uchii et al., 2009; OIE, 2015). Furthermore, no CyHV-3-associated mortalities have been reported in any fish or other animal anywhere in the world (Gotesman et al., 2013, Michel et al., 2010, OIE, 2012).

The CSIRO-Australian Animal Health Laboratory has conducted susceptibility trials using bath and intra-peritoneal injection methods alongside carp controls (McColl et al., 2016). The species selected represent a broad range of taxa with a breadth of evolutionary relationships with the order Cypriniformes, including:

  • the native siluriform blue catfish (Neoarius graeffeii) and freshwater eel-tailed catfish (Tandanus tandanus);
  • the anguilliform short-finned eel (Anguilla australis);
  • the perciform Murray cod (Maccullochella peelii), golden perch (Macquaria ambigua), silver perch (Bidyanus bidyanus);
  • the smaller-bodied olive perchlet (Ambassis agassizii) and carp gudgeon (Hypseleotris )
  • the native salmoniform common jollytail (Galaxias maculatus) and Australian smelt (Retropinna semoni);
  • the introduced rainbow trout (Onchorhynchus mykiss),
  • the native atheriniform crimson-spotted rainbowfish (Melanotaenia duboulayi);
  • the native mugiliform sea mullet (Mugil cephalus);
  • the native clupeiform bony herring (Nematalosa erebi);
  • the native petromyzontiform Short-headed lamprey (Mordacia mordax).

A representative mammal (mouse), bird (chicken), crustacean (yabby Cherax destructor), reptiles (eastern water dragon, Intellagama lesueurii) and Macquarie short-necked turtle (Emydura macquarii). The amphibians Peron’s tree frog (Litoria peroni) and spotted marsh frog (Limnodynastes tasmaniensis) have also been tested.

None of these species showed any signs of infection by CyHV-3.

Research conducted to date also demonstrates that European carp present in Australia are highly vulnerable to the carp virus, that the virus only causes disease in European carp, and that all other species are not susceptible.

Research proposed under the National Carp Control Plan will further build our understanding of possible risks and benefits of carp biocontrol. A thorough, systematic quantitative assessment of social, economic and ecological risks is proposed, in addition to a robust and transparent process for quantifying expected benefits and costs of carp biocontrol in Australia.

The carp virus will also not be released until it has been independently evaluated by regulators in two government departments against two regulated review processes and assessed under the Biological Control Act and approved by Chief Veterinary Officers of all jurisdictions 

References

  • Bretzinger, A., Fischer-Scherl, T., Oumouna, M., Hoffman, R. and Truyen, U. (1999). Mass mortalities in koi carp, Cyprinus carpio, associated with gill and skin disease. Bulletin of the Eurasian Association of Fish Pathologists 19, 182 – 185.
  • Gilad, O., Yun, S., Adkison, M. A., Way, K., Willits, N. H., Bercovier, H. and Hedrick, R. P. (2003). Molecular comparison of isolates of an emerging fish pathogen, koi herpesvirus, and the effect of water temperature on mortality of experimentally infected koi. Journal of General Virology 84, 2661-2668.
  • Gotesman, M., Kattlun, J., Bergmann, S. M. and El-Matbouli, M. (2013). CyHV-3: the third cyprinid herpesvirus. Diseases of Aquatic Organisms, 105, 163-174.
  • Haenen, O. and Hedrick, R. (2006). Koi herpesvirus workshop. Bulletin- European Association of Fish Pathologists, 26.
  • Haenen, O., Way, K., Bergmann, S. M. and Ariel, E. (2004). The emergence of koi herpesvirus and its significance to European aquaculture. Bulletin of the Eurasian Association of Fish Pathologists 24, 293 – 307.
  • Hedrick, R. P., Gilad, O., Yun. S., Spangenberg, J. V., Marty, G. D., Nordhausen, R. W., Kebus, M. J., Bercovier, H. and Eldar, A. (2000). A herpesvirus associated with mass mortality of juvenile and adult koi, a strain of common carp. Journal of Aquatic Animal Health 12, 44 – 57.
  • Matsui, K., Honjo, M. I. E., Kohmatsu, Y., Uchii, K., Yonekura, R. and Kawabata, Z. I. (2008). Detection and significance of koi herpesvirus (KHV) in freshwater environments. Freshwater Biology, 53, 1262-1272.
  • Michel, B., Fournier, G., Lieffrig, F., Costes, B. and Vanderplasschen, A. (2010). Cyprinid herpesvirus 3. Emerging Infectious Diseases, 16, 1835 - 1843.
  • Perelberg, A., Smirnov, M., Hutoran, M., Diamant, A., Bejerano, Y. and Kotler, M. (2003). Epidemiological description of a new viral disease afflicting cultured Cyprinus carpio in Israel. The Israeli Journal of Aquaculture, 55(1), 5-12.
  • Sunarto, A. and Cameron, A. (2005). Response to mass mortality of carp: an Indonesian experience. FAO fisheries proceedings, 2005, 87-105.
  • Uchii, K., Matsui, K., Iida, T. and Kawabata, Z. (2009). Distribution of the introduced cyprinid herpesvirus 3 in a wild population of common carp, Cyprinus carpio Journal of Fish Diseases, 32, 857-864.
  • World Organisation for Animal Health (OIE; 2014 ). Manual of diagnostic tests for aquatic animals.
  • World Organisation for Animal Health (OIE; 2015). World Animal Health Information Database (WAHID). http://www.oie.int/wahis_2/public/wahid.php

 

 

What will everything eat when carp numbers are significantly reduced?

Non-scientific

Some Australian native species including Murray cod, Eastern water rats, cormorants and long-nosed fur seal prey on the pest fish species carp. Consequently, there is a need to understand what might happen if carp numbers are significantly reduced following possible implementation of the National Carp Control Plan.

Though there are only few studies which have examined the importance of carp as a food source in Australian ecosystems, available research suggests that they do not comprise a large part of the diet of most species. This is partly because juvenile carp often live in different habitat types to predators such as adult Murray cod. Also, carp quickly grow to a size that is too large for most predatory species to eat.

A recent Queensland study demonstrated that reducing carp numbers can cause subsequent explosions in biomass levels of other native prey items including zooplankton and small-bodied native fish. This work indicates that reducing carp numbers may increase food available for predatory species, not decrease it. This, in turn, may result in healthier populations of species which eat these prey items, including popular native angling species. 

 

Scientific

Carp make up a significant proportion of the biomass of fish in many Australian rivers (Harris et al., 1997; SRA Unpublished Data; Lintermans, 2007), and potentially significant reductions in carp abundance and biomass levels if a carp biocontrol program were to eventually proceed have prompted some to ask the question "what will species that currently prey on carp eat?". The insinuation is that without carp, these species might not have sufficient food available and starve. But what does the available research say?

Whilst there is not a significant body of research available on the contribution that carp make to the diet of native Australian species, what information does exist suggests that they generally do not form a significant dietary component. In fact, Koehn et al. (2004) suggests the rapid expansion of carp within Australia may have been assisted by lack of predatory pressure. This is largely because the rapid growth rate of carp enables them to quickly reach a size that precludes their consumption by most predators.

Nevertheless, some species of native fish, waterbirds, and charismatic fauna do prey on carp to some extent. In particular, Australia’s largest predatory freshwater fish, the Murray cod, has been shown to predate upon carp. A dietary study by (Ebner, 2006) found 35% of Murray cod sampled (all cod >500mm total length) contained carp. Similarly, Baumgartner (2005) found Cyprinidae spp. (carp and Goldfish) constitute up to 25% of Murray cod prey occurrence in cod sampled from the Murrumbidgee River.

Examination of Murray cod stomachs from Rivers of the Southern Murray Darling Basin found <7% of Murray cod stomachs sampled contained carp (Doyle et al., 2012). Carp up to 410 mm total length have been recorded in stomach of large Murray cod. Doyle et al. (2012) attributes the low occurrence of carp in the diet of Murray cod to the variation in the habitat utilisation between early life stages of carp, which primarily inhabit shallow floodplain-type habitats, and Murray cod that prefer main channel habitats.

Golden perch and Australian bass also consume small carp, though infrequently, and each species would be incapable of consuming adult carp due to their gape size limitations (Ebner, 2006, Doyle et al., 2012). The critically endangered Trout cod has also been shown to predate carp, however carp made up <1% of their prey (Baumgartner, 2005).

Terrestrial vertebrates that have been shown to predate upon carp and other Cyprinids include feral cats (Jones and Coman, 1981), the Eastern water rat (Woollard et al., 1978) and cormorants (Miller, 1979), however in each instance European carp constitute a small proportion of the diet. Hughes et al. (1983) suggests carp within billabongs may provide a reliable food source for Australian Pelicans. Koehn et al. (2004) suggests with few effective predators, sequestered detrital carbon, rather than passing up through subsequent trophic levels of macroinvertebrates and smaller fish (Bunn and Davies, 1999), may become ‘locked’ away from the trophic chain for the lifespan of a carp (up to 50 years) (Bănărescu and Coad, 1991).

So some native Australian species do prey on carp. However, it is important to note that native species which currently prey on this pest species have not always relied upon them for food. In years gone by when carp were absent or in much lower numbers in Australian waterways native prey items including zooplankton, invertebrates, small fish, biofilms were more abundant. Ebner et al. (2006) suggested major shifts in prey availability have influenced the ecology of Murray cod and the structure and function of the food web in the rivers of the Murray-Darling Basin.

The decreased diversity of native prey species provides opportunity for Murray cod to exert a larger per capita effect on carp (Pimm, 1982, Ebner, 2006). However in microcosm trials both Murray cod and golden perch consumed carp relatively infrequently compared to native prey species (Doyle et al., 2012). There are very few published studies which provide an insight into potential alterations to food web dynamics that may result from significant reductions in carp biomass within Australian aquatic ecosystems.

Prey switching of carp’s dominant predator, the Murray cod, may exert increased pressure upon other native species and decapods, though the ecosystem will eventually reach a predator/prey equilibrium (McColl et al., 2014). Furthermore, the reduction of carp may allow the proliferation of native prey items of predatory species including Murray cod returning the ecosystem closer to its former state before the proliferation of carp in the 1960s. Indeed the findings of Gehrke et al. (2010) showed that after a significant reduction in carp biomass within several experimental wetlands the biomass of small-bodied native fish increased by up to three times the biomass of carp removed. In the Queensland study, carp biomass was reduced within two of four lagoons, removing 43% and 33% of carp biomass, 34 and 26 kg per hectare respectively. The other two lagoons remained untouched, for comparison. In the two lagoons where carp were controlled, native fish biomass increased by 90 kg per hectare, roughly three times the biomass of carp removed. Added to this, large zooplankton populations (e.g. Boekella and Daphnia) increased 10 times and populations of aquatic insects and crustaceans also boomed. In the two lagoons where nothing was done, populations of native fish, zooplankton, aquatic insects or crustaceans did not change.

What this work suggests is that native fish are much more efficient in their use of food resources than carp (producing three times the biomass) and that removing carp will likely increase the food available for predatory fish and waterbirds, not decrease it. This should ultimately lead to bigger, healthier populations of popular native angling species and waterbirds.

References

  • Banarescu, P. and Coad, B. (1991). Cyprinids of Eurasia. Cyprinid Fishes. Springer.
  • Baumgartner, L. J. (2005). Effects of weirs on fish movements in the Murray-Darling Basin, University of Canberra.
  • Bunn, S. and Davies, P. (1999). Aquatic food webs in turbid, arid-zone rivers: preliminary data from Cooper Creek, western Queensland. A Freeflowing River: the Ecology of the Paroo River, 67-76.
  • Doyle, K., Mcphee, D. and Wallter, G. (2012). ‘Can native predatory fishes control invasive carp in south-eastern Australia’. In: Forum Abstracts: Cap management in Australia - State of knowledge. Canberra: Invasive Animals Cooperative Research Centre.
  • Ebner, B. (2006). Murray cod an apex predator in the Murray River, Australia. Ecology of Freshwater Fish, 15, 510-520.
  • Gehrke, P. C., St Pierre, S., Matveev, V. and Clarke, M. (2010). Ecosystem responses to carp population reduction in the Murray-Darling Basin. Project MD923 Final Report to the Murray-Darling Basin Authority
  • Harris, J. H. and Gehrke, P. C. (1997). Fish and rivers in stress: the NSW rivers survey, Cronulla, NSW, NSW Fisheries Office of Conservation and the Cooperative Research Centre for Freshwater Ecology, in association with NSW Resource and Conservation Assessment Council.
  • Jones, E. and Coman, B. (1981). Ecology of the feral cat, Felis catus (L.), in South-Eastern Australia I. Diet. Wildlife Research, 8, 537-547.
  • Koehn, J. D. (2004). Carp (Cyprinus carpio) as a powerful invader in Australian waterways. Freshwater Biology, 49, 882-94.
  • Lintermans, M. (2007). Fishes of the Murray-Darling Basin: an introductory guide, Murray-Darling Basin Commission, Canberra.
  • McColl, K., Cooke, B. and Sunarto, A. (2014). Viral biocontrol of invasive vertebrates: Lessons from the past applied to cyprinid herpesvirus-3 and carp (Cyprinus carpio) control in Australia. Biological Control, 72, 109-117.
  • Miller, B. (1979). Ecology of the Little Black Cormorant, Phalacrocorax sulcirostris, and Little Pied Cormorant, melanoleucos, in Inland New South Wales I. Food and Feeding Habits. Wildlife Research, 6, 79-95.
  • Pimm, S. L. (1982) Food webs,
  • Woollard, P., Vestjens, W. and Maclean, L. (1978). The ecology of the eastern water rat Hydromys chrysogaster at Griffith, NSW: food and feeding habits. Wildlife Research, 5, p. 59.

 

 

Don't Murray Cod rely on carp for food?

Non-scientific

While Carp currently make up around 25-35% of the diet of Murray cod, research has shown that cod prefer native prey items if given a choice. If carp numbers are reduced, Murray cod will switch foods. This behaviour is common in many fish species.

Studies have also shown that a reduction of carp can allow small native fish, and the microscopic food they eat, to flourish. This makes the food web healthier and more natural.

One study showed that native bony bream biomass increased 240% and 1130% after carp reduction in two experimental lagoons, while native gudgeon biomass increased by more than 1600% in one lagoon. Lagoons where no carp control occurred showed no increase in native fish over the same period. 

 

Scientific

Australia’s largest predatory freshwater fish, the Murray cod, has been shown to eat carp. A dietary study by (Ebner, 2006) found 35% of Murray cod sampled (all cod >500 mm total length) contained carp. Similarly, Baumgartner (2005) found cyprinids (carp and goldfish) constituted up to 25% of prey occurrence in Murray cod sampled from the Murrumbidgee River.

Examination of Murray cod stomachs from rivers of the Southern Murray-Darling Basin found <7% of Murray cod stomachs sampled contained carp (Doyle et al., 2012). Doyle et al. (2012) attributes this low occurrence of carp in the diet of Murray cod to differences in habitat utilisation between early life stages of carp that primarily inhabit shallow floodplain-type habitats, and Murray cod that prefer main channel habitats. Given the large gape of Murray cod and the species’ ability to attain up to 180cm in length (Lintermans, 2007), large Murray cod are likely to be the only fish predator of adult carp. Carp up to 410 mm total length have been recorded in stomachs of large Murray cod (J. Stocks, pers. comm.). Golden perch and Australian bass may also consume small carp, though these species would be incapable of consuming adult carp due to their gape size limitations (Ebner, 2006; Doyle et al., 2012). The critically endangered trout cod has also been shown to eat carp, but carp made up <1% of their prey (Baumgartner, 2005).

Australian freshwater ecosystems are now significantly modified, with native fish biomass significantly reduced, and carp are now the dominant species within many Australian rivers (Harris and Gehrke, 1997; SRA unpublished data; Lintermans, 2007). Ebner et al. (2006) suggests these major shifts in prey availability have influenced the ecology of Murray cod and the structure and function of the food web in the rivers of the Murray-Darling Basin. The decreased diversity of native prey species provides opportunity for Murray cod to exert a larger per capita effect on carp (Pimm, 1982; Ebner, 2006). However, in microcosm trials, both Murray cod and golden perch consumed carp relatively infrequently compared to native prey species (Doyle et al., 2012).

Fish regularly shift diet in response to varying availability of food resources (Werner and Gilliam, 1984). If carp numbers were greatly reduced, prey switching of carp’s dominant predator, the Murray cod, may exert increased pressure upon other native species and decapods, though the ecosystem should eventually reach a predator/prey equilibrium (McColl et al., 2014). Further, the reduction of carp may allow the proliferation of native prey items of Murray cod, thus returning the ecosystem closer to its former state before the proliferation of carp in the 1960’s. Indeed the findings of Gehrke et al. (2010) showed that after a significant reduction in carp biomass within several experimental wetlands the biomass of small-bodied native fish increased by up to three times the biomass of carp removed. Bony bream (Nematalosa erebi) biomass increased by 240% and 1130% in the two experimental lagoons, while gudgeon (Hypseleotris spp.) biomass increased by more than 1600% in one lagoon. Native fish in control lagoons showed no increase over the same period. Similarly, anecdotal evidence from western New South Wales suggests that macroinvertebrates may increase in abundance following carp reduction (Ellis, 2016).

Koehn et al. (2004) suggests that, with few effective predators of carp in the ecosystem, detrital carbon that is sequestered in the bodies of carp, rather than passing up through subsequent trophic levels of macroinvertebrates and smaller fish (Bunn and Davies, 1999), may become ‘locked away’ from the trophic chain for the lifespan of a carp (up to 50 years) (Bănărescu and Coad, 1991). Equally, significant reduction in carp numbers within Australian rivers may enable nutrients to once again become available to Australian native species.  It should be noted, however, that other invasive species (redfin perch, goldfish, oriental weather loach) may also benefit from reduced carp numbers.

References

  • Banarescu, P. and Coad, B. (1991). Cyprinids of Eurasia. Cyprinid Fishes. Springer.
  • Baumgartner, L. J. (2005). Effects of weirs on fish movements in the Murray-Darling Basin, University of Canberra.
  • Bunn, S. and Davies, P. (1999). Aquatic food webs in turbid, arid-zone rivers: preliminary data from Cooper Creek, western Queensland. A Freeflowing River: the Ecology of the Paroo River, 67-76.
  • Doyle, K., Mcphee, D. and Wallter, G. (2012). ‘Can native predatory fishes control invasive carp in south-eastern Australia’. In: Forum Abstracts: Cap management in Australia - State of knowledge. Canberra: Invasive Animals Cooperative Research Centre.
  • Ebner, B. (2006). Murray cod an apex predator in the Murray River, Australia. Ecology of Freshwater Fish, 15, 510-520.
  • Ellis, I. (2016). Lock 8 Wetland 780 fish nursery project on Ta-Ru Land. NSW Department of Primary Industries – Fisheries, Buronga.
  • Gehrke, P. C., St Pierre, S., Matveev, V. and Clarke, M. (2010). Ecosystem responses to carp population reduction in the Murray-Darling Basin. Project MD923 Final Report to the Murray-Darling Basin Authority
  • Harris, J. H. and Gehrke, P. C. (1997). Fish and rivers in stress: the NSW rivers survey, Cronulla, NSW, NSW Fisheries Office of Conservation and the Cooperative Research Centre for Freshwater Ecology, in association with NSW Resource and Conservation Assessment Council.
  • Koehn, J. D. (2004). Carp (Cyprinus carpio) as a powerful invader in Australian waterways. Freshwater Biology, 49, 882-94.
  • Lintermans, M. (2007). Fishes of the Murray-Darling Basin: an introductory guide, Murray-Darling Basin Commission, Canberra.
  • McColl, K., Cooke, B. and Sunarto, A. (2014). Viral biocontrol of invasive vertebrates: Lessons from the past applied to cyprinid herpesvirus-3 and carp (Cyprinus carpio) control in Australia. Biological Control, 72, 109-117.
  • Pimm, S. L. (1982). Food webs,
  • Werner, E. E. and Gilliam, J. F. (1984). The ontogenetic niche and species interactions in size-structured populations. Annual Review of Ecology and Systematics, 15, 393–425

 

 

Will the virus affect other species?

Non-scientific

Specificity to the target organism is a fundamental pre-requisite for a biological control agent. Research on the carp virus’s specificity to carp has been conducted both in Australia and internationally, but some uncertainty remains, and the NCCP has recommended additional work in this area to support evidence-based decision making. 

Disease caused by the carp virus has only been reported in European Carp (Cyprinus carpio) and hybrids thereof (e.g. hybrids of carp and Goldfish). However, viral infection is not always accompanied by clinical signs of disease, and knowing whether the carp virus is causing these less obvious “sub-clinical” infections in any native species is clearly important for decision-making on future directions for carp control in Australia. Evidence in this area is equivocal. On one hand, research that searched for viral messenger RNA (mRNA)—a key indicator of viral replication, and therefore infection—has found no evidence of infection in any non-carp species. On the other hand, international researchers have concluded that some non-carp fishes could be susceptible to carp-virus infection after trials using other methods. For example, non-carp fishes can be exposed to the virus, kept in isolation for varying periods of time, then housed with susceptible carp. Subsequent emergence of infection and/or disease in carp can be interpreted as evidence that the non-carp fish were infected, and passed the infection to carp. Likewise, carp virus DNA has been detected in a range of non-target species, although this does not necessarily indicate infection, and may simply reflect virus particles in or on the body of a non-target animal following exposure to high doses of the virus during testing. Finally, in non-target species susceptibility trials involving Australian native fish, some species had higher mortalities after exposure to the virus than were recorded in control groups (those not exposed to the virus). While subsequent testing of these individuals found no evidence of viral infection, the exact cause of death was not determined. 

In response to these questions and uncertainties, the NCCP commissioned a review of best practice in viral challenge trials using the carp virus. This review concluded that some non-target species could potentially be infected by the carp virus. In response, the NCCP commissioned additional viral challenge trials. Again, these did not find viral mRNA in the tested non-target species, indicating they were not infected. However, water-quality issues precluded testing of Rainbow Trout in these trials. Additionally, mortalities were recorded in in both test (i.e. exposed to the virus) and control (not exposed to the virus) fish that, while apparently not due to infection by the virus, could not be definitively attributed to other causes. Given these considerations, and consistent with a precautionary approach to this important issue, the NCCP has recommended additional research testing the virus’s specificity to carp as part of further decision-making on future directions for carp biocontrol.
 

 

Scientific

Refer to non-scientific response above.

 

 

Native catfish are the closest Australian native fish relatives to carp. Does this mean that catfish could be vulnerable to the virus?

Non-scientific

Viruses are generally most likely to jump between closely related species. Australia does not have any native cyprinids (the group of fishes to which carp belong). Catfishes are the native Australian fish most closely related to cyprinids. Therefore, it could be reasonable to infer that if the virus was going to jump to a native Australian species, native catfish could be a likely candidate. However, while catfish is the most closely related to the common carp group, the two are not particularly closely related in an evolutionary sense. There is no evidence that native Australian catfish are susceptible to the carp virus. As noted in a previous FAQ, the NCCP has recommended additional work assessing the species specificity of the carp virus.

 

Scientific

The most closely related taxonomic order to the one carp belongs to is Siluriformes (catfish), leading some to question whether their level of relatedness is sufficient to create risk of species jump.

To understand this question, a brief journey into the science of phylogenetics is necessary. Phylogenetics investigates the evolutionary origins and relationships of organisms at varying levels of taxonomic organisation. Thus, phylogenetic ‘relatedness’ among organisms is defined by how recently (in evolutionary terms) two or more groups diverged from a common ancestor and emerged as taxonomic entities recognisable in their modern forms.

Carp and catfish belong to two distinct taxonomic orders, Cypriniformes and Siluriformes respectively. Cypriniformes and Siluriformes form part of a broader phylogenetic grouping of fishes called the Otophysi. The two other otophysian orders are Gymnotiformes (electric eels and American knifefishes), and Characiformes, which includes piranhas and tetras.

Otophysi forms one of two series in the superorder Ostariophysi. A superorder is a fine-scale taxonomic ranking above Order and below Class (see below), and encompasses greater technical detail than is required for a general understanding of the evolutionary relationship between carp and catfish.

The Otophysi are generally considered to be monophyletic, meaning that, in the distant evolutionary past, they arose from a single common ancestor before diverging into different species and moving across the Earth’s surface to occupy their present geographical ranges (Briggs, 2005. Otophysian evolution has been debated, and is complicated by a span of more than 100 million years for which no fossil evidence has been discovered (Briggs, 2005; Santini et al., 2009; Nakatani et al., 2011; Chen et al., 2013).

Nonetheless, molecular evidence suggests that Cypriniformes emerged as a recognisable taxonomic entity between 130 and 186 million years ago, while Siluriformes diverged somewhat earlier, between 162 and 198 million years ago (Nakatani et al., 2011). The fossil record provides relatively more recent divergence dates, with the oldest-known cypriniform fossil being about 61 million years old and the oldest siluriform fossil being between 83.5 and 88.6 million years old (Chen et al., 2013).

Divergence inferences from the fossil record can only be based on available fossils and cannot account for undiscovered material or species that no left fossilised remains. Consequently, divergence times estimated from the fossil record are usually underestimated (Anderson, 2012). The most recent common ancestor of carp and catfish therefore lived tens, and perhaps hundreds, of millions of years ago. Thus, catfish are indeed the Australian native fishes most closely related to carp, but this not imply that the evolutionary relationship is particularly close.

Broad description of taxonomic relatedness

The degree of taxonomic relatedness between carp and catfish may be contextualised by a general overview of the taxonomic hierarchy (the scheme scientists use to classify living things). From broadest (i.e. least related) to narrowest (i.e. an individual species), the taxonomic hierarchy is:

Kingdom: Kingdom is the broadest level of biological classification. For example, all multicellular animals, whether mosquitos or elephants, are classified into the Kingdom Animalia. Taxonomists in Australia, Great Britain, and several other countries generally recognise five kingdoms; Animalia, Plantae, Fungi, Protista, and Monera, while American taxonomists sometimes divide the Kingdom Monera (bacteria) into two kingdoms (Archaeabacteria and Eubacteria), making a total of six kingdoms.

Phylum: The Phylum is another very broad level of classification. For example, the Phylum Chordata includes all vertebrates and some invertebrates. The defining features of chordates (animals within the Phylum Chordata) are possession, at some stage in the life-history, of:

  • Pharyngeal slits
  • A dorsal nerve chord
  • A notochord
  • A post-anal tail

Thus, human beings (and indeed all mammals), birds, reptiles, amphibians, fish, and some invertebrates, such as sea squirts, all belong in the Phylum Chordata.

Class: There are approximately 107 animal classes, although this number can vary based on taxonomic revisions. As an example of the degree of relatedness implied by this taxonomic rank, human beings belong in the class Mammalia, along with all other mammals (i.e. whales, seals, dolphins, cats, dogs, horses, cows, mice etc).

Focussing specifically on fish, the class Actinopterygii, to which carp and catfish both belong, includes all fishes apart from sharks, rays, and jawless fishes (lampreys, hagfish). Thus, for example, carp, catfish, barramundi, all tunas, marlin, mullet, gudgeons, gobies, coral trout, Murray cod, and approximately 24,000 other fish species are all actinopterygians.

Order: Order is the taxonomic rank at which carp (order Cypriniformes) and catfish (order Siluriformes) diverge. To place the concept of order in context, human beings belong in the order Primates, along with lemurs, lorises, tarsiers, monkeys, and apes. Domestic dogs belong in the order Carnivora, along with cats (including the big cats), seals, walruses, weasels, skunks, hyaenas, and many other predatory mammals.

Family: At this taxonomic rank, carp and catfish have now diverged. The order Cypriniformes is divided into 11-12 families, with common carp (Cyprinus carpio) belonging to family Cyprinidae. The order Siluriformes, to which catfish belong, contains more than 30 families. The two catfish species tested for susceptibility to Cyprinid herpesvirus 3 belong to two separate families, Ariidae (forktail catfishes) and Plotosidae (eel-tailed catfishes).

Genus: Organisms sharing this taxonomic rank are closely-related. Introduced common carp are the only species from the genus Cyprinus occurring in Australia. Human beings belong to the genus Homo. Modern humans (i.e. us) are the only extant (surviving) representatives of this genus.

Species: Individuals within a species can interbreed to produce fertile offspring. The common carp is a species; Cyprinus carpio, while modern humans are also a species; Homo sapiens. A scientific name for a species thus comprises the genus name (e.g. Cyprinus), which is shared by all species within that genus, and the specific epithet (e.g. carpio). Taxonomists are sometimes interested in defining sub-species, a finer classification again, but this level of detail is unnecessary for the present discussion.

References

  • Anderson, J.S. (2012). Fossils, molecules, divergence times, and the origin of Salamandroidea. Proceedings of the National Academy of Sciences, 109, http://www.pnas.org/cgi/doi/10.1073/pnas.1202491109
  • Briggs, J.C. (2005). The biogeography of otophysan fishes (Ostariophysi: Otophysi): a new appraisal. Journal of Biogeography, 32, 287 – 294.
  • Chen, W-J., Lavoué, S. & Mayden, R.L. (2013). Evolutionary origin and early biogeography of otophysan fishes (Ostariophysi: Teleostei). Evolution, 67, 2218 – 2239.
  • Mccoll, K. A., Sunarto, A., Slater, J., Bell, K., Asmus, M., Fulton, W., Hall, K., Brown, P., Gilligan, D., Hoad, J., Williams, N. and Crane, M. St J. (2016). Cyprinid herpesvirus 3 as a potential biological control agent for carp (Cyprinus carpio) in Australia: susceptibility of non-target species. Journal of Fish Diseases, 40, 1141-1153.
  • Nakatani, M., Miya, M., Mabuchi, K., Saitoh, K. & Nishida, M. (2011). Evolutionary history of Ostophysi (Teleostei), a major clade of the modern freshwater fishes: Pangaean origin and Mesozoic radiation. BMC Evolutionary Biology,11: 177 http://www.biomedcentral.com/1471-2148/11/177
  • Santini, F., Harmon, L.J., Carnevale, G. & Alfaro, M.E. (2009). Did genome duplication drive the origin of teleosts? A comparative study of diversification in ray-finned fishes. BMC Evolutionary Biology 9:194 doi:10.1186/1471-2148-9-194

 

 

Why weren't all Australian native species (& sizes) tested?

Non-scientific

Representatives of most major groups of freshwater fish that carp interact with across their distribution in Australia have been tested (see the list of species tested and see figure below). 

Diagram of the Fish family tree

 

Scientific

Refer to the non-scientific response above.

 

Do genetic changes in the virus pose an unacceptable risk?

Non-scientific

Refer to the scientific response below.

 

Scientific

At approximately 295,000 base pairs, CyHV-3 has the largest genome in the family Alloherpesviridae (Fournier and Vanderplasschen, 2011; Davison et al., 2013). Viruses with large DNA genomes are generally all stable and less likely to undergo mutations that lead to host switching than are smaller RNA viruses. This high stability is supported by the high similarity of all CyHV-3 isolates at the sequence level (>99%).

Further evidence in support of this assessment of low risk includes the lack of reports that CyHV-3 has extended its host range beyond common and koi carp in those countries where it is endemic since first recognized in the mid-1990s (Aoki et al., 2007, Hedrick et al., 2000), and a lack of evidence that closely related cyprinid herpesviruses, CyHV-1 (Sano et al., 1985) and CyHV-2 (Jung and Miyazaki, 1995) have extended their host range since discovery. Nonetheless, the NCCP has recommended additional work assessing the species specificity of the carp virus.

References

  • Aoki, T., Hirono, I., Kurokawa, K., Fukuda, H., Nahary, R., Eldar, A., Davison, A. J., Waltzek, T. B., Bercovier, H. and Hedrick, R. P. (2007). Genome sequences of three koi herpesvirus isolates representing the expanding distribution of an emerging disease threatening koi and common carp worldwide. Journal of Virology, 81, 5058-5065.
  • Davison, A. J., Kurobe, T., Gatherer, D., Cunningham, C., Korf, I., Fukuda, H., Hedrick, R. P. and Waltzek, T. B. (2013). Comparative genomics of carp herpesviruses. Journal of Virology, 87, 2908 – 2922.
  • Fournier, P.G. and Vanderplasschen, A. (2011). Cyprinid herpesvirus 3: an interesting virus for applied and fundamental research. Bulletin de l’Académie Vétérinaire de France, 164, 353-358.
  • Hedrick, R., Gilad, O., Yun, S., Spangenberg, J., Marty, G., Nordhausen, R., Kebus, M., Bercovier, H. and Eldar, A. (2000). A herpesvirus associated with mass mortality of juvenile and adult koi, a strain of common carp. Journal of Aquatic Animal Health, 12, 44-57.
  • Jung, S. J. and Miyazaki, T. (1995). Herpesviral haematopoietic necrosis of goldfish, Carassius auratus (L.). Journal of Fish Diseases, 18, 211-220.
  • Sano, T., Fukuda, H., Furukawa, M., Hosoya, H. and Moriya, Y. (1985). A herpesvirus isolated from carp papilloma in Japan. Fish Shell Pathol, 32, 307-311.

 

 

Can you 100% guarantee the virus will not mutate if released?

Non-scientific

Predicting viral evolution is inherently difficult. However, some general patterns have emerged that suggests there is a low probability of the carp virus mutating or otherwise evolving in ways that would enable it to infect a new host species (including Australian native species). This low risk is partly because the Carp virus is a type of virus – a double-stranded DNA virus - for which mutation of a type and magnitude that would enable infection of a new host species is not a primary mechanism of viral evolution, and partly because common carp are not closely related to any Australian native species.

Because nothing in science is 100% guaranteed, the possibility of the carp virus eventually evolving a new host association cannot be excluded, however such a process would operate over longer timescales than this control strategy would require if approved. 

 

Scientific

Refer to the non-scientific response above.

 

Will 2,000,000 tonnes of carp die in 2 days?

Non-scientific

No. Carp biomass and epidemiological research under the NCCP indicates that the virus would likely need to be introduced into targeted carp sub-populations to effectively cause outbreaks. The virus transmits most effectively when carp are in physical contact and only causes disease in carp within a fairly narrow temperature range. These factors mean that major, approximately simultaneous carp mortalities across broad geographic areas are unlikely.

For more details on specific research projects, please visit our research page

 

Scientific

Refer to the non-scientific response above.

 

 

Informing possible release

 

When and where might the virus be released?

Non-scientific

As a national-scale assessment of the feasibility of carp biocontrol, the NCCP has provided a strategic overview of the considerations and approaches that could inform virus deployment strategies if carp biocontrol were to proceed. Specific recommendations about times and places for virus release are beyond the NCCP’s scope, and would be developed at the regional scale much later in the planning process, if governments decide to proceed towards implementation. 

In general, carp-biocontrol effectiveness is likely to be maximised, and risks most effectively managed, if virus release occurred during a year that was neither particularly dry nor subject to flooding, and during a period of low to moderate carp population density. In terms of release locations, various potential release strategies involving staged deployment of the virus into different portions of the Murray-Darling Basin and coastal catchments have been developed and reported in the NCCP. However, these strategies are general guidelines for planning only. If carp biocontrol eventually proceeds, considerable regional- and local-scale planning would be needed to develop detailed release strategies, with consideration given to local climate and weather patterns, river heights and flows, and carp aggregating behaviour among other factors.

 

Scientific

Refer to the non-scientific response above.

 

Could the virus be released in a controlled manner (i.e. staging to reduce the impact of a clean-up)?

Non-scientific

In the simplest terms, two broad approaches exist for deployment of the carp virus, if a carp biocontrol program proceeds. The first is broadscale deployment, which would aim to introduce the virus into carp populations right across the species’ Australian distribution more or less simultaneously (i.e. over a single spring/summer season). The second is a more staged approach, in which carp sub-populations within defined regions would be targeted for biocontrol and carcass management activities before operations move on to the next region. 

The first option (broadscale) has some advantages (particularly in terms of assisting in mitigating the potential development of herd immunity), but would be logistically very difficult and likely to result in overstretching available resources. Therefore, the NCCP has recommended a more staged approach, in which particular portions of the Murray-Darling Basin and affected coastal catchments and the interconnected carp sub-populations they contain are targeted for virus deployment and carcass-management activities. Importantly, these regions are large enough to retain some benefits of the broadscale approach. In contrast, a very fine-scale approach to releasing the virus, in which (for example), only individual waterbodies or river reaches were targeted for deployment at any one time, would largely relinquish many of these advantages and could foster development of herd immunity and/or rebuilding of carp numbers through immigration from areas that had not yet been targeted.

The order in which different portions of the Murray-Darling Basin and the various coastal catchments could be targeted for deployment could vary depending on a range of considerations, and section 3 of the NCCP (Implementation strategy) describes these permutations. In general though, release strategies would need to be shaped around the biology of carp and the virus. In particular, the emergence of temperatures that enable infection and disease in carp, and the onset of carp aggregating behaviour will dictate optimum times for biocontrol operations in particular regions. In general, permissive temperatures for infection and disease in carp will occur from spring through to early summer in most portions of the species’ Australian distribution, with northern portions of carp’s Australian range tending to experience these conditions earlier in the season than more southern areas.

 

Scientific

Refer to the non-scientific response above.

 

What will happen to the dead carp?

Non-scientific

Dead carp will occur in waterways where the virus is released. The extent of the dead carp and how water quality will respond to dead carp was the subject of NCCP research.  This NCCP research indicates that significant water quality impacts are not expected in many waterbodies.

There will however need to be management and possible removal of dead carp in high water quality risk locations. 

A critical challenge for the NCCP has been to demonstrate how we can manage dead carp in a way which avoids impacts on water quality, people, livestock and native species. To meet this challenge, the NCCP undertook research, and talked to experts and local communities about how to respond to dead carp biomass.

The ideas that researchers, stakeholders and experts come up with were tested and refined through regional case studies which workshopped how the virus release can be managed in a specific region. The case studies involved all the relevant authorities, stakeholders who might be impacted and people with local knowledge about carp and their waterways.

The NCCP identified a range of methods to respond to the build-up of dead carp at a wide range of locations including:

  • regulating water flows to flush, move or dry out water bodies where dead carp biomass is located
  • removal or clean-up of dead carp biomass with boats, nets, booms, pumps and specifically engineered machinery
  • the movement of dead carp to low risk sites, and
  • leaving the dead carp in situ where there are no impacts.

The specific chosen methods for managing dead carp will depend on local conditions and arrangements.

Dead carp that are removed from the waterways will be transported to regional processing facilities wherever possible. The NCCP has a specific research project which will recommend how the dead carp biomass can be used. Where is it not possible to use the dead carp, they will be disposed of at approved waste disposal sites.

Virus response will be managed through co-ordinated regional, state and national bodies that bring together relevant government agencies and local authorities. The community and commercial sector will also be involved in the response to the virus release. 

 

Scientific

Like any worthwhile endeavour, carp biocontrol presents some challenges. Maintaining water quality for use by people, stock, and native species is one such challenge. The NCCP recognises the importance of this task, and our approach to developing a practical, effective, and flexible clean-up strategies is outlined below.

Learning from the past

Fish kills in freshwater ecosystems occur world-wide, with many causes (e.g. Monette et al., 2006; Hoyer et al., 2009; Polidoro and Morra, 2016). Research on fish kills has tended to focus on identifying causes (e.g. Thronson and Quigg, 2008; Moustaka-Gouni et al., 2017) and ecological consequences (e.g. Starling et al., 2002; Sayer et al., 2016) of fish deaths. Nonetheless, published and unpublished case studies consider clean-up action to protect water quality following fish kills (La and Cooke, 2011). Researchers will be engaged under the NCCP to systematically survey fish-kill clean-up methods worldwide, providing insights into what works, what doesn’t, and likely challenges and ensure that past experience informs proposed approaches.

Quantifying risk

Intuitively, we can all understand that major carp mortality events entail some risks to water quality. However, understanding the exact nature and magnitude of these risk may require a specialised approach. Research commissioned under the NCCP will include a scientific risk assessment quantifying risks associated with the proposed carp biocontrol program, including the clean-up. Hayes et al. (2007) provide an overview of the methods used in scientific risk assessment.

Biomass estimates: how many carp are there, and where are they?

Successful clean-up requires understanding carp abundance and distribution at several spatial scales, from continental through to particular habitat types. The NCCP completed a multi-method biomass study, providing the most accurate picture ever developed of carp distribution and abundance in Australia. Methods for biomass estimation that were used included:

  • capture-recapture studies
  • acoustic and radio-tagging
  • collation and statistical interrogation of all pre-existing carp abundance datasets
  • physical measurement of carp biomass when lakes and wetlands are drained as part of ecological remediation works
  • environmental DNA (e-DNA, a suite of methods that enable detection of a species and estimation of its abundance based on DNA shed into the water)

This multi-method approach will enable cross-checking and triangulation, enhancing the accuracy and rigour of resulting biomass estimates.

An ecosystem perspective on clean-up requirements

Planning the clean-up requires knowledge of the virus’s behavior in wild carp populations, including seasonal patterns of viral latency and re-emergence (Eide et al., 2011; Xu et al., 2013). To enable this understanding, the NCCP developed an epidemiological model of the carp virus’s behaviour across all 29 river catchments of the Murray-Darling Basin was developed. The model identifies where carp mortality events are likely, allowing for response planning.

The hydrological models, developed and tested over many years, also examine the effects of varying levels of carp biomass on dissolved oxygen levels in a range of aquatic habitat types. Mosley et al. (2012) provide an example of a similar modelling process. These models will be complemented by detailed experimental studies in real ecosystems (see Boros et al., (2014) for an example of this kind of experiment). Additional NCCP research investigated nutrient interception pathways in freshwater ecosystems, identifying options for avoiding blue-green algae blooms. Together, these research projects will enable response planning that safeguards water quality. For further reading in these areas, Brookes et al. (2005) discuss nutrient interception pathways, while Carmichael and Boyer (2016) review health impacts of blue-green algae.

How to eat an elephant: compartmentalising clean-up

Successful control of a pest species over a large geographic range can be logistically challenging, and this is certainly true of carp control in Australia. Common carp are now present in every Australian state and territory except the Northern Territory, making up more than 80% of fish biomass in some river systems, and up to 93% in some areas (Harris and Gehrke, 1997). Recent NCCP research has updated this knowledge with actual biomass estimates by region. Logistically, it would impractical to seek to employ a simultaneous pest control strategy for common carp across the species’ distribution; a phased approach is required.

The need to phase any release and clean up strategy also presents some clear challenges. In particular, how to compartmentalise such a large, geographically, climatically and hydrologically diverse landscape. The world’s largest rat extermination program in South Georgia offers some useful insights here. The aim of this program was to eradicate brown rat (Rattus norvegicus) from a 170km long, 10-40km wide sub-Antarctic island 1400km east of the Falkland Islands through introduction of 183 tonnes of poison over 224 square miles (580km2). Through robust metapopulation research of the target species (Robertson and Gemmell, 2004) it was learned that the island’s unique climate and topographical attributes resulted in several isolated rat populations separated by large glaciers. This knowledge enabled development of a staged program for implementation, in which the island was divided into a number of treatment zones, which were treated individually (Figure 1). Using this staged, methodical strategy the project team were able to progressively move across the island, treating rats in each zone, testing effectiveness in each zones before moving on until eventually the entire island was treated successfully. This is a noteworthy accomplishment clearly considered impossible by some in 1980 (Poncet et al., 1980), who reported brown rats to be “an established part of the wildlife of South Georgia”, and also reported that “no management procedures would be possible to reduce or control the existing rat population even if this were thought desirable”. The successful outcome delivered in spite of earlier pessimism highlights the value of adopting an evidence-based strategy, coupled with a ‘can do attitude’ when tackling significant pest control challenges.

 

 

Figure 1. Glaciers enable South Georgia to be divided into discrete zones for the purpose of rodent control (Figure reproduced from Poncet and Poncet, 2009).
Figure 1. Glaciers enable South Georgia to be divided into discrete zones for the purpose of rodent control (Figure reproduced from Poncet and Poncet, 2009).

 

While there is no evidence of genetic structuring of carp in Australia, the discontinuous nature of Australia’s Murray-Darling Basin resulting from extensive installation of flow regulating infrastructure may offer a means via which the release of CyHV-3, and subsequent clean-up of carp biomass may be logically compartmentalised and phased (see Figure 2). Under the National Carp Control Plan opportunities are being explored to utilise these assets to separate waters into discrete treatment zones, enabling carp to be treated within each zone in a staged manner.

 

 

Figure 2. Dams and weirs present throughout the Murray-Darling Basin (in green). Opportunities will be explored to use these compartmentalise reaches into zones, enabling progressive treatment for the control of carp (source: Murray-Darling Basin Authority)
Figure 2. Dams and weirs present throughout the Murray-Darling Basin (in green). Opportunities will be explored to use these compartmentalise reaches into zones, enabling progressive treatment for the control of carp (source: Murray-Darling Basin Authority)

 

Using flow

Many Australian rivers are highly regulated, with locks, weirs, and dams controlling water movement (Growns, 2008). The NCCP is working with river managers to identify ways that flows can be manipulated to assist release and clean-up and maintain water quality.

Boots on the ground: the logistics

Results from these research projects will show us what needs to be done. Expert help will then be enlisted to work out how we do it. The NCCP will form a Critical Issue Advisory Group composed of experts from areas including military and transport logistics, commercial carp harvesting, and large-scale human- and animal-health responses. These experts will develop detailed strategies for rapidly responding to carp mortality events, including identifying equipment and personnel needs.

References

  • Boros, G., Takács, P. and Vanni, M. J. (2015). The fate of phosphorus in decomposing fish carcasses: a mesocosm experiment. Freshwater Biology, 60, 479-489.
  • Brookes, J. D., Aldridge, K., Wallace, T., Linden, T. and Ganf, G. G. (2005). Multiple interception pathways for resource utilization and increased ecosystem resilience. Hydrobiologia, 552, 135-146.
  • Carmichael, W. W. and Boyer, G. L. (2016). Health impacts from cyanobacteria harmful algae blooms: implications for the North American Great Lakes. Harmful Algae, 54, 194-212.
  • Eide, K. E., Miller-Morgan, T., Heidel, J. R., Kent, M. L., Bildfell, R. J., LaPatra, S., Watson, G. and Jin, L. (2011). Investigation of koi herpesvirus latency in koi. Journal of Virology, 85, 4954-4962.
  • Growns, I. (2008). The influence of changes to river hydrology on freshwater fish in regulated rivers of the Murray-Darling basin. Hydrobiologia 596, 203-211.
  • Harris, J. H. and Gehrke, P. C. (1997). Fish and Rivers in Stress. The NSW Rivers Survey. Fisheries Research Institute & the Cooperative Research Centre for Freshwater Ecology. Sydney.
  • Hayes, K. R., Kapuscinkski, A. R., Dana, G., Li, S. and Devlin, R. H. (2007). ‘Introduction to environmental risk assessment for transgenic fish’. In: Environmental risk assessment of genetically modified organisms, Vol 3. Methodologies for transgenic fish (Kapuscinski, A.R., Hayes, K.R., Li, S., Dana, G., Hallerman, E.M. & Schei, P.J., eds), pp. 1-28. CAB eBooks.
  • Hoyer, M. V., Watson, D. L., Willis, D. J. and Canfield Jr., D. E. (2009). Fish kills in Florida’s canals, creeks/rivers, and ponds/lakes. Journal of Aquatic Plant Management, 47, 53-56.
  • La, V. T. and Cooke, S. J. (2011). Advancing the science and practice of fish kill investigations. Reviews in Fisheries Science, 19, 21-33.
  • Monette, S., Dallaire, A. D., Mingelbier, M., Groman, D., Uhland, C., Richard, J-P., Paillard, G., Johannson, L. M., Chivers, D. P., Ferguson, H. W., Leighton, F. A. and Simko, E. (2006). Massive mortality of common carp (Cyprinus carpio carpio) in the St. Lawrence River in 2001: diagnostic investigation and experimental induction of lymphocytic encephalitis. Veterinary Pathology, 43, 302-310.
  • Mosley, L. M., Zammit, B., Leyden, E., Heneker, T. M., Hispey, M. R., Skinner, D. and Aldridge, K. T. (2012). The impact of extreme low flows on the water quality of the lower Murray River and Lakes (South Australia). Water Resource Management, 26, 3923-3946.
  • Moustaka-Gouni, M., Hiskia, A., Genitsaris, S., Katsiapi, M., Manolidi, K., Zervou, S-K., Christophoridis, C., Triantis, T.M., Kaloudis, T. and Orfanidis, S. (2017). First report of Aphanizomenon favaloroi occurrence in Europe associated with saxitoxins and a massive fish kill in Lake Vistonis, Greece. Marine and Freshwater Research, 68, 793-800.
  • Polidoro, B. A. and Morra, M. J. (2016). An ecological risk assessment of pesticides and fish kills in the Sizaola watershed, Costa Rica. Environmental Science and Pollution Research, 23, 5983-5991.
  • Poncet, S., Poncet, L., Poncet, D., Christie, D., Dockrill, C. and Brown, D. (2011). Introduced mammal eradications in the Falkland Islands and South Georgia. Island invasives: eradication and management. IUCN, Gland, Switzerland, pp.332-336
  • Robertson, B. C. and Gemmell, N. J., (2004). Defining eradication units to control invasive pests. Journal of Applied Ecology, 41, 1042-1048.
  • Sayer, C. D., Davidson, T. D., Rawcliffe, R., Landgon, P. G., Leavitt, P. R., Cockerton, G., Rose, N. L. and Croft, T. (2016). Consequences of fish kills for long-term trophic structure in shallow lakes: implications for theory and restoration. Ecosystems, 19, 1289-1309.
  • Starling, F., Lazzaro, X., Cavalcanti, C. and Moreira, R. (2002). Contribution of omnivorous tilapia to eutrophication of a shallow tropical reservoir: evidence from a fish kill. Freshwater Biology, 47, 2443-2452.
  • Thronson, A. and Quigg, A. (2008). Fifty-five years of fish kills in coastal Texas. Estuaries and Coasts, 31, 802-813.
  • Xu, J-R., Bently, J., Beck, L., Reed, A., Miller-Morgan, T., Heidel, J. R., Kent, M. L., Rockey, D. D. and Jin, L. (2013). Analysis of koi herpesvirus latency in wild common carp and ornamental koi in Oregon, USA. Journal of Virological Methods, 187, p.372.

 

 

What risk do dead carp pose to water quality - in particular dissolved oxygen and blue-green algae?

Non-scientific

Live carp reduce water quality by stirring up mud as they feed and excreting nutrients into surrounding waters, which in turn promotes blue-green algae blooms. Dead carp can also have impacts if left in a waterway. In sufficient numbers, dead carp can reduce oxygen levels in water, and as they decompose they can release nutrients into surrounding water, causing algal blooms and changes in water chemistry.

Research undertaken under the National Carp Control Plan has helped to improve current understanding of how different quantities of dead carp impact on water quality in the variety of habitats that carp inhabit in Australia. This research shows that in areas with flowing water (e.g. main river channels), decomposing carp would be unlikely to compromise water quality. In contrast, in still-water areas (e.g. off-channel waterways such as billabongs and wetlands), decomposing carp could reduce oxygen levels and increase the risk of blue-green algal blooms, especially in places where carp density exceeds about 300 kg/ha. Effectively managing carp carcasses in these locations would be important if carp biocontrol were eventually to proceed. For more details on specific research projects, please visit our research page

 

Scientific

Studies show that living carp muddy waters, increase nutrient levels (potentially promoting blue-green algae blooms), and reduce abundance of water plants (macrophytes), invertebrates (e.g. aquatic insects and crustaceans), and some fish species (Vilizzi et al., 2014, 2015; Weber and Brown, 2009). For example, Weber and Brown (2009) found that carp increased water turbidity (muddiness) in 91% of surveyed studies, reduced invertebrates in 94%, and reduced macrophytes in 96% of surveyed studies (Weber and Brown, 2009). A more recent meta-analysis supported these results, finding strong evidence for carp impacts on all the same ecosystem components (Vilizzi et al., 2015).

Dead carp can also have impacts if left in a waterway in sufficient numbers. Large organic matter inputs—including from dead fish—can result in low dissolved oxygen concentrations as microbes consume oxygen during respiration while decomposing the organic matter (King et al. 2012; Brookes and Hipsey, 2019). In the Murray-Darling Basin (MDB) in particular, large organic matter inputs are usually associated with flood events, during which vegetation litter is inundated and dissolved organic carbon (DOC) is leached from the substrate (Whitworth et al. 2014). While the effects of vegetation-derived DOC on dissolved-oxygen concentrations have been well documented in the MDB (Gehrke et al., 1993; McMaster and Bond 2008), the effects of a large organic matter inputs in the form of fish carcasses on dissolved oxygen were poorly understood, leading the NCCP to commission research in this.

Water-quality research under the NCCP indicates that, at the carp densities mapped through many Australian waterways, impacts to habitats with flowing water (e.g. river channels) are unlikely, even if most dead carp were left to decay in place (Brookes and Hipsey, 2019). Conversely, waterways with low or no flow (e.g. wetlands, billabongs), are more susceptible to declines in water-quality caused by decomposing carp, particularly if carp density in these habitats exceeds about 300 kg/ha (Brookes and Hipsey, 2019). Similarly, environments with high temperatures, or that are thermally stratified (i.e. a water column that divides into distinct layers characterised by different water temperatures) could be similarly affected, though these factors are also most likely to occur in the absence of flow, and are therefore most likely to arise in off-channel habitats as well (Brookes and Hipsey, 2019). Low or zero flows, high temperatures, and thermal stratification are the primary risk factors for reductions in water quality following carp kills because these phenomena create the conditions needed for blue-green algae to grow, and, in the case of high temperatures, reduce the capacity for the water to hold dissolved oxygen. If carp biocontrol proceeds in Australia, areas with the characteristics described here (high carp densities, low or no flows, high temperatures and/or thermal stratification) would need to be targeted to carcass-management activities.
 

References

  • Gehrke, P. C., Revell, M. B., Philbey, A. W. (1993). Effects of river red gum, Eucalyptus camadulensis, litter on Golden perch, Macquaria ambigua. Journal of Fish Biology, 43, 265-279.
  • Brookes, J.D. & Hipsey, M.R. (2019). Water quality risk assessment of carp biocontrol for Australian waterways. Final Report to FRDC, Canberra, November 2019, 152 pp, CC BY 3.0. FRDC project numbers 2017-055 and 2017-056, combined to form NCCP research project 9.
  • King, A. J., Tonkin, Z. and Lieshcke, J. (2012). Short-term effects of a prolonged blackwater event on aquatic fauna in the Murray River, Australia: considerations for future events. Marine and Freshwater Research, 63, 576-586.
  • McMaster, D. and Bond, N. (2008). A field and experimental study on the tolerances of fish to Eucalyptus camaldulensis leachate and low dissolved oxygen concentrations. Marine and Freshwater Research, 59, 177-185.
  • Vilizzi, L., Tarkan, A. S. and Copp, G. H. (2015). Experimental evidence from causal criteria analysis for the effects of common carp Cyprinus carpio on freshwater ecosystems: a global perspective. Reviews in Fisheries Science and Aquaculture, 23, 253-290.
  • Vilizzi, L., Thwaites, L. A., Smith, B. B., Nicol, J. M. and Madden, C. P. (2014). Ecological effects of common carp (Cyprinus carpio) in a semi-arid floodplain wetland. Marine and Freshwater Research, 65, 802­­-817.
  • Weber, M. J. and Brown, M. L. (2009). Effects of common carp on aquatic ecosystems 80 years after “Carp as a Dominant”. Reviews in Fisheries Science, 17, 524-537.
  • Whitworth, K. L., Baldwin, D.S. and Kerr, J. L. (2014). The effect of temperature on leaching and subsequent decomposition of dissolved carbon from inundated floodplain litter: implications for the generation of hypoxic blackwater in lowland floodplain rivers. Chemistry and Ecology, 30, 491-500.

 

 

If infected by the virus, could carp change their behaviour to reduce mortality and impact the overall effectiveness of the control program?

Non-scientific

A previous study (Rakus et al., 2017) reported that carp infected with the carp virus appeared to change behaviour in lab trials and would congregate around heating elements. On this basis, it is possible that, if the carp virus were eventually released in Australia, infected carp may seek out warm water refuges, reducing the overall program effectiveness. The overall impacts of this behaviour on the effectiveness of a potential carp biocontrol program in Australia is unknown, as the prevalence of “thermal refuges” (i.e. areas of a waterbody with temperatures exceeding those in which the virus effectively causes disease) would vary greatly among waterbodies. For more details on specific research projects, please visit our research page.

References

  • Rakus K, Ronsmans M, Forlenza M, et al. Conserved Fever Pathways across Vertebrates: A Herpesvirus Expressed Decoy TNF-α Receptor Delays Behavioral Fever in Fish. Cell Host & Microbe. 2017;21(2):244-253. doi:10.1016/j.chom.2017.01.010. 

 

Scientific

Rakus et al. (2017) reported that carp infected with the carp virus appeared to change behaviour in lab trials, and would congregate around heating elements in trials. On this basis they postulated that carp may seek out warmwater refuges within Australian waterways, in doing so reducing overall program effectiveness. This is a possibility, however is also being studied under the NCCP to enable this risk to be better understood.

Researchers under the NCCP conducted epidemiological modelling to better understand patterns of viral transmission, spread, and mortality. Epidemiological knowledge was also required to predict the locations and environmental conditions in which major carp mortalities are likely, and equally, areas where sub-optimal outcomes may be experienced. The predictive capacity necessary for planning both release and clean-up was provided primarily by the NCCP’s epidemiological modelling project.

For more details on specific research projects, please visit our research page.

References

  • Rakus K, Ronsmans M, Forlenza M, et al. Conserved Fever Pathways across Vertebrates: A Herpesvirus Expressed Decoy TNF-α Receptor Delays Behavioral Fever in Fish. Cell Host & Microbe. 2017;21(2):244-253. doi:10.1016/j.chom.2017.01.010.

 

 

Would the possible release of the carp virus produce ongoing mass fish kills?

Non-scientific

Following initial deployment of the virus (if it ever occurs), infection, disease, and death would be expected to move through an infected carp sub-population over approximately four to eight weeks, coinciding with water temperatures within the permissive range for the disease caused by the carp virus (approximately 16–28 °C) (Technical Paper 2; NCCP research project 4). Major carp kills occurring simultaneously across large geographic areas are not expected, as the demonstrated importance of physical contact as a transmission mechanism (NCCP research project 6) should ensure that the virus spreads relatively gradually through targeted sub-populations.
This means that after any possible initial virus deployment, ongoing strategic virus release may be required based on an adaptive management approach.

 

Scientific

Epidemiological modelling has been undertaken to evaluate the impact of CyHV-3 in a biocontrol program. It was shown that direct contact between carp was necessary for efficient transmission of the virus. Studies using feral carp held in aquaria demonstrated highly efficient transmission when fish were cohabited to allow close/direct contact for short periods of time and with a small proportion of infected individuals. Conversely, water-borne transmission of the virus was inefficient. Infection by immersion was achieved by using high concentrations of cell culture amplified virus. However, a concentration of CyHV-3 that was sufficiently high to establish infection by immersion was rarely generated by infected carp, even when aquarium water was contaminated by carp with clinical signs of disease and very high viral loads. The quantities of CyHV-3 in tissues, skin swabs and water were monitored throughout the course of infection from pre-clinical stages, through the development of lesions and after death or recovery. Despite very high loads of CyHV-3 in tissues and skin mucus, the concentration of virus released into water rarely reached a concentration that was needed to establish infection. These results confirm that epidemiological models for CyHV-3 transmission should focus on transmission by fish-to-fish contact. The role of water-borne transmission under field conditions is unlikely to contribute significantly to the spread of CyHV-3 where the large volume of natural waters result in a high dilution of virus and there are adverse conditions for the maintenance of infective virus.

The requirement for physical contact between carp to ensure transmission presents both opportunities and challenges. The need for physical contact to ensure effective transmission contributes to a geographically and seasonally restricted outbreak pattern that facilitates carcass management. However, transmission through physical contact also means that engineering disease outbreaks of sufficient magnitude to knock down carp populations may be challenging.

 

Are ongoing outbreaks and low oxygen events inevitable?

Non-scientific

If the carp virus is approved for deployment, ongoing major carp kills occurring across large geographic areas are not expected. This is because direct physical contact has been identified as important for allowing transmission of the virus (NCCP Research Paper 6, Project Number 2020-104). This means that if the virus is ever approved for release, it is likely to spread relatively gradually through targeted sub-populations, and ongoing strategic virus release will likely be required based on an adaptive management approach to achieve high levels of knock-down.

In locations where high levels of carp infection are achieved broadscale and long-term water-quality impacts are unlikely, but are possible in some habitat types. In flowing river channels dead carp are unlikely to compromise water quality beyond acceptable levels. However in still or slow flowing areas water quality could be reduced – especially where carp densities are high (>300 kilograms per hectare). Physical reduction of carp number in these areas prior to virus deployment could both enhance carp control success and mitigate risks to water quality.

 

Scientific

    What is known regarding inevitability of ongoing outbreaks?

    Epidemiological modelling that was conducted as part of this program demonstrated that direct contact between carp was necessary for efficient transmission of the virus. Studies using feral carp held in aquaria demonstrated highly efficient transmission when fish were cohabited to allow close/direct contact for short periods of time and with a small proportion of infected individuals. Conversely, water-borne transmission of the virus was inefficient. Infection by immersion was achieved by using high concentrations of cell culture amplified virus. However, a concentration of CyHV-3 that was sufficiently high to establish infection by immersion was rarely generated by infected carp, even when aquarium water was contaminated by carp with clinical signs of disease and very high viral loads. The quantities of CyHV-3 in tissues, skin swabs and water were monitored throughout the course of infection from pre-clinical stages, through the development of lesions and after death or recovery. Despite very high loads of CyHV-3 in tissues and skin mucus, the concentration of virus released into water rarely reached a concentration that was needed to establish infection. These results confirm that epidemiological models for CyHV-3 transmission should focus on transmission by fish-to-fish contact. The role of water-borne transmission under field conditions is unlikely to contribute significantly to the spread of CyHV-3 where the large volume of natural waters result in a high dilution of virus and there are adverse conditions for the maintenance of infective virus.

    The requirement for physical contact between carp to ensure transmission presents both opportunities and challenges. The need for physical contact to ensure effective transmission contributes to a geographically and seasonally restricted outbreak pattern that facilitates carcass management. However, transmission through physical contact also means that engineering disease outbreaks of sufficient magnitude to knock down carp populations may be challenging.

     

    What is known regarding inevitability of low oxygen events? 

    Broadscale and long-term water-quality impacts are unlikely, but impacts may occur in some habitat types: Research has identified and investigated likely impacts of decomposing carp on water quality. Water-quality impacts depend on dead-carp densities and their distribution in waterways, so water-quality research is built on carp mortality predictions generated by epidemiological modelling. Risks investigated included declines in dissolved oxygen, undesirable nutrient increases, harmful algae blooms, proliferation of disease-causing microbes, and impaired capacity to treat water. These variables are relevant for understanding the potential implications of carp kills for both ecosystem health and water use by humans and livestock.

    In flowing river channels, carp decomposition is unlikely to compromise water quality beyond acceptable tolerances. However, in still or slow-flowing areas away from main channels, water quality could be reduced, especially when carp densities exceed 300 kilograms per hectare (kg/ha). Reducing high-density sub-populations by targeted physical removal prior to virus deployment could both enhance carp control success and mitigate risks to water quality by reducing the total number of dead carp resulting from disease outbreaks. Unregulated dryland rivers in the northern MDB face particular water-quality risks, as these waterways dry to isolated pools that provide drought refuges for threatened species, endure extended low- or zero-flow periods, and already experience impaired water quality. Virus-induced carp kills (with associated in-situ carcass decomposition) under cease-to-flow conditions in these systems could result in fish kills if not appropriately managed, yet detecting outbreaks a nd managing carp carcasses (for example, through physical collection) present particular challenges in these generally remote and sparsely populated areas.

    Water treatment is unlikely to be compromised at the carp densities expected in most areas. However, water treatment and disinfection would become untenable at very high carp densities (approximately 2000 kg/ha). Carp densities of this magnitude are rare in Australian ecosystems, but could potentially occur in ‘point-source’ form if dead carp accumulate in small areas as a result of water currents or wind.

    Proliferation of harmful bacteria, including those that cause botulism, is possible following carp kills, particularly if water quality more broadly is degraded. Outbreaks of bacterial disease have not been reported in Australia following fish kills, but this risk remains possible, and the biology of botulism outbreaks in particular makes predicting them difficult. Managing carp carcasses would provide the most effective mitigation measure against outbreaks of bacterial disease including botulism. 

     

     

    Community 

     

    What is the National Carp Control Plan?

    Non-scientific

    The Fisheries Research and Development Corporation (FRDC) lead a ~$10.4 million program, on behalf of the Australian Government, to assess the feasibility of using a virus called Cyprinid herpesvirus 3 (CyHV-3, the carp virus) as a biological control agent to control invasive European Carp (Cyprinus carpio, carp) in Australia. The NCCP focused on assessing the feasibility of using the virus to control carp while minimising impacts to industries, communities, and the environment should a carp-virus release go ahead.

     

    Scientific

    Refer to the non-scientific response above.

     

     

    Will the carp virus infect humans?

    Non-scientific

    Multiple lines of evidence show that the carp virus does not infect humans.

    First, a report to the European Commission by the Scientific Committee on Animal Health and Welfare found that there is no evidence of ANY fish virus ever being transmitted to humans.

    Second, researchers have attempted to culture the carp virus on human cell lines, and cell lines of other primates (i.e. apes and monkeys) without success. In other words, even deliberate and concentrated attempts to infect human and other primate cells with the carp virus have been unsuccessful.

    Third, the virus’s history in carp aquaculture globally provides a practical demonstration of the carp virus’s inability to infect humans. People in thirty-three countries where carp virus is present have been repeatedly exposed to the carp virus without a single documented case of infection by the virus. This exposure has included clean-up from virus outbreaks, when workers have close, repeated contact with carp that are shedding large quantities of the virus. In addition infected fish are regularly eaten and the absence of infections under these conditions provides confidence that the virus is not transmissible to humans. 

     

    Scientific

    There are multiple lines of evidence demonstrating that CyHV-3 will not infect humans:

    • The virus has been described since the 1990’s and is now present in over thirty-three countries. Fishers, aquaculturists and Koi enthusiasts come into contact with the virus on a regular basis through interaction with water and/or fish carrying virus particles, and no adverse effects have been documented.
    • A significant though unquantified proportion of carp sold internationally for human consumption are vaccinated with a weakened strain of the virus, and no human health concerns have been raised in relation to consumption. Israel alone produces 5 – 6,000 tonnes of carp per annum for human consumption, of which the majority is vaccinated (Pers Comm. Arnon Dishon.)
    • Carp aquaculturists in some countries harvest farmed carp immediately upon observing clinical signs of CyHV-3 and sell infected fish at a reduced price (Pers. Comm. Ayi Santika). Despite this no human health concerns have been raised in relation to human consumption.
    • There are no documented instances of closely related CyHV-1 (carp pox virus), or CyHV-2 (goldfish hematopoietic necrosis virus) causing issues for humans.
    • McColl et al. (2016) have tested mice as a model mammal species and confirmed that the virus did not replicate within inoculated mice.
    • Dishon (2007) attempted to infect cell cultures of homoeothermic, mostly mammalian origin both at 37°C and 22°C which did not result in either cytopathic effect or presence of virus by PCR. Cells included embryonic chick cells (CEF), XC (a rat cell line), HeLa (human cell line) and CV-1 (monkey origin cell line). Importantly, there has been no evidence of any fish virus causing disease in humans (European-Commission, 2000).
    • All fishers, aquaculturists, researchers, fisheries managers and community groups surveyed from Indonesia, the United States, United Kingdom, Israel and Japan during a recent international study tour confirmed that they have never experienced any health issues, including respiratory, skin, eye or oral irritation/sensitisation as a result of contact with the virus in either its wild or attenuated form.

    References

    • Dishon, A. (2007). Cyprinid Herpesvirus Type 3, Modified Live Virus Product code 1443.20 VS Memorandum 800.109 MS & MCS testing report submission.
    • European Commission (2000). Health and Consumer Protection Directorate-General, Scientific Committee on Animal Health and Animal Welfare, 2000, Assessment of zoonotic risk from Infectious Salmon Anaemia virus.
    • McColl, K. A., Sunarto, A., Slater, J., Bell, K., Asmus, M., Fulton, W., Hall, K., Brown, P., Gilligan, D., Hoad, J, WIlliams, N, Crane, M. (2016). Cyprinid herpesvirus 3 as a potential biological control agent for carp (Cyprinus carpio) in Australia: non-target species testing. Journal of Fish Diseases, 40, 1141-1153.

     

     

    Can't we just eat them all?

    Non-scientific

    Apart from the fact that some Australian states prohibit the possession of carp, there has historically been relatively little interest in the species as a table-fish in Australia. However, there is no doubt that carp are seen as a useful food source by other nations, particularly those suffering from poor food security. The Plan investigated options for the wise use of carp biomass, irrespective of the control method used.

    For more details on specific research projects, please visit our research page

     

    Scientific

    Refer to the non-scientific response above.

     

    Can't we have a big carp muster?

    Non-scientific

    Carp musters are great fun, and help to engage communities in pest control, however research demonstrates that angling events only reduce populations by approximately 0.5% - 1.8% which unfortunately is not enough to reduce ecosystem impacts caused by carp. 

     

    Scientific

    Carp musters are great fun, and help to engage communities in pest control, however research demonstrates that this will not result in a lasting reduction in carp numbers (Norris et al., 2013).

    For example, Norris et al., (2013) reported carp angling to be the least effective of all harvesting methods examined, with population reductions ranging from 0.5%–1.8% across angling competitions examined. Norris et al., (2007) also estimated population reduction by anglers in the Goondiwindi Carp Cull to be 0.5% compared to 13.4% for electrofishing (Norris et al., 2007). Similarly, in 2008, anglers in the Goondiwindi Carp Cull removed 40 carp from Rainbow Lagoon, equivalent to 1.9% of the estimated population, and much lower than the catches provided by other methods (Norris et al., 2013).

    Models presented by Thresher (1997) and Brown and Walker (2004) demonstrate that unless carp populations can be a reduced by a large percentage, physical removal is unlikely to offer an effective method for carp control. On this basis, Gehrke et al. (2010) suggest that low-cost carp angling events provide an effective method for promoting community awareness of issues surrounding carp in the Murray-Darling Basin, but their effectiveness in reducing carp populations and environmental impacts is low.

    References

    • Brown, P. and Walker, T. I. (2004). CARPSIM: stochastic simulation modelling of wild carp (Cyprinus carpio) population dynamics, with applications to pest control. Ecological Modelling, 176, 83-97.
    • Gehrke, P. C., St Pierre, S., Matveev, V. and Clarke, M. (2010). Ecosystem responses to carp population reduction in the Murray-Darling Basin. Canberra, Australia: Murray-Darling Basin Authority.
    • Norris, A., Hutchison, M. and Chilcott, K. (2007). Goondiwindi Carp Cull. Progress report Invasive Animals CRC Progress Report, Project 10.U.8. May 2007. Unpublished report QDPI&F and Invasive Animals CRC. 16 pp.
    • Norris, A., Chilcott, K. and Hutchison, M. (2013). 'The role of fishing competitions in pest fish management'. In: PestSmart Toolkit. Invasive Animals Cooperative Research Centre, Canberra.
    • Thresher, R. E. (1997). 'Physical removal as an option for the control of feral carp populations'. In: Roberts J and Tilzey R (eds) Controlling carp: exploring options for Australia. Proceedings of a workshop 22–24 October 1996, Albury. CSIRO and Murray- Darling Basin Commission. Pp. 58-73.

     

     

    Can't the dead carp be used as fertilizer?

    Non-scientific

    Carp are currently used to make fertilizer in Australia , and it may be possible to do so on a larger scale if the carp virus is used to reduce carp numbers and impacts.

    Researchers explored options for utilisation of carp biomass under the National Carp Control Plan.

    For more details on specific research projects, please visit our research page

     

    Scientific

    Refer to the non-scientific response above.

     

    Carp are worth money. Can't we sell them?

    Non-scientific

    It is true that carp are worth money, however the concept of selling carp for profit is more complex than it sounds. Carp are one of the most farmed and consumed freshwater fish species worldwide.  Because of this, the average global price of Carp is very low: approximately $1.50 per kilo. This makes it extremely difficult to harvest, chill and export carp profitably. 

     

    Scientific

    Refer to the non-scientific response above.

     

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