Assessing the impacts of gillnetting in Tasmania: implications for by-catch and biodiversity
University of Tasmania (UTAS)
Principal Investigator: Jeremy Lyle
Key Words: Gillnet, by-catch, post release survival, ecological risk assessment, fishing practices, fisher motivations and attitudes
Summary: Gillnet fisheries target a range of habitats, including reef and non-reef areas, and land a wide diversity of fish species, with over 90 taxa reported in commercial catch returns. The recreational gillnet fishery
targets much the same species as the commercial sector and there is considerable overlap between sectors in the areas fished. For both sectors comparatively few species account for the majority of the landings.
Catches in the Banded Morwong fishery are dominated by the target species (>85%), only Bastard Trumpeter and Longsnout Boarfish are of any significance amongst the other species harvested. The general graball net fisheries target a range of species with Bastard Trumpeter, Blue Warehou and Australian Salmon key components of the catch. Bastard Trumpeter, Blue Warehou and Atlantic Salmon (escapees from fish farms) comprise the main species retained by the recreational gillnet sectors. Catches in the commercial small mesh and recreational mullet net fisheries although low, are dominated by Australian Salmon, „Pike‟ (Snook and Longfin Pike) and Yelloweye Mullet. The difference in catch composition between graball and small mesh nets is due to mesh selectivity, along with the prohibition of setting recreational mullet nets over reef.
In each of the gillnet fisheries a component of the catch is not retained (by-catch), either because of regulation (size or catch limits, closed seasons for selected species, prohibited or protected species) or because of market and/or fisher preferences. The by-catch component, as a proportion of total catch numbers was found to be relatively high; 52% for Banded Morwong fishers, 49% for the general graball fishery, 66% for the small mesh fishery and 35% for the recreational gillnet fishery, although the latter may be an underestimate as it is based on self-reported information. A wide diversity of species that included target species comprised the by-catch component, but in terms of overall contribution to by-catch numbers relatively few species accounted for the bulk of the discards. The main non-target by-catch species included Draughtboard Shark, Marblefish, Bluethroat Wrasse, Leatherjackets and Skates/Rays.
Discard rates for by-catch species tended to exceed 80%, whereas discard rates for species typically targeted or retained as by-product typically ranged between 10 – 20%.
Capture condition (based on an assessment of physical damage and responsiveness) and delayed mortality rates (based on tank survival trials) of gillnet caught fish varied between species and were influenced by operational factors including soak time and in some instances season. Several species were particularly resilient, suffering minimal physical damage and low rates of initial and delayed mortality, and experienced high overall post release survival (PRS) rates (>85%) irrespective of soak duration. Species in this category included Banded Morwong, Bastard Trumpeter, Marblefish, Draughtboard Shark, Purple Wrasse, Leatherjackets, Longsnout Boarfish and Skates/Rays. Species with moderately high PRS rates (70 – 85%) included Bluethroat Wrasse, Elephantfish, Whitespotted Dogfish and Bluestriped Goatfish. Southern Sand Flathead, Gummy Shark and Jackass Morwong had lower PRS rates (50 – 70%), while survival rates for a suite of other species including Blue Warehou, Australian Salmon and Atlantic Salmon were quite poor (< 50%).
A number of interactions with threatened, endangered and protected species (TEPS) were observed in this study. Fur Seals were commonly observed in the vicinity of gillnets and the majority of direct interactions with the nets involved provisioning (removal and consumption of entangled fish); there were no instances involving entanglement of seals. Entanglement and drowning of seabirds (Cormorants and Penguins) in gillnets was observed, though such incidences were rare making it difficult to identify contributing factors. In Macquarie Harbour, the endangered Maugean Skate was regularly caught in gillnets set in depths of between about 5-15 m. Although the majority of individuals captured were in excellent condition and lively when released, a small proportion of those captured in overnight deployments were either in poor condition or had died, confirming some by-catch mortality in these longer soak times.
Analyses of historic gillnetting data and underwater visual census data revealed that there have been some changes to species abundance and species composition over the past 20 years but, on the whole, this has been dominated by the decline in Banded Morwong abundance and inter-annual variability in the abundance of Bastard Trumpeter and Blue Warehou. Marblefish abundances have declined in most regions since the mid-1990s despite being rarely retained and having high post release survival. Previous fishing and poor handling practices may have resulted in higher than expected by-catch mortality.
Overnight netting was a common practice for recreational fishers prior to its prohibition in all areas apart from Macquarie Harbour. This ban appears to have had a significant impact on netting effort, not only has it achieved a marked reduction in the proportion of overnight sets but there has been a substantial reduction in overall recreational netting effort. Virtually all recreational gillnet fishers engage in other types of recreational fishing, only a small proportion identified gillnetting as their main recreational fishing activity or that they would consider giving up fishing altogether if they could not gillnet. There was general agreement amongst recreational fishers that recent management changes had been effective in improving fishing practices and in reducing wastage and by-catch.
A formal ecological risk assessment was conducted based on four sub-fisheries that make up the Tasmanian gillnet fishery. These are the large mesh graball (Banded Morwong) sub-fishery, the general graball net fishery, comprised of reef and non-reef sub-fishery components, the latter occurs predominately within shark refuge areas, and the small mesh fishery, which includes commercial small mesh and recreational mullet net components.
Level 1, Scale, Intensity and Consequence Analysis identified that target, by-catch/by-product and TEPS components had consequence scores above moderate for several hazards (principally „capture by fishing‟, fishing without capture‟ and „external hazards‟). By contrast habitats and communities were judged to be impacted with low consequence by each of the gillnet fisheries and thus were not considered in the Level 2 Productivity Susceptibility Analysis (PSA) assessment.
The PSA identified a number of species at high risk, each specific to a sub-fishery and a result that reflects differences in mesh selectivity as well as differences in the spatial coverage of the fisheries. Bastard Trumpeter was the only species ranked as high risk in the graball (reef) sub-fishery, largely because inshore reefs represent the core habitat for juveniles and sub-adults and the species is particularly susceptible to gillnet capture. None of the species that interacted with the graball (Banded Morwong) subfishery were ranked as high risk, predominantly due to the high level of selectivity achieved for the target species by the large mesh size. Atlantic Salmon and Rainbow Trout were ranked as having high
vulnerability in the non-reef sub-fishery but, being introduced exotics, this represents a positive ranking, with fishing pressure contributing to their removal from the environment. Maugean Skate and Whitespotted Dogfish were also identified as high vulnerability species; the former has a highly restricted distributional range, presumed low population size and key biological attributes are unknown, and the latter on the other hand is amongst the least productive chondrichthyan species known. Within the small mesh fishery, the Great Cormorant, Rock Flathead and Snook were ranked as having high vulnerability, although low catches and wide distribution outside of Tasmania waters suggest the actual vulnerabilities for the fish at least may not be as high as implied by this analysis. Of the marine mammals, other seabirds and other chondrichthyans considered in the PSA most were ranked as medium vulnerability, mainly due
to low productivity levels.
1. Synthesise available gillnetting information, with particular reference to links between operational parameters and catch composition
2. Determine catch composition and levels of by-catch associated with the main commercial gillnet fisheries
3. Assess implications of recent management changes on recreational netting practices
4. Assess the relationships between gillnet soak times, capture condition and by-catch survival
5. Evaluate the impacts of gillnetting on the biodiversity of key inshore ecosystems and potential strategies to mitigate these impacts