Project number: 2016-047
Project Status:
Budget expenditure: $143,302.00
Principal Investigator: Rob Kenyon
Organisation: CSIRO Oceans and Atmosphere Hobart
Project start/end date: 30 Jun 2016 - 29 Jun 2019


The project addresses a fundamental need that decisions about WRD in southern GoC catchments are based on comprehensive information. Understanding the importance of individual rivers and cyclic river flows to support the life cycle of juvenile banana prawns and other fisheries species is critical. River flow sustains ecological services that support Northern Prawn Fishery (NPF) target species, commercial, recreational and indigenous fisheries species, and species with high conservation/cultural value.
The report “Our North, Our Future: White paper on developing Northern Australia” (2015) highlights interest in developing irrigated agriculture across northern Australia. Our research will support decision making about alternative strategies for managing water resources effectively (quantifying trade-offs) for both agriculture and marine production, and biodiversity conservation (both species and habitats).
The NESP project will determine the contribution of riverine productivity in the southern Gulf of Carpentaria to catch in the NPF fishery to assess the importance of rivers likely to be impacted by water development as a fishery source. Outcomes from the TRaCK (2008/09) prawn abundance and estuarine production studies from the Norman River showed that with abundant postlarval recruitment to estuarine nursery habitats, estuaries sustain strong populations of juvenile banana prawns. Measured indicators of estuarine productivity do not limit rapid individual prawn growth (size and body weight). But high predation in estuarine habitats reduces the population that contributes to offshore fishery recruitment if prawns are not cued to emigrate by floodflows`
A need remains to test this relationship in other rivers within the GOC catchment. Simultaneous measurement of primary production and prawn abundance has not been achieved, apart from in the Norman River in 2008/09 (Burford et al. 2010). Prawn sampling requires specialist expertise to standardise effort across river systems, and Kenyon provides the expertise needed. Additionally the NESP project will synthesize historical data available from surveys of fishery recruitment (Kenyon et al. 2015).


1. Synthesize historical data available on surveys of the fishery and recruitment of prawns.
2. Contribute to the sampling design for field trips to the southern Gulf of Carpentaria to estimate juvenile prawn densities across estuarine nursery habitats.
3. Undertake field sampling across estuaries in the southern Gulf of Carpentaria to estimate prawn densities and explore linkages to primary production in rivers with different characteristics and catchment features compared to the Norman River, expanding knowledge to other GOC rivers.
4. Contribute to data analysis from field sampling effort (Objective 2) and provide advice on sample sorting and analysis.

Final report

ISBN: 978-1-925994-05-6
Authors: R.Kenyon M.Burford A.Jarrett S.Faggotter C.Moeseneder R. Buckworth
Final Report • 2020-05-12 • 5.36 MB


The objectives of this project were to use historical data and derived knowledge from banana prawn research in the Gulf of Carpentaria (GoC) to identify knowledge gaps and examine estuarine juvenile banana prawn abundance in a subset of Gulf estuaries where water development is planned.  The field trips and associated sampling were undertaken jointly with the Northern Australian Environmental Resources’ (NAER) NESP Project 1.4 ‘Links between Gulf Rivers and Coastal Productivity’ (NESP colleagues assisted with the prawn sampling).  NESP 1.4 was jointly funded through Griffith University and the Australian Government (via NESP).  A major objective was to undertake field sampling of the estuaries of a representative subset of GoC rivers to obtain estimates of abundance of juvenile banana prawns within the estuarine habitats at the same time as NESP 1.4 undertook benthic and water column productivity measurements, and macrobenthos sampling.  In addition, an objective was to be able to compare the estimates of prawn abundance and habitat productivity between the rivers.  Using NESP data on habitat availability, prawn numbers in each estuary were estimated.  Our studies show that interannual variability in juvenile prawn densities drives variability, rather than river to river productivity differences. 
The distribution and abundance of juvenile banana prawns within each of the Mitchell, Gilbert and Flinders river estuaries was sampled for the first time since the 1970s.  In addition, tributary habitats (creek and creeklets among the estuarine mangrove forests) were sampled for the first time.  These tributary habitats had not been sampled by Staples (1979); the team only sampled shallow bank habitats along the main river channel. The importance of tributary habitats was recognised in the 1990s during banana prawn research in the Embley and Mission River estuaries (see Vance et al. 1990).  Together with measurement of the salinity regimes of each estuary, prawn distribution and abundance provide enhanced understanding of the effect of change in freshwater flows on estuarine banana prawn behaviour.  Trawl samples were taken during November 2016 and 2017 (the late-dry season), during the critical September to February recruitment window for juvenile banana prawn settlement, survival and growth in tropical Australian estuaries.  After the wet season of 2019, a subset of the suite of sample sites was trawled in each of the three rivers.  In addition, within the Flinders River only, sampling was undertaken during the wet season in February and after the wet season in May 2018.
Prawn abundance
In November 2016, estuarine juvenile banana prawn habitats within the Mitchell, Gilbert and Flinders Rivers supported abundances of juvenile prawns that ranged from 1.34 ± 0.48 to 1.85 ± 1.11 prawns m-2, not significantly different densities between the three estuaries.  These results match the concurrent estuarine productivity experiments which showed that the water column and benthic productivity measured in each estuary also was not consistently different in 2016. In 2017, the abundance of juvenile prawns in the Gilbert River was higher (4.52 ± 2.03 prawns m-2), though not significantly different than abundances in the other two rivers which were <1 prawn m-2. Despite a different trend in average prawn abundance, the water column and benthic productivity measured in each estuary in 2017 remained equivalent between estuaries. Juvenile banana prawn abundance in Australia’s tropical rivers is highly variable temporally between the same sites over different years and spatially between sites in proximity (Staples 1980a, Staples and Vance1985, Vance et al. 1998).  These trends were characteristic of the prawn abundances found in the Mitchell, Gilbert and Flinders Rivers.  
The collaborating NESP project 1.4’s ecological productivity experiments suggested that none of the estuaries exhibit high levels of productivity or levels that differ strongly between estuaries in a manner that would support high abundances of juvenile prawns in one estuary or the other.  In addition, the productivity experiments demonstrate nutrient paucity within estuaries and suggested that monsoon-cued critical inputs of nutrients via riverine/estuarine connectivity and unregulated flood flows are needed to sustain downstream ecosystem services.
The NESP 1.4 made estimates of the aerial and linear extents of mangrove habitats in the three estuaries and found that the Mitchell River estuary supported the largest areas of mangroves, followed by the Gilbert River estuary and the Flinders River estuary. Scaling up prawn abundance by the area of mangrove habitat, overall population estimates in each of the estuaries ranged from 76.2 (± 27.3) million juvenile prawns in the Mitchell River to 13.4 (± 8.0) million in the Gilbert River in November 2016.  In November 2017, the estimates ranged from 32.7 (± 14.7) million juvenile prawn in the Gilbert River to 5.8 (± 2.3) million juvenile prawns in the Flinders River in 2017. Using linear extent of the intertidal ‘mangrove mudbank’ is less speculative, with estimates of 1.9 (± 0.7) million juvenile prawns in the Mitchell River to 0.6 (± 0.4) million in the Gilbert River in 2016; and 1.5 ( ±0.7) million juvenile prawns in the Gilbert River to 0.3 (± 0.1) million in the Flinders River in 2017.
Prawn distribution within an estuary
In 2017, within the brackish Gilbert River estuary, juvenile prawns moved downstream from their benthic recruitment habitats in the creek/mangrove matrix in the upper reaches of estuarine tributaries to extensive mudbank/mangrove habitats in the lower reaches of river estuarine, and where density-dependent predation likely was lower.  In November 2017, 5.95 ± 2.92 prawns m-2 (80%) of juvenile prawns were found in the estuarine upper-tributaries while ~20% (1.44 ± 0.38 prawns m-2) occupied main river habitats. In contrast, in the Gilbert River in November 2016, 96% of juvenile banana prawns (2.34 ± 1.41 prawns m-2) were found in the upper-tributaries while only 4% of juvenile prawns (0.15±0.06 prawns m-2) were found in the main river habitats downstream. 
In 2017, low-level river flows and brackish estuarine conditions cued juvenile prawns to move downstream to be found at higher relative abundances in the lower-estuary main river habitats. In the Gilbert River, the higher prawn abundance in the main river habitats was matched by salinity-cued emigration of a cohort of juvenile prawns from the river to nearshore habitats (~5-8 mm CL, but mostly 5-7 mm CL).  The restricted size range (3mm CL) and small size of the juvenile prawns suggested freshwater-cued early-season emigration of juvenile prawns that would be subject to lower predation rates offshore (Lucas et al. 1979; Gwyther, 1982), to survive and grow and contribute to good catches of large-sized commercial prawns early in the fishing season. Significant emigration from the Flinders River was found as well.  The size range of the emigrants was larger than those from the Gilbert River.  Given the near-marine salinity of the Flinders estuary and the large size of the emigrants, ontogenetic emigration rather than freshwater-cued emigration was more probable from the Flinders River.   
Banana prawn samples of both benthic juveniles and emigrants have been taken using the same gear as was used during the TRaCK project which sampled the Norman and Flinders River in the south-east Gulf of Carpentaria in the 2000s.  The same gear was used historically in the Norman River in the 1970s and the Embley River in the 1980s and 1990s.  The Gilbert River  has a large meandering main river channel with shelving mangrove-lined mudbanks on the inner reach of each meander.  Several mud-substrate creeks and many small tributaries branch off the main river channel.  These are critical first-settlement habitats for immigrating banana prawn post-larvae moving upstream on flood tides.  The Mitchell River is the least similar of the three rivers.  Its lower estuary has sandy substrates and exposed sandbanks and supported lower densities of juvenile banana prawns.  However, muddy tributaries branch off the main river channel and the other delta-channels that form three openings to the Gulf of Carpentaria.  These tributaries support high abundances of juvenile banana prawns and contribute to offshore prawn stocks. 
Bio-geochemical tracers–estuary to offshore
In November 2016 and 2017, and February and May 2018 juvenile banana prawns from each of the three GoC river estuaries were analysed for bio-geochemical trace element signatures.  Sediment samples were collected from each of the prawn sample sites and analysed using the same technique. As well, juvenile banana prawns (and sediment samples) were collected from otter trawls undertaken close inshore adjacent to the river estuaries and analysed for bio-geochemical trace element signatures of recent emigrants.  During January 2017 and 2018, sediment and banana prawn samples were collected from deeper sites offshore from each of the Mitchell, Gilbert and Flinders river estuaries in the eastern and south-eastern Gulf of Carpentaria and the trace element signatures of offshore prawns and sediment were analysed.
Over each year and set of samples, trace element analyses have shown that the offshore marine sediment and offshore prawns show a similar signature among and between locations.  The mixed offshore signature differed from the estuarine signatures and no link to juvenile prawns from a particular estuary to those caught offshore of that estuary had been identified.  Each of the estuaries has a different signature identified using the bio-geochemical tracers.   However, the strongest signal identified as part of these analyses was between the ‘upper’ and ‘lower’ estuary, a trend found in each estuary. 
Historical research from the NPF has shown that recently-settled post larval banana prawns use estuarine tributary mangrove/mudbank habitats as their preferred transition habitat to their benthic juvenile phase.  Juveniles forage and grow in the tributaries before moving downstream and emigrating.  Juvenile banana prawns in the Mitchell, Gilbert and Flinders rivers follow the same ecological cycle.  In addition, historical research has shown that a reduction in monsoon-driven river flows due does reduce fishery catch and the economic performance of the NPF (Vance et al, 1998, 2003; Kenyon et al. 2018).  Water resource development in GoC catchments and current riverflow-dependent fisheries such as the NPF are likely to successfully co-exist in Australia’s wet-dry tropics if water is harvested from monsoon-season high-flows only. During the annual wet season, high flow volumes dominate the catchment and during strong wet seasons, the highest flows may dominate the capacity of in-stream or off-stream dams. Significant volumes of water can be harvested from high-level flood flows as a small proportion of the total flow volume resulting in minimal impact on the downstream ecosystem services provided by the flood flow. The quantum of water reaching the estuary would be of similar magnitude to an unregulated river. 
In contrast to the harvest of water from high flows, the seasonality, magnitude and duration of low flows must be maintained.  The harvest of low-flood flows diverts a large proportion of the total flow volume with major impacts on downstream ecosystem services.  Both the magnitude and duration of low flows are reduced with a disproportionate impact on offshore banana prawn catch (Broadley et al. in press).  Both seasonal low flows and low-level floods in years of poor wet season rainfall require protection.  Water Resource Plans and Resource Operation Plans should incorporate trigger levels of flow below which environmental flows must be maintained.  Trigger levels should be based on quantitatively modelled flows estimated from ecosystem-based research and that withstand scrutiny by water managers and water users.  NPF management need to engage with the Water Resource Development process via stakeholder consultation processes to promote a legislature that specifies water management protocols and infrastructure design that maintain ecosystem services downstream to minimise impacts on fishery production.

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