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).
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.
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.
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 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.
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.