Understanding the relationship between commercial prawn species population dynamics, fishing patterns and climate in the Shark Bay World Heritage area in Western Australia
Department of Primary Industries and Regional Development (DPIRD) WA
There is growing concern the sustainability of prawns may be at increased risk due to a higher proportion of small prawns in commercial catches and fishery-independent recruitment surveys. The brown tiger prawn stock experienced its lowest recruitment in 3 years. The causes of change in prawn size and the magnitude of recruitment remain unknown, but we can identify some plausible hypotheses. These include; slowing of growth with lower winter temperatures, a reduction in productivity, changes to timing of spawning, total fishing pressure and intra-annual fishing patterns. It is possible that changes are influenced by interactions between some or all of these. A higher proportion of smaller prawns in commercial catches has continued despite adjustments to management. In addition, the location of prawns during the season has become less predictable. There is an urgent and immediate need to understand the mechanisms underlying these changes to make the necessary management changes to ensure long-term sustainability. The approach will be to use existing datasets to test hypotheses about potential associations between physical (e.g. temperature, rainfall), biological (e.g. seagrass cover), and fishery (e.g. prawn recruitment) variables that we would expect to occur under plausible cause-effect scenarios. These scenarios will be developed through discussions between researchers with different skill sets and insights from prawn fishers. The study will adopt a multi-disciplinary approach to utilise expertise of researchers with skills beyond conventional fisheries science and management, including oceanography, ecology, data science and mathematics, and research specific to the SBPMF and synthesis of the fishing industry. The objectives clearly address both WARAC and the Prawn IPA key priorities of adaptive approaches to changing climate. The objectives are supported by DPIRD managers and some sections of the Shark Bay prawn trawl industry. An independent review of the prawn fishery conducted in April 2019 by Malcolm Haddon provided recommendations regarding research priorities. The review found the objectives addressed a number of key uncertainties identified in the review and suggested they were urgent.
1. Understand the impact of changing temperature and other environmental parameters (e.g. seagrass, flooding events) on the reproductive cycles, growth and distribution patterns of western king and brown tiger prawns
2. Understand the relative contribution of autumn and spring spawning stock to the catch and prawn size composition to assess why there are more small prawns in the fishery at the start of the year and whether harvesting large volumes of small prawns impact subsequent recruitment.
3. Develop performance indicators for the Shark Bay Prawn Harvest Strategy (HS) (which is currently based on abundance indices), if appropriate, that are related to size of prawns.
4. Determine if information on ecological processes and primary productivity are sufficient to construct predictive models of prawn carrying capacity.