Prawn fisheries throughout Australia are intensively fished and some have shown signs of overfishing. In some cases, the current stocks of prawns are now lower than those which would produce maximum yields. Prawn stocks can vary greatly from year to year because of environmental fluctuations and this leads to highly variable catches. Fishery managers must therefore adopt conservative harvest strategies to prevent fishers reducing stocks to dangerous levels in years when recruitment is low. However, the harvesting and processing sector tend to be on average, over-capitalised, in order to cope with years of high recruitment. Enhancement of prawn stocks through releasing juvenile prawns has the potential to reduce fluctuations in stocks. It provides a possible way of adjusting the catching and processing capacity to more stable levels of prawn stocks, which would reduce the need for over-capitalisation.
The enhancement of Australian penaeid prawn fisheries has the potential to be a useful management tool to increase fishery yields, rebuild over-exploited stocks, and reduce fluctuations in catch due to variable recruitment. It also has the ability to improve the management of fisheries by collecting more precise information about the biological characteristics of the stock (e.g. survival and growth, production in nursery grounds, migration pathways and factors affecting fluctuation in populations). For stock enhancement to be successful, the biology and ecology of the target animal must be thoroughly understood (including the production of the postlarvae/juveniles, environmental requirements, carrying capacity, and all factors that contribute to mortality), and methods must be available to monitor and assess the success of the releases. Much ecological information for stock enhancement is now available for many commercially important species of penaeid prawn in Australia, and novel approaches to tagging prawns (e.g. stable isotopes, rare alleles and reporter genes), release strategies, and assessment of carrying capacity are being developed.
Most of the preliminary assessments of the costs and benefits of prawn stock enhancement in Australia have not assessed a particular fishery or region in detail – they have to some extent developed generic models. For our knowledge on how to enhance prawn stocks in Australia to progress further, it is essential to develop, apply and refine bioeconomic models to a specific region and fishery. For the reasons outlined above (see background), the Exmouth Gulf Prawn Fishery is an ideal location to focus on applying the concepts and the simple model that have been developed from different studies around Australia. The much more intensive study outlined in this proposal will also help to evaluate enhancement projects for other prawn fisheries around Australia.
The beneficiaries of stock enhancement would be expected to contribute to the costs of research and monitoring, and ultimately pay for the enhancement. Therefore, stock enhancement must be cost-effective and a cost-benefit analysis using a bioeconomic model, is an essential part of any enhancement project. Bioeconomic models need to be developed in the early stage of the feasibility study. If the outcomes are favourable for enhancing tiger prawns in Exmouth Gulf, it will be used to optimise the design and management of the trial enhancement program proposed for Stage 2 of the full project.