Management of a marine renewable resource involves selecting a trade-off between conflicting objectives related to conservation and utilization. This problem is complicated by uncertainty about the current status and productivity of the resource being managed, and hence about the implications of alternative management measures. A general quantitative framework for evaluating these trade-offs in the face of uncertainty is developed. This framework allows for uncertainty about the current state of the resource and the observational error associated with future data. It can assess the performances of a variety of harvest strategies based on setting total allowable catches (TACs). These include constant catch, fixed escapement and constant fishing effort strategies. It is possible to constrain the changes in TAC from one year to the next. This framework is illustrated using the eastern stock of gemfish.

It is necessary to quantify the status of the resource to apply this framework. In this project, the historical trends in, current status of, and productivity of the eastern gemfish population is evaluated using two age-structured assessment approaches tailored to the specifics of the gemfish resource. These methods take account of the two-fishery nature of the resource, explicitly consider sex-­structure, and use the catches, the catch rates in the winter fishery, the length frequency data and the age-length keys. This resource is estimated to have declined markedly during the 1980s as a consequence of unsustainable catches and a long series of weak year-classes. However, the assessments cannot distinguish among alternatives for the relationship between spawning stock size and future recruitment. The two assessments arrive at different conclusions regarding the size of the resource relative to AFMAs harvesting target of 40% of virgin level.

The research program supported by this grant was directed at determining the number and distribution of subpopulations or genetic stocks which contribute to Australian fisheries for barramundi, northern sharks, and narrow-barred Spanish mackerel. Starch gel electrophoresis was used to reveal patterns of genetic variation in each species.

The analysis of approximately 4,500 barramundi collected at locations from the Ord River in Western Australia to the Rockhampton area of Queensland revealed the existence of at least 14 genetically discrete stocks. The significant amount of genetic subdivision characterizing Australian barramundi indicates that fisheries regulations and management policies must be focused on individual barramundi stocks in order to be maximally effective. The number and location of reserves, possible stocking of hatchery reared fish, and selective breeding of captive barramundi should all be based upon maintaining and exploiting the naturally occurring genetic heterogeneity of barramundi populations indicated by this study.