Spatial modelling and assessment of the Victorian southern rock lobster (Jasus edwardsii) fishery
As management of Australian southern rock lobster fishery becomes more advanced so must the stock assessment techniques used to provide scientific management advice. A management plan was developed in 2003 specifying limit and target reference points for available and spawning biomass, and these will be assessed annually using the length-structured model. The current modelling is conducted for each of Victoria’s two fishing Zones with no spatial separation within each Zone. From assessment work conducted by MAFRI it is clear that there are variations in growth, movement and size at onset of maturity within each Zone. Failure to incorporate this variation into modelling of assessments, would substantially reduce the reliability of model projections. Continued development of the rock lobster assessment model with emphasis on risk assessment, spatial dynamics, including variations in growth were given priority for Victoria’s Research needs (Research Needs and Priorities for Fisheries in Victoria 2001/02-2005/06 Eds. G. Newman and D.C. Smith, 2002) and ranked as essential in the Rock Lobster Fishery Management Plan (Fisheries Victoria, Management Report Series No. 1, June 2003). The current model uses a single set of growth parameters throughout the fifty-year time series of fishery data. It is possible that growth has changed during the history of the fishery due to density-dependent factors, and this proposal seeks to test the sensitivity of the assessment and its forecasts to such changes. The examination of sensitivity is critical for the assessment of the current biomass against reference points which are based on the biomass during the early stages of the fishery. The current model was recently modified to allow for the introduction of Marine Protected Areas (MPAs), which are currently being introduced in Victoria’s marine waters. The modelling needs to refine the assessment of MPAs and this will be addressed by increasing the spatial resolution. Further development of the assessment modelling to include the application of Management Strategy Evaluation (MSE) techniques also rates as high/essential within Victoria’s Research priorities as it will enhance the ability to evaluate alternative policies using the model. The application of MSE techniques will assist in determining the robustness of the assessment to violations of its assumptions, identify directions for future monitoring, and evaluate current decision rules and reference points. An MSE would also provide a formal basis to evaluate management issues such as Marine Protected Areas, size limits etc.. One key issue that requires formal evaluation using an MSE approach is the trade-off between the benefits of conducting assessments at fine spatial scales given spatial variation in biological characteristics and the cost in terms of data requirements.
1. Extend the current Bayesian assessment of rock lobsters off Victoria by allowing for greater spatial resolution within each management Zone.
2. Extend the assessment to allow for density-dependent growth, information from a larval settlement index, and changes over time in the Legal Minimum Length.
3. Evaluate the assessment model and potential decision rules using the Management Strategy Evaluation approach.
Principal Investigator: David K. Hobday and Andr` E. Punt