How much spatial structure can data for rock lobster off Victoria, Australia support?

Stock assessments based on fitting, sex- and size-structured population dynamics models, combined with projections of the future implications for available biomass and egg production, currently form the basis for scientific management advice for rock lobster, Jasus edwardsii, off Victoria, Australia. Traditionally, assessments have been conducted for two zones (east and west) off Victoria. However, recent analyses indicate finer-scale spatial resolution in growth and the probability of being mature as a function of size. Assessments were therefore conducted for six regions, as well as for the two conventional zones, and Bayesian model selection criteria were used to compare alternative model formulations. The results suggest that better fits to the data can be obtained by conducting assessments by region than by zone. The general qualitative conclusion of past stock assessments is that the rock lobster populations off Victoria are depleted to below the target reference point of 40% of the available biomass in 1951 and this conclusion is robust to how the data are treated spatially. However, the time-trajectories of egg production and available biomass differ among the regions, with the areas in the centre of Victoria being more depleted than those at the extremes.