Further development and progress towards evaluation of an Antarctic toothfish (Dissostichus mawsoni) stock model for
the Ross Sea

This report outlines the development of a Bayesian sex and age structured population model for the assessment of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea (Subareas 88.1 and 88.2), and initial progress towards evaluation of spatially explicit models. Three model scenarios were investigated. The first scenario considered the Ross Sea fishery as a single homogeneous area (single-area). The second and third scenarios (2-area and 3-area respectively) split the Ross Sea into either two or three discrete areas, with migrations of fish between areas. The 2-area model appeared to provide a better representation of the some of the observations, although there was a lack of any observations defining migration between areas. The 3-area model, showed many problems in fitting the data adequately. In both of the spatial models, there was a lack of useful data on area splits, migration rates, or migration patterns.
Simulations experiments suggested that reliable, albeit uncertain, estimates of biomass could be obtained from each model when the operating model was the same as the estimation model. However, the single area estimation model was strongly negatively biased when using tag-recapture data simulated from the 2-area model.
All the multi-area models investigated here could do with considerable improvement. The choice of area (i.e., the boundary between areas), selectivity functions (i.e., either domed or logistic), selectivity types (i.e., either age based or length based), and migration ogives need further investigation. Additional tag-recapture data, specifically data that allows movement rates between areas to be quantified, are required to develop more realistic stock structure hypotheses.