Assessment models for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea including data from the 2005/06 season

We provide an update of the Bayesian sex and age structured population stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea (Subareas 88.1 and SSRUs 88.2A–B), using revised catch, CPUE, catch-at-age and tag-recapture data from New Zealand vessels. The updated reference case resulted in a much higher estimate of initial biomass, though this appeared to be almost entirely due to the impact of the catch-at-age data from the shelf fishery. Parameterising the model to better explain these data resulted in estimates of initial and current biomass that were very similar to those presented in 2005. The inclusion of the 2005 tag-release and the 2006 tag-recapture data had a small effect that slightly modified estimates from the models, suggesting that the new data had a similar pattern to that of previous years.
Preliminary models using tag-recapture data from all vessels are also presented. Inclusion of all vessels tagging data resulted in a more optimistic assessment, and was due to the increased numbers scanned but smaller increase in the reported numbers recaptured. The reason for this is unclear, however, a number of the recaptured tags were not able to be linked to a release due to non-availability of some release data from non-New Zealand vessels. Further investigation is recommended.
Overall, model fits to the data were adequate, with the tag-release and recapture data providing the most information on stock size. Monte-Carlo Markov Chain (MCMC) diagnostics suggested little evidence of non-convergence. MCMC estimates of initial (equilibrium) spawning stock abundance (B0) for the preferred model were 80 510 t (95% C.I.s 59 920–119 920 t), and current (B2006) biomass was estimated as 87% B0. Estimated yields, using the CCAMLR decision rules, were estimated to be 3046 t.