ASSESSMENT MODELS FOR ANTARCTIC TOOTHFISH (DISSOSTICHUS MAWSONI) IN THE ROSS SEA FOR THE YEARS 1997–98 TO 2008–09

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, catch-at-age, and tag-recapture data for the 2008–2009 season. The reference model using the selected trips tag data and the revised maturity ogive gave a similar, but slightly lower, estimate of initial biomass to that from the 2007 base case. Two sensitivity models were run; one updated the 2007 base case assessment using New Zealand tag data only with the 2007 maturity ogive whilst the second use the selected trips data set with the 2007 maturity ogive. Overall, model fits to the data were adequate, and, as in previous assessments, the tag-release and recapture data provided the most information on stock size. Monte-Carlo Markov Chain (MCMC) diagnostics suggested little evidence of non-convergence in the key biomass parameters, although there was some evidence of non-convergence in the annual shift parameters for the shelf fishery. MCMC estimates of initial (equilibrium) spawning stock abundance (B0) for the 2009 reference model were estimated as 62 080 t (95% C.I.s 56 020–70 090 t), and current (B2007) biomass was estimated as 79.9% B0 (77.7–82.2%). Estimated yields for the reference case, using the CCAMLR decision rules, were 2850 t.