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    Assessment models for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea including data from the 2006/07 season

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    A. Dunn and S.M. Hanchet (New Zealand)
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    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 2007 season. The updated reference model resulted in a slightly lower estimate of initial biomass than the 2006 base case. The inclusion of the 2007 recaptures of 2006 tags released had the most substantive impact on the model estimates.
    Models using tag-recapture data from all vessels are also presented. Inclusion of all vessels tagging data resulted in a more optimistic assessment. The more optimistic estimate was probably due to the lower recapture rate by non-New Zealand vessels, particularly in the early years. The reason for this is unclear, and may be related to different distributions of fishing effort by different vessels, to poorer survival of tagged fish, or to poorer detection rates. However, if data from all vessels was restricted to 2007 recaptures of 2006 releases, then model estimates were more similar to the 2007 reference model.
    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. MCMC estimates of initial (equilibrium) spawning stock abundance (B0) for the 2007 reference model were estimated as 71 200 t (95% C.I.s 59 570–87 900 t), and current (B2007) biomass was estimated as 81.9% (78.4–85.4%). Estimated yields for the reference case, using the CCAMLR decision rules, were 2700 t.
    Similar estimates of initial biomass and yield were obtained for models where the north fishing selectivity was assumed to be logistic (median B0=78 480 t, yield=2988 t), and where the 2007 recaptures from all vessels were added to the New Zealand tag-release and recapture data (median B0=81 100 t, yield=3099 t). If tag-release and recapture data for all vessels in all years were used, then the estimated initial biomass was higher (B0=110 130 t), and corresponding yields were also higher (yield=4200 t).