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    Assessment models for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea for the years 1997–98 to 2010–11

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    S. Mormede, A. Dunn and S.M. Hanchet (New Zealand)

    We provide an update of the Bayesian sex and age structured population stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (Subareas 88.1 and SSRUs 88.2A–B), using revised catch, catch-at-age, and tag-recapture data for the 2010–2011 seasons. The 2011 reference model using the selected trips tag data gave a similar, but slightly higher estimate of initial biomass than the 2009 base case. Retrospective analysis suggests that this is partly as a result of the increased number of vessels in selected data set and partly as a result of the 2010 and 2011 observations. Two sensitivity models are presented; the first considers the effect of including possible unaccounted mortality from lost gear, and the second uses tag release and recapture data from all vessel trips.

    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 2011 reference model were estimated as 73 870 t (95% credible intervals 69 070–78 880), and current (B2011) biomass was estimated as 80.0% B0 (95% credible intervals 78.6–81.3).The estimated yield, using the CCAMLR decision rules, was 3282 t