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    Preliminary assessment models of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea including data from the 2005/06 season

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    Dunn, A. & Hanchet, S.M.

    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), by including tag-recapture data for 2005–06 from New Zealand vessels. The updated assessment resulted in a similar, though slightly higher, estimate of initial and current biomass as the 2005 base case model. The inclusion of the 2005 release and the 2006 recapture data had the effect of only slightly modifying the estimates from the models, suggesting that the new data had a similar pattern to that of previous years.

    Model sensitivities using tag-recapture data from all vessels, revised biological parameters, and an alternative method of parameterising the tag-release data are also presented. Inclusion of ‘all vessels’ tagging data resulted in a much more optimistic assessment, but was due to the increased numbers scanned, but smaller increase in the reported numbers recaptured. It is likely that this assessment would change once the outstanding tag release and tag recapture data are used within the analysis.

    The assessment using the revised biological parameters was slightly more optimistic in terms of B0 than the 2006 reference case, with the use of the lower M (0.13 y-1 rather than 0.15 y-1) resulting in a more optimistic estimate of B0. However, as the changes in growth, length-weight, and M all imply slightly lower productivity, the yield estimate from the revised parameters model are likely to be slightly lower than for the 2006 reference case.

    As for the 2005 base cases, model fits to the data were mostly adequate, with the tag-release and recapture data providing the most information on stock size.