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    Assessment models for Antarctic toothfish (Dissostichus mawsoni) in SSRU 882E including data from the 2005/06 season

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    A. Dunn and S.M. Hanchet (New Zealand)
    Agenda Item(s)

    We provide an update of the Bayesian sex and age structured population stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the SSRU 88.2E, using revised catch, CPUE, catch-at-age and tag-recapture data from New Zealand and all vessels. The updated reference case resulted in a slightly higher estimate of initial biomass than in 2005, though this was mostly due to a different assumption of the level of natural mortality (M=0.13 y-1), and the revised length-weight and growth relationships. There was little difference between model runs using the New Zealand tag-release and recapture data and the all vessel tag-release and recapture data (B0= 10 300 t and 9 530 t respectively).
    Model fits to the data were adequate, with the tag-release and recapture data providing the most information on stock size, but none of the data sets had much information about the maximum size of the stock. Monte-Carlo Markov Chain (MCMC) diagnostics suggested no evidence of non-convergence. Reference case MCMC estimates of initial (equilibrium) spawning stock abundance (B0) were very uncertain, with the median estimated as 10 300 t (95% credible intervals 5 340–25 210 t), and current (B2006) biomass estimated as 91.4% B0 (95% C.I.s 83.4–96.5%).
    Estimated yields, using the CCAMLR decision rules, were estimated to be 352 t for the reference case and 366 t for the all vessels case, assuming a future fishing selectivity equal to the maturity ogive.

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