ASSESSMENT MODELS FOR ANTARCTIC TOOTHFISH (DISSOSTICHUS MAWSONI) IN SUBAREA 88.2 SSRU E FOR THE YEARS 2002/03 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 SSRU 88.2E, using revised catch, catch-at-age and tag-recapture data from New Zealand vessels and for selected trips. The New Zealand vessels model resulted in a higher estimate of initial biomass than the same model for 2006, but the estimated initial biomass was very imprecise. The selected trips data set resulted in a more precise estimate of initial biomass, although there was some conflict in the likelihoods of the tag recapture observations from different years. The use of the selected trips with the revised maturity ogive resulted in a greater similarity between the yields resulting from an assumption that the future fishing selectivity was equal to the current fishing selectivity and the assumption that future fishing selectivity was equal to the maturity ogive. Model fits to the data were adequate, with the tag-release and recapture data providing the most information on stock size, although there were conflicts between information in the recapture data from different years. Monte-Carlo Markov Chain (MCMC) diagnostics suggested no evidence of non-convergence in any of the models. The selected trips with the revised maturity ogive MCMC estimates of initial (equilibrium) spawning stock abundance (B0) were uncertain, with the median B0 estimated as 7 540 t (95% credible intervals 5 870–10 020 t), and current (B2009) biomass estimated as 80.7% B0 (95% C.I.s 75.34–85.5%). Estimated yields from this model were 361 t, assuming future fishing selectivity equal to the current fishing selectivity.