A single-area stock assessment model of Antarctic toothfish (Dissostichus mawsoni) in SSRU 88.2E for the 2004/05 season

This report outlines a Bayesian sex and age structured population stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in SSRU 88.2E. The model structure was assumed to be the same as used for the Antarctic toothfish in the Ross Sea (see Dunn et al. 2005b), except that catch removals were modelled as a single fishery.
Model fits to the data were adequate, with the tag-release and recapture data providing the most information on stock size, but with the catch-at-age data excluding preventing very low estimates of stock size. Monte-Carlo Markov Chain (MCMC) diagnostics suggested some evidence of non-convergence, particular in the declining right-hand limb of the fishing selectivity parameters.
MCMC estimates of initial (equilibrium) spawning stock abundance (B0) were very uncertain, with the median estimated as 7720 t (95% credible intervals 3760–22 240 t), and current (B2005) biomass estimated as 91.8% B0 (95% C.I.s 83.1–97.1%).
Estimated yields, using the CCAMLR decision rules, were estimated to be 670 t. However, as the fishing selectivity was estimated to be to the right of the maturity ogive, there was a significant biomass of mature, but ‘invulnerable’ fish in the projection period. Moreover, the mean catch that could actually be taken under this scenario was 450 t. Hence, two revised methods for assessing risk are also considered. The first assumed that the future fishing selectivity was equal to the maturity ogive, and hence the yield was calculated as 273 t. The second used the model estimate of the vulnerable biomass as the reference biomass in the risk evaluation, and hence the yield was calculated as 218 t.