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

This report outlines a 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). A range of model sensitivities were examined, with the base case assuming the Ross Sea fishery as a single homogeneous area (single-area) with catch removals from three spatially based fisheries (shelf, slope, and north).
Model fits to the data were adequate, with the tag-release and recapture data providing the most information on 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. However, multi-chain comparisons and sensitivities with fixed selectivity ogives suggested that this would have little impact on the estimates of key model parameters and yield estimates.
MCMC estimates of initial (equilibrium) spawning stock abundance (B0) were 69 400 t (95% credible intervals 47 700–111 900 t), and current (B2005) biomass was estimated as 88% B0 (95% C.I.s 83–93%). Estimated yields, using the CCAMLR decision rules, were estimated to be 2964 t although continued catch at current levels (3207 t) would not be expected to breach the 50%B0 threshold until 2029.