We provide an update of the Bayesian sex and age structured population stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (Subareas 88.1 and SSRUs 88.2A–B), using revised catch, catch-at-age, and tag-recapture data for the 2012–2013 seasons. Several models were examined including an update of the 2011 base case (R1), a model using the revised data selection method, new maturity ogive, and new data weighting method (R2), and a model which used a logistic selectivity for the fishery in the North. We recommend model R2 be used to provide catch limits for the stock because this model reflects the best science and methodologies available.
Retrospective analysis and MPD profiles suggest that the tag recapture data from the last few years tend to push the model toward a higher biomass, in particular the data from the 2012 tagged fish. We expect this is due to higher catches in SSRU88.1K which has a comparably high biomass but a shorter catch history (therefore fewer tags available for recapture) than the other slope SSRUs 88.1H and I. Fish released or recaptured in SSRU K have also consistently shown higher rates of movement between SSRUs and lower rates of recapture in the location of release, perhaps indicative of a more mobile population in this location.
In contrast the updated data selection algorithm resulted in less tag data being selected for input into the stock assessment, which tended to push the model toward lower biomass. The updated maturity ogive a slightly positive effect, and the new data weighting had no effect on model biomass.
Sensitivity runs showed that cryptic biomass is not an issue for this stock assessment, and that the data from the sub-adult survey series is expected to be helpful in estimating year class strengths in the future.
Overall, model fits to the data were adequate, and, as in previous assessments, the tag-release and recapture data provided the most information on stock size. Monte-Carlo Markov Chain (MCMC) diagnostics suggested little evidence of non-convergence in the key biomass parameters, although there was some evidence of non-convergence in the annual shift parameters for the shelf fishery. MCMC estimates of initial (equilibrium) spawning stock abundance (B0) for the 2013 reference model were estimated as 68 790 t (95% credible intervals 59 540–78 470); and current (B2013) biomass was estimated as 74.8% B0 (95% C.I.s 71–78). The estimated yield, using the CCAMLR decision rules, was 3044 t.