A Bayesian sex- and age-structured integrated stock assessment for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (Subareas 88.1 and SSRUs 88.2A-B) is updated and revised using the most recent available data for the fishery. The assessment uses reported catch (C2), and tag-recapture data for 1998–2019 and age and abundance data from the Ross Sea Shelf survey for 2012–2019. Tag releases for 2001–2018 and commercial fishery age frequencies for 1998–2018 are also included.
Models estimates using the updated data for 1998–2017, new data from 2018 and 2019, revised growth and length-weight parameters, and a similar model structure as in 2017 estimated the equilibrium pre-exploitation spawning stock biomass to be about 71 730 t (95% CIs 65 890–78 730 t) and the current stock status to be 66% B0 (63–69% B0).
Model estimates of the pre-exploitation mature (spawning) stock biomass and current spawning stock biomass was slightly lower than that from the 2017 assessment due (i) to increases in the number of tags recaptures in 2018 and 2019; (ii) two years of additional fishing as the population approaches the target of 50% B0; and (iii) revised length-weight and growth parameters. Sensitivity runs showed that the results were robust to assumptions of fleets-as-areas structure, older fish ogive choices, and the choice of tag dispersion rates. We also consider the recommendations from the CCAMLR Independent Stock Assessment Review for Toothfish in 2018, using updated growth and length-weight parameters, and sensitivities to estimate natural mortality and evaluate the effect of alternative levels of tag over-dispersion.
Overall, model fits to the data were adequate, and, as in previous assessments, the mark-recapture data provided the most information on stock abundance. Markov chain Monte-Carlo (MCMC) diagnostics suggested no evidence of non-convergence in the key biomass parameters. Patterns of residuals in the age-frequency proportions suggested some evidence of either strong and weak year class strength patterns before the period where year classes are estimated in the base model (2003–2013) or changing fishing selectivity over the course of the fishery. While sensitivity analyses suggested that these did not adversely impact estimates of current status, they should be investigated in future work
The precautionary yield using the CCAMLR decision rules and the applying catch split under CM 91-05 between north of 70° S (19%), south of 70° S (66%), and the SRZ (15%) was 3140 t. Hence, we recommend that the catch limit be set at a total 3140 t for the Ross Sea region in the 2019/20 and 2020/21 fishing seasons, with a catch limit of 588 t for north of 70° S, 2042 t for south of 70° S, and 464 t for the SRZ.