We update the Bayesian sex- and age-structured integrated stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (RSR; Subareas 88.1 and Small-Scale Research Units (SSRUs) 88.2A-B) using the most recent available data for the 2024 season.
The previous assessment model was updated with catch data for 1998–2024, tag-release data for 2004–2023 and tag-recapture observations for 2005–2024, fishery catch-at-age composition data for 1998–2023, and abundance and age composition data from the Ross Sea Shelf Survey (RSSS) for 2012–2023. The model structure was the same as that used for 2023 using Casal2, except that the non-informative selectivity priors used Student’s-t distributions and the RSSS catchability coefficient was modified to be a free parameter.
The 2024 base case model Markov chain Monte Carlo posterior estimated B0 as 77 920 t (95% CIs 72 060–84 690 t) and the current stock status (B2024) as 65.2% B0 (95% CIs 62.3–68.1 B0). While there had been no discernible trend or drop in recent recruitment, the precautionary yield was calculated using the CCAMLR toothfish decision rules and applying a catch split of 19% for the area north of 70° S, 66% for south of 70° S, and 15% in the Special Research Zone. This resulted in a catch limit of 3278 t from the 2024 base case model assuming that future recruitment was similar to that from the most recent estimated 10 years (2009–2018). Yields assuming recruitment that was similar to all estimated years (2005–2018) would have resulted in a catch limit of 3460 t. A preliminary exploration of spatial Chapman’s was undertaken. This suggested an expected level of reduction of toothfish abundance over time in the western side of the south of 70° S management area, while the other areas displayed some interannual variability but little overall trend.
The application of a U-based harvest rate resulted in a higher potential catch limit, as the current stock status was estimated to be above the target of 50% B0. The revised CCAMLR toothfish decision rules, proposed by WG-SAM-2024 to use the lower value of the constant catch projection and the U-based calculation, would not result in a different recommendation of precautionary yield.
Sensitivity models using logistic selectivities for the N70 fishery, and either up- or downweighting the different groups of observations (fishery catch-at-age compositions, tag recapture observations, or the RSSS abundance) did not lead to significantly different results, other than down weighting the RSSS abundance resulted in a slightly higher initial biomass (B0) and slightly lower current stock status.
An evaluation of potential Harvest Control Rules found that either a constant U-based or the tested ramp rules could be used to maintain the stock status at the target levels, with different rules having different trade-offs between catch, stability and risk or stock depletion. These rules provide an alternative method for managing toothfish fish stocks and we recommend that further work on developing harvest control rules within Management Strategy Evaluations be considered.
Based on these results, we recommend that the 2024 base case model with recent (10-year) recruitment be used for the provision of management advice, leading to a proposed catch limit of 3278 t for RSR Antarctic toothfish for the 2024/25 and 2025/26 seasons