We update the Bayesian sex- and age-structured integrated stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (Subareas 88.1 and Small-Scale Research Units (SSRUs) 88.2A-B) using the most recent available data for the Antarctic toothfish fishery. The assessment model employs reported catch for 1998–2021, tag-release data for 2001–2019 and associated tag-recapture observations for 2002–2020, commercial fishery age frequencies for 1998–2020, abundance observations from the Ross Sea Shelf Survey (RSSS) for 2012–2021, and age observations from the RSSS for 2012–2020.
The assessment model used slightly revised catch data and observations for 1998–2019, and new data and observations for 2020 and 2021. Observational data (tag releases, tag recaptures, and age/length data) from vessel trips that had been quarantined since the previous assessment were also excluded. The model structure was the same as that employed in 2019. Additional data that should be available to enable an update of this assessment to be reviewed by the 2021 meeting of the Working Group on Fish Stock Assessment (WG-FSA) will include age data from the 2021 RSSS, tags released in 2020, and tags recaptured in 2021.
The 2021 model (R1.1) maximum posterior density (MPD) estimated the equilibrium pre-exploitation mature (spawning) stock biomass (B0) as 79 140 t, and the current stock status (B2021) as 61.9%. Markov chain Monte Carlo (MCMC) estimated B0 as 78 530 t (95% CIs 72 090–86 470 t) and the current stock status (B2021) as 62.0% B0 (95% CIs 58.9–65.2% B0). The estimated status in 2019 from the updated model was almost the same as that estimated in the 2019 assessment (B2019 was 66.0% in the 2019 model and was 65.7% in the updated 2021 model).
The key outcome of the sensitivity analyses was that exclusion of the initial three years of tag-release data (2001–2003) and associated tag-recapture data resulted in almost no differences in estimated B0 or current biomass and improved the overall model fit. Although exclusion of the initial three years of tag-release data and associated tag-recapture data did not improve the Pearson residuals of the catch-at-age data (and decreased the negative log likelihood for the remaining tag-recapture observations), results suggested that a model without the early tag data could be used as the base case model for the 2021 assessment of Ross Sea Antarctic toothfish. The main advantage of doing this would be to reduce the number of partitions within the assessment model, thereby decreasing the computer memory allocation required and processing time to evaluate the model. We recommend that the sensitivity excluding the 2001–2003 tag data (R1.2) be employed as the base case for the 2021 stock assessment.
We recommend that future development should include, inter alia, aggregation of the very old fish (> 35 years) in the observations as a plus group, recruitment of fish into the model at age four or five, and division of fisheries into discrete temporal periods.