Here, we provide diagnostic plots for the 2021 assessment for Ross Sea region Antarctic toothfish (Dissostichus mawsoni) presented in Grüss et al. (2021a), following the recommendations of WG-SAM-2015 (SC-CAMLR-XXXIV 2015 Annex 5). The stock assessment is described in Grüss et al. (2021a), and a detailed description of the stock area, stock assessment methods and the stock assessment parameters is given in Grüss et al. (2021b).

This paper reports on the update of 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 RSR Antarctic toothfish fishery. The assessment model uses catch and effort data for 1998–2021, tag-release data for 2001–2020 and associated tag-recapture observations for 2002–2021, 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 2012–2021. Observational data (tag releases, tag recaptures, and age/length data) from vessel trips that had been quarantined since the 2019 assessment were also excluded. The model structure was the same as that used in 2019.

The 2021 model (R1.1) maximum posterior density (MPD) estimated the equilibrium pre-exploitation spawning stock biomass (B₀) as 78 892 t, and the current stock status (B₂₀₂₁) as 62.7%. Markov chain Monte Carlo (MCMC) estimated B₀ as 78 373 t (95% CIs 71 999–85 663 t) and the current stock status (B₂₀₂₁) as 62.7% B₀ (95% CIs 59.9–65.6% B₀). The estimated status in 2019 from the 2021 assessment (66.3%) was almost the same as that estimated by the 2019 assessment (66.0%).

Following recommendations from the Working Group on Statistics, Assessments and Modelling (WG-SAM), the sensitivity analyses focused on the diagnostics of excluding the initial three years or six years of tag-release data (2001–2003 or 2001–2006) and associated tag-recapture data. The key outcome of the sensitivity analyses was that exclusion of the initial three years of tag-release data (i.e., for 2001–2003) and associated tag-recapture data (in R1.2) resulted in almost negligible differences in model fits to tag or age data, slightly improved likelihood, and showed almost no differences in estimates of B₀  or current biomass in the R1.1 and R1.2 models.

The precautionary yield calculated using the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) 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 was 3495 t with the base case model (R1.1). We recommend a catch limit of 3495 t for RSR Antarctic toothfish in the 2021/22 and 2022/23 fishing seasons.

The Amundsen Sea region is operating under a research plan developed by the Scientific Committee in 2014 designed to provide data to support a two-area stock assessment model. The slope and shelf area within the Amundsen Sea (Small-Scale Research Units 882C-G) is showing improvements in tag recaptures within the four research blocks. However, the seamounts in the north (Small-Scale Research Unit 882H) have shown a lack of spatial representation within the seamount complex, decreasing catch limits, catches exceeding catch limits, and limited recapture data. Although age data are becoming available from other Members, there are no agreed criteria for aggregating age data among laboratories. We recommend that Members continue to make age data available and hold a workshop on age determination and aggregation of age data for Dissostichus spp., following the recommendation by WG-FSA-19 (SC-CAMLR 38, Annex 7, Paragraph 4.83).

This paper summarises fishing catch and fishing effort in the Ross Sea region (Subarea 88.1 and Small-Scale Research Units (SSRUs) 88.2A-B)) together with biological characteristics of the catch of Antarctic toothfish (Dissostichus mawsoni) up to and including the 2020–21 fishing season. The implementation of the Ross Sea region Marine Protected Area (RSrMPA) from 1^st December 2017 has concentrated subsequent fishing on the continental slope south of 70° S, with recent fishing effort extending east into SSRUs 88.2A-B. Scaled annual length and age distributions of the catch show several modes of strong recruitment progressing through time on the slope (south of 70°), while the size and age distributions in the north have not changed. There was a small change in the sex ratio of Antarctic toothfish, with a gradual pattern of more males caught in all areas until 2015. The number of Antarctic toothfish recaptured in 2020–21 was higher than the average annual number over the past decade, likely a consequence of the concentration of fishing effort on the Ross Sea slope with the implementation of the RSrMPA. In 2021, a tagged fish was recaptured after 17 years at liberty.

The time series of relative abundance and age structure from the Ross Sea shelf survey provides information about year class strength, variability, and autocorrelation, and hence is an important input into the Ross Sea stock assessment.

The objectives of the survey were to (i) continue monitoring trends in Antarctic toothfish (Dissostichus mawsoni) recruitment in the southern Ross Sea (core strata A–C); (ii) continue monitoring trends in abundance of larger (large sub-adult and adult) toothfish and biological characteristics in two areas of importance to predators: McMurdo Sound and Terra Nova Bay in alternate years; and (iii) collect and analyse a wide range of data and samples from these areas (e.g., demersal fish, benthic invertebrates, stomach and tissue samples, acoustic data), which will contribute to the Research and Monitoring Plan for the Ross Sea region MPA.

The estimated relative biomass index of toothfish showed an increase to the second highest in the survey time series in 2021. Biological data and samples were collected from more than 3,100 toothfish; samples and measurements were also collected on bycatch species. A total of 268 toothfish were tagged and released with an 89.6% tagging length overlap statistic. The catch limit of 65 t was not exceeded, mainly because catches in Terra Nova Bay were lower than in previous years. Collected toothfish otoliths were aged and included in the 2021 stock assessment.

This paper examined eggs and larval transport pathways of Antarctic Toothfish under extreme Southern Annual Mode (SAM) events in the continental slope-shelf regions of East Antarctica using a particle tracking model. Ocean currents datasets were calculated using a coupled ocean-sea ice-shelf model output “COCO.” Model simulation results indicated transport pathways of particles released from the east side slopes in the East Antarctic regions were different between the positive and negative SAM phases. While the Antarctic slope current (ASC) generally prevailed southwestward transports of particles from slopes to inner shelves regardless of release regions under both extreme SAM events, particles released from the northeast slopes were transported offshore by the Antarctic Circumpolar Current (ACC) or inner shelves by the combination of ACC and local recirculation system. The result suggests extreme SAM events caused the change in the transport of egg and larvae in the east slopes of the East Antarctic region.

The diet of Antarctic toothfish, Dissostichus mawsoni, collected from the 88.1 research subarea was analyzed by metabarcoding their stomach contents. As a result of metabarcoding analyzes of 436 stomach contents, which have been collected for three years (2018, 2019, and 2021), a total of 184 haplotypes and 38 species were obtained. As shown in previous studies, the main diet of D. mawsoni was fish species and relatively much lower proportions of cephalopods and crustaceans. Among them, both Macrourus caml and Cryodraco antarcticus were the two most abundant prey species, ranging from 25.41 % to 83.96 % with 64.43 % on average, supporting their importance as a diet of D. mawsoni in the 88.1 subarea. In the previous study, Chionobathyscus dewitti, Macrourus caml, and Macrourus whitsoni were the most dominant prey items in 88.3 and 58.4 subarea. Clustering analysis showed that prey compositions clearly reflected geographical distance, indicating that metabarcoding analysis of D. mawsoni stomach contents would be one of the efficient methods to monitor ecosystem sustainability in each research area. However, in addition to the geographical distance, the water depth where the fish were collected may also be one of the critical factors for the difference in the prey compositions. Long-term surveys should be conducted for statistical significance.

The stocks of Antarctic toothfish, Dissostichus mawsoni were a relatively economically and ecologically important fishery resource in the waters of Antarctic. However, there is not enough data and information to assess the robust stocks and deliberate proper measurements for sustainable utilize, especially relate to diet and feeding strategy which may lead to think about ecosystem based stock assessments and fishery managements. Therefore, the Korean scientists analyze diet composition and feeding strategy of D. mawsoni in the research block 88 as a part of Korean research plan for the exploratory longline fishery for Dissostichus spp. during 2021 season. This study was carried out based on the results of stomach content identification of the D. mawsoni caught in the research block 88 in CCAMLR conversion area from December 2020 to January 2021. The diet composition and feeding strategy of D. mawsoni were studied using 548 specimens (54 to 183 cm in body length) in the research block 88, respectively. D. mawsoni is a carnivore and piscivorous fish that mainly consumed fishes. Macrourus caml, M. whitsoni, and M. spp. of Macrouridae was dominant taxa. Chionobathyscus dewitti of Channichthyidae were the second largest dietary component in the fish diet of D. mawsoni. Its diet also included small quantities of mollusks, crustaceans, stones, and cnidarians. In this study, fishes were the dominant prey item in all size classes (<100 cm, n=65; 100-120 cm, n=83; 120-140 cm, n=72; 140-160 cm, n=99; ≥160 cm, n=32). The graphical method for feeding strategy revealed that D. mawsoni is an opportunistic and specialized predator on fishes and showed narrow niche width.

This paper presents the results from a three-year longline fishing research survey (2017 - 2019), conducted to improve understanding of Dissostichus spp. population connectivity, biological characteristics and spatial structure across CCAMLR Subareas 48.2 and 48.4. The results provide evidence linking D. mawsoni with the Antarctic continental shelf and indicate a potential D. mawsoni spawning region in Subarea 48.2. The movements of recaptured tagged fish indicate potential connections with the Lazarev Sea (Subarea 48.6) as well as the southern South Sandwich Islands. The results contribute to the information available for further refinement of the D. mawsoni stock hypothesis.

В 2018 г. Китай представил в АНТКОМ свой первый документ, касающийся разработки плана проведения исследований и мониторинга МОР АНТКОМ, по которому поступило много замечаний и предложений (CCAMLR–XXXVII/32). Для того чтобы помочь продвинуть дискуссии по этому вопросу, Китай пересмотрел этот документ с научной точки зрения, серьезно приняв во внимание поступившие в прошлом году комментарии.