The paper provides a preliminary examination of catches and data holdings for bycatch species in the Ross Sea toothfish fishery. Bycatch species composition varied between management areas; however landings of most species groups were generally highest in SSRUs 88.1H and 88.1I in the S70 management area. As found in other areas of the Convention, macrourids were the most commonly observed bycatch group by both weight and numbers. Macrourids, skates, icefish, eel cods and morid cods comprised almost 99.5% of the total bycatch by weight. A large amount of length, weight, sex, and gonad stage data, as well as material for ageing (i.e., otoliths for finfish, whole individuals or caudal thorns for skates), has been collected for key species since 2014, in particular the starry skate Amblyraja georgiana, violet cod Antimora rostrata, the macrourids Macrourus caml and M. whitsoni, and for the most common eel cod species in the Ross Sea, Muraenolepis evseenkoi.

Based on the findings of this paper, we recommend:

1. A revised data collection plan be developed to direct continued collection and support analysis of data for bycatch species. Such a plan should balance data collection with other observer commitments, and be aligned with both a revised fishery-based research plan as well as the broader objectives of the Ross Sea region Marine Protected Area Research and Monitoring Plan.
2. Characterisations of inter alia recent trends in catch, effort, catch rates, and biological information should be compiled for commonly observed species so that quantitative risk assessments can be developed assess the risk of fishing on these species.
3. To assist with the cataloguing and analyses of biological data:

• i)     The "Revision of CCAMLR catch reporting forms" e-group explore ways to indicate whether non-otolith tissue samples where retained on the biological form; and
• ii)     The e-group also explore ways to indicate whether individuals recorded in the biological form were tagged.

4. That the Secretariat periodically summarise data available for bycatch species from toothfish fisheries in other areas and incorporate this information into fishery reports.

The Casal2 stock assessment software package is being developed to implement an updated generalised age-structured population model that allows more flexibility in specifying the population dynamics, parameter estimation, and model outputs than CASAL.

The package is approaching a development point where it can be considered for use by CCAMLR for tag-based toothfish assessments. Casal2 models can be built with user-defined categories such as area, sex, and maturity, and has been constructed to allow for alternative parameterisations of processes and observations to be compared between models. The structural elements of a model in Casal2 are generic and user defined. Casal2 models can be used for a single population with a single anthropogenic event (i.e., a single fishery), or for multiple species and populations, areas, and/or anthropogenic or exploitation methods.

Casal2 is open-source and is available at https://github.com/NIWAFisheriesModelling/Casal2. Comparisons of CASAL and Casal2 in a few stock assessments for New Zealand species show that they provide the same results for estimation of key productivity parameter, but more validation is required.

A Casal2 workshop will be held later in 2021 for scientists who wish to engage in the development and testing of Casal2, and we invite Members to participate in this workshop and in an e-group to develop test cases for presentation at WG-SAM in 2022.

The medium-term research plan for the Ross Sea toothfish fishery was adopted by the Scientific Committee in 2014. A subsequent data collection plan for fishing vessels and observers was implemented for the 2015/16 to 2019/20 fishing seasons. In 2019/20 and 2020/21, bycatch sampling was extended with a focus on the skate tagging programme. In 2022, the 5-year reports of activities in support of the Research and Monitoring Plan for the Ross Sea region Marine Protected Area are expected to highlight progress and information gaps that could be incorporated into a revised fishery based research and data collection plan for fishing vessels and observers. New Zealand proposes a workshop to:

1. Identify fishery based medium-term research objectives,
2. Develop an associated fishery data collection plan to meet the research objectives,
3. Identify high priority non-Olympic fishery surveys or research activities, and
4. Identify voluntary programmes to develop and test novel data collection mechanisms

The current management structure in the Amundsen Sea region aims to develop the data required for an integrated stock assessment for toothfish in SSRUs 882C–H. Results are promising for the southern area (SSRUs 882C–G) but yielding inadequate and deteriorating information in the north (SSRU 882H). In SSRU 882H, the current fishery dynamics are likely to generate successively lower catch limits, constraining tag releases and tag recaptures, and failing to index abundance of the seamount complex population. This paper describes a range of options to improve current fishery dynamics and range in complexity of design, coordination and monitoring required, and in the likelihood of success in producing tag-based biomass estimates.

We recommend that:

1. Management of SSRUs 882C–G remains unchanged,
2. That FSA and the Scientific Committee recommend that Members collaborate intersessionally to develop a management plan structuring fishing in SSRU 882H to better address scientific objectives for a focus topic at WG-SAM-2022.

Updated version of the stock annex.

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.