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    Assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region to 2020/21

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    Document Number:
    A. Grüss, A. Dunn and S. Parker
    Submitted By:
    Mr Nathan Walker (New Zealand)
    Approved By:
    Mr Nathan Walker (New Zealand)

    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.