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    Revised assessment models for Patagonian toothfish in research block 58.4.4b_1 (SSRU 58.4.4bC) for the years 1990–2014

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    Document Number:
    K. Taki (Japan), S. Mormede (New Zealand) and T. Ichii (Japan)
    Submitted By:
    Dr Kenji Taki (Japan)

    We presented five sensitive runs of single-sex age-structured CASAL models for stock status assessments of Dissostichus eleginoides in the research block 58.4.4b_1 in the last WG-SAM meeting. IUU catch was estimated within the models using disease mortality functionality. Among them, the plausible model was considered as OB_1b model, which removed tag and CPUE observations in 2008, because the fits to CPUE in 2008 and MPD profiles for tags in the same year was spurious in OB_1a model which included all observations. The Working Group, however, requested the further examination of the sensitivity to the 2008 tag data, along with the IUU selectivity modeled as a double normal function. The Working Group also requested that projections be presented to WG-FSA for this assessment that examine the consequences of different harvest levels for the time to recovery to the target level.

    We revised to retain tag data in 2008 for OB_1b and other relevant models following the recommendations. The total IUU catches in the revised OB_1b model were estimated to be 71 tonnes, which corresponded 43 % of that (163 tonnes) in the same model of the last WG-SAM report. The median MCMC estimates of the initial and current biomass were 610 and 330 tonnes in the revised OB_1b model. The CCAMLR yield was calculated at 30 tonnes. Any harvest level lead to the CCAMLR risk 2 value > 50% in the beginning of 35 year projection period, and the CCAMLR yield 30 tonnes lead to the period as 24 years (between 2015 and 2038). We estimated the depredation rates by killer whales in the separated document. When depredation rate of 25 % by killer whales constantly occurs, the CCAMLR yield should be reduced to 22.5 tonnes.