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    An integrated stock assessment for the Heard Island and the McDonald Islands Patagonian toothfish (Dissostichus eleginoides) fishery (Division 58.5.2)

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    Document Number:
    P. Ziegler, D. Welsford, W. de la Mare and P. Burch (Australia)
    Submitted By:
    Mr Doug Cooper (CCAMLR Secretariat)

    This paper presents an updated assessment for Patagonian toothfish (Dissostichus eleginoides) at the Heard and the McDonald Islands (Division 58.5.2) with data until mid August 2014. This paper presents a bridging analysis and proposes a new base case assessment model which is based on the best available estimates of model parameters, the use of abundance estimates from a random stratified trawl survey (RSTS), an estimated survey catchability coefficient q, and auxiliary commercial composition data to aid with the estimation of year class strength and selectivity functions of sub-fisheries.

    Compared to the 2013 assessment, this assessment takes into account the recommendations of WG-FSA-2013, SC-CAMLR-2013 and WG-SAM-2014, and incorporates (a) new fishery observations up to 2014 including new ageing data from the 2012, 2013 and 2014 random stratified trawl surveys (RSTS) and commercial ageing data from 2013, (b) a Beverton-Holt stock-recruitment relationship, (c) a simplified model structure, (d) an updated ageing error matrix, (e) an updated growth model, and (f) a prior for the survey catchability q which has been estimated from the ratio of survey fish abundance using the swept area method and the proportion of tag-recaptures in survey catch on the main trawl ground. All model runs were conducted with the CASAL version that was agreed on by WG-SAM-14. These changes substantially improved the internal consistency and stability of the assessment model.

    The new base case assessment model indicates that the virgin spawning stock biomass (SSB0) is higher than that estimated in 2013, with an MCMC estimate of 136 714 tonnes (95% CI: 96 982-186 401 tonnes). Estimated SSB status in 2014 was 0.72 (95% CI: 0.63-0.81), and survey q was estimated at 0.32 (95% CI: 0.23-0.47). Using this model, a catch limit of 6110 tonnes satisfies the CCAMLR decision rules. The projected stock remains above the target level for the entire projection period.