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    Preliminary investigations into a two-area stock assessment model for Antarctic toothfish (Dissostichus mawsoni) in the Amundsen Sea Region

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    S. Mormede, A. Dunn and S.M. Hanchet (New Zealand)
    Submitted By:
    Ms Doro Forck (CCAMLR Secretariat)

    In 2013, the Scientific Committee of CCAMLR could not achieve consensus on a stock status for Antarctic toothfish in the Amundsen Sea Region (ASR - Subarea 88.2 SSRUs 88.2C-H) and identified several areas for further work. This work was carried out and presented to WG-SAM in 2014, who noted that the assessments were unable to fit the patterns in the recaptures of tagged fish seen in SSRU 88.2H. As a consequence, WG-SAM requested further runs be carried out with emigration estimated in the model. However, the models that mimicked emigration in a single area assessment model failed to provide adequate fits to the observed tag recapture data (Mormede et al. 2014b). As an alternative approach, we present the preliminary development of a two-area stock assessment model.

    A two-area model is a method for allowing explicit modelling of movement (migration) of fish into an assessment model, allowing the modelling of the populations in the southern ASR (88.2C-G) and the northern ASR (88.2H), as well as modelling the movement of fish between these two areas.

    The inclusion of two-areas and migrations into the assessment model has both provided a more plausible explanation of the population structure as well as enabling a better account of the observed patterns in the tag recapture data. However, in this preliminary model some difficulties still remain. While the local biomass in the north and the migration to and from the northern ASR were able to be discerned, the biomass in the southern ASR and the associated proportion of that biomass migrating northward could not be resolved. This was because the parameters that represent the initial biomass in the southern ASR and the proportion of that biomass moving north were highly correlated, with little data to inform estimation of biomass in the southern ASR. As a result, estimates of the biomass and current stock status of the population from these models remains highly uncertain.
    This paper is presented for consideration by CCAMLR and may contain unpublished data, analyses, and/or conclusions subject to change. Data in this paper shall not be cited or used for purposes other than the work of the CAMLR Commission, Scientific Committee or their subsidiary bodies without the permission of the originators and/or owners of the data.

    Further development of these approaches is recommended, and will need to be further considered by WG-SAM in 2015. Furthermore, because reliable estimation of biomass in the southern ASR is currently not possible due to lack of data, we recommend that the acquisition of additional data to provide an index of abundance for the southern ASR be given a high priority.