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    Investigating emigration in stock assessment models of Antarctic toothfish (Dissostichus mawsoni) in Subarea 88.2 SSRUs 88.2C–H

    Request Meeting Document
    Document Number:
    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 Subarea 88.2 SSRUs 88.2C-H and identified several areas for further work. This was presented to WG-SAM in 2014, and found that the models were unable to fit patterns in the recaptures of tagged fish seen in the SSRU 88.2H fishery. The patterns showed a sharp decay rate of a cohort of tagged fish with few being recaptured after more than 3 years at liberty, a steepening of the decay rate over time, and a trend for increasing proportions of tagged fish caught over time.

    To help understand the population dynamics which could explain the observed patterns in the tag recapture data we carried out a series of simple simulations in R. This suggested that the observed pattern for that tagging data in SSRU 88.2H was only reproduced in scenarios that included both immigration and emigration,  combined with moderate to high exploitation rates

    Within a single area model, emigration can be mimicked by treating it either as a constant biomass of removals or as an additional mortality rate. Both of these approaches were unsuccessful in achieving an adequate fit to the tag data. Clearly, the steep decline in the recapture rates of a cohort of tagged fish through time cannot be explained solely by emigration (a process that includes both tagged and untagged fish), but requires a significant amount of immigration each year to explain the remaining data. Models that include more than one area may be required to model both immigration into SSRU 88.2H and the subsequent emigration back to SSRUs 88.2C–G.

    As requested by WG-SAM we have developed models for (i) SSRU 88.2H using tag recaptures from 1 year at liberty; (ii) SSRU 88.2H using tag recaptures from 3 years at liberty; (iii) SSRUs 88.2C-H using tag recaptures from 1 year at liberty, but excluding tags from the south; and (iv) SSRUs 88.2C-H using tag recaptures from 3 years at liberty, but excluding tags from the south. For each model we provide estimates of biomass and yields based on the CCAMLR decision rules. However, we caution that none of these models provide an adequate explanation of the observations available — none of the model captured either the sharp decay in the number of tagged fish recaptured with increasing time at liberty, nor the steepening in that decay over time.