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    Calculating effective releases and recaptures for stock assessments based on tag detection and tagging mortality indices

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    Número de documento:
    S. Mormede (New Zealand)
    Presentado por:
    Mr Doug Cooper (Secretaría de la CCRVMA)

    Vessel-specific indices of tag detection rates and tagging mortality rates have been developed using a methodology which controls for the spatial and temporal variability of fishing operations by pairing each individual tag release or recapture event with all other fishing events which occurred in the same time and place (i.e. within a specific distance and in the same fishing season). The method showed that when the confounding effects of variable fishing location and time were controlled, there were nonetheless significant differences between vessels with respect to the tag detection index in the Ross Sea region. This index was subsequently used in 2013 to select the tagging data to be included in the stock assessment for the Ross Sea region and Amundsen Sea region, with the understanding that additional work should be undertaken to further develop this method.

    Previous applications of the pairwise method utilised an index of tag detection with an arbitrary threshold to determine which tag data to include in the stock assessment, with respect to both tag release and tag recapture data. Here we present new results of simulations which indicate that the index of tagging mortality rate previously generated using this method is as useful as the index of tag detection rate. We further develop a method to estimate effective tag releases and effective numbers of fish inspected for tags based on those indices. This method allows the use in the stock assessment of tag data from all vessels, via direct application of the continuous indices, i.e. without reliance on arbitrary vessel selection thresholds. This method accounts for differences in tagging mortality and tag detection rates among vessels, and therefore is less likely to over-estimate the stock biomass than if tag data were used indiscriminately.

    We recommend that the method proposed be used in the Ross Sea region stock assessment and in the Amundsen Sea region stock assessment.