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    Preliminary investigations of an assessment model for skates and rays in the Ross Sea

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    A. Dunn, S.M. Hanchet, S.L. Ballara and M.P. Francis (New Zealand)
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    This report presents the data and preliminary results from developmental model for Antarctic skates in SSRUs 88.1H, 88.1I, 88.1J, & 88.1K of the Ross Sea. The developmental model attempted to create a catch history of all skates and rays in the Ross Sea, and integrate these data with the available observational data (including tag-recapture data) into a single integrated stock assessment model. We conclude that aspects of the catch history were very uncertain, including the species composition, the weight and number of skates caught, the proportion discarded, and the survival of those tagged or discarded. The size composition of the commercial catch was also very uncertain because of the low numbers sampled each year. Most aspects of the tagging data were also uncertain including the actual numbers of skates released, the initial mortality of tagged skate, the tag loss rate, and the numbers of skates scanned for tags. While updated summaries of the numbers of skate tag releases and recaptures have been reported, these data are still preliminary, and further work is required. Lastly, there is great uncertainty over the biological parameters including age and growth, natural mortality, steepness, and size and age at maturity. The applicability of a general model, such as presented here, to a multi-species catch has not been investigated. While is it plausible that a general model may be adequate if the productivity parameters of the different species of skates and rays are similar, we conclude that additional research is required to investigate the usefulness of such models. We also make a number of suggestions for areas where better data are required. These include recommending work that would improve species identification, increasing the detection rate of tagged skates, increasing the number of skates measured and sexed, validating estimates of age and growth, revising the skate tagging protocols, and undertaking more extensive skate survivorship experiments.