Updated SCAA/ASPM assessment of Gulf of Maine Cod

Butterworth, Doug S; Rademeyer, Rebecca A

Updated SCAA/ASPM assessment of Gulf of Maine Cod

Working Paper

2008

Publisher

University of Cape Town

Department

Marine Resource Assessment and Management Group

Faculty

Faculty of Science

Abstract

The ASPM (SCAA) assessments presented in Butterworth and Rademeyer (2008a) are updated through the addition of data for two more years, with the plus group extended from age 7 to age 8 on AIC grounds. Based largely on AIC considerations (though for technical reasons these are admittedly approximately calculated), the best assessment selected is that with a Ricker stock recruitment function and dome shaped selectivity. Amongst a number of sensitivity tests, an early gear change, use of the Baranov form rather than Pope’s approximation, and commencing the assessment in different years (all prior to abundance index data becoming available) do not lead to any differences of note in estimates of key quantities. A simulation study shows the ASPM estimator to introduce only a slight bias towards a domed shape when the underlying reality exhibits asymptotically flat selectivities. Assessment variants which force flat selectivity in NEFSC surveys and the commercial fishery at large ages are not simply less preferred, but indeed strongly rejected under the AIC model selection criterion (e. g. relative AIC-weights of less than 10-13 for the standard M=0.2 specification). Such variants are not compatible with the low proportions of older cod in surveys and commercial catches – a feature for which cogent explanation needs to be offered before they might be accepted as providing a reliable basis for assessment. The greater rate of decline of commercial selectivity for old cod compared to that for the NEFSC surveys provides indirect confirmation of some dome effect, though further evidence from other sources would be desirable. The assessment can hardly distinguish different values of M, though increasing M above 0.2 suggests a lesser downward selectivity slope at large ages and a better resource status. Search over a range of stock recruitment relationships suggests the Ricker form to be preferred, though without completely eliminating the Beverton-Holt form in AIC terms. Under the best assessment, the stock is estimated at present to be at some 80% of its MSY level in terms of spawning biomass, with most variants suggesting somewhat higher levels than this.

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