Research Document - 2007/021

Year-class effects in 3Ps cod catch and index data

By Shelton, P.A. and D.C.M. Miller

Abstract

Age-disaggregated catch and population index data contain information on recruitment (number of young at an initial age) and the cumulative effects of mortality. In this research document these two effects in combination are considered to comprise the year-class effect. Methods exist for disentangling these two effects from survey and catch data e.g. Adaptive Framework for Sequential Population Analysis (ADAPT), but where such methods have not been applied for some reason, there is still value in looking at the combined effect within a data series and between data series. If recruitment is low and/or mortality is high, the year-class effect will be diminished. Significant year-class effects were found in all indices and the commercial catch data available for assessing the 3Ps cod stock. In addition, year-class effects are coherent across most data sets. Most of the data support a conclusion of a diminished year-class effect in the recent period, likely through some combination of poor recruitment and/or heavy fishing. Simulation results indicate that significant year-class effects are unlikely to be generated or obscured by random error, unless the effect is small and the error is large. Failure to achieve a quantitative assessment of 3Ps cod in 2006 led to provision of qualitative advice. This advice is likely to have less impact on decision-making than an accepted quantitative assessment. Although Virtual Population Analysis (VPA) based methods such as ADAPT may provide unsatisfactory fits to 3Ps cod data, significant and coherent year-class effects suggest that this is a relatively information-rich stock and if ADAPT fits are not satisfactory, alternative quantitative assessment approaches should be urgently considered.

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