Research Document - 2007/081

Initial investigations on potential impacts of reductions in spawn survey and bio-sampling program effort on herring stock assessments and management advice

By Haist, V., and J. Schweigert

Abstract

B.C. herring stock assessments are supported by a rich database that encompasses surveys of spawning stock abundance and biological data that span over a 56-year history. While responsibility for data collection has changed over time, recent changes may affect the ability to maintain herring data acquisition programs at previous levels. This has raised concern about the potential impact of degradation in the herring data collection systems on stock assessments and management advice.

This paper uses simulation methods to investigate how different levels of sampling effort in the biological sampling spawn surveys program might impact herring stock assessments. Specifically, the question of how different the 2006 stock assessments would have been if data collection programs had changed some time in the recent past is addressed.

Results of the simulations allow some general conclusions about the potential impacts of reductions in bio-sampling and spawn survey program effort on herring stock assessments and management advice. Program changes that effect only the bio-sampling data would likely have relatively minor impact on estimates of spawning stock biomass but would have greater effects on stock forecasts and estimates of potential yields, though without obvious bias. Reduced effort for spawn survey programs introduces a negative bias to the spawn survey data, which translates to comparable levels of negative bias in estimates of spawning abundance. The use of a model that synthesizes both the spawn and age-composition data allows some compensation in stock forecasts, particularly when there are only a few years of biased spawn data. For simulations that included both the bio-sampling and spawn survey data quality effects, stock forecast estimates were more strongly affected and had larger negative biases than when only reduced spawn survey data quality was simulated.

Results of the simulations are primarily intended for illustrative purposes and do not quantify the direct effect of various levels of sampling effort on stock assessments and management advice. Evaluation of the trade-offs between sampling effort (and associated costs) and stock assessment precision and accuracy is beyond the scope of this analysis. Analyses to pursue those objectives would require development of a fully prescribed operating model.

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