Research Document - 2004/011

An evaluation of alternative age-structured models for risk assessment of Pacific herring stocks in British Columbia

By Fu, C., Schweigert, J. and Wood, C.C.

Abstract

Recent declines in two of five major stocks have raised concerns about the assessment and management of Pacific herring in British Columbia. Fisheries and Oceans Canada has committed to a science-based review of the stock assessment and fishery management framework for the herring stocks. As a first step in this review process, we have developed a new age-structured model (NASM) in a Bayesian context. The purpose of this paper is to compare its performance with that of the existing age-structured model (EASM), first, by conducting simulation-estimation experiments and second, by reconstructing the dynamics of the five major herring stocks.

The simulation-estimation experiments indicate that the EASM only performs as well as the NASM if, as has been assumed previously, the conversion factor (q) required to scale the survey index of spawn to total spawning biomass changed abruptly at a known date (e.g., the year the survey design changed). In this case, model selection becomes difficult because no single model performs consistently better on all measures. However, if the underlying q changed randomly, the new model NASM-3 that fixes q but estimates annual variations in natural loss (M) performs best. When the underlying mean M shifted drastically from one period of time to another (i.e., regime shift), NASM-3 and NASM-4, which account for annual variations in M perform significantly better than other models. However, if M changed only randomly around a constant level, the advantage of estimating annual variations in M becomes less obvious. Overall, NASM-3 seems promising based on the simulation results.

In applying the four models (NASM-3, NASM-4, EASM-1, and EASM-2) to the five herring stocks, parameter estimates differed among models just as expected from the simulation results for three of the five stocks (QCI, GS and WCVI); for these stocks we are reasonably confident in identifying the most plausible reconstructions. The promising NASM-3 model tends to overestimate the proportion at age 3 in all the five stocks suggesting that the selectivity functions are not entirely appropriate. If this model is to be used for future stock assessment and risk assessment, a further investigation into different selectivity functions is warranted.

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