Research Document - 2007/047

A risk assessment framework for Pacific herring stocks in British Columbia

By Schweigert, J.F., C. Fu, C.C. Wood and T.W. Therriault

Abstract

A risk assessment framework was developed with the goal of determining conservation limit reference points for harvested species as part of the Department of Fisheries and Oceans initiative on Objectives Based Fisheries Management (OBFM). In Pacific Region, Pacific herring (Clupea pallasi) was one of two pilot species chosen based on the availability of extensive biological and fisheries data. A critical component of the risk assessment was the development of a population dynamics model of Pacific herring stocks reflecting the best current understanding of fishery and environmental impacts to assess current abundance and make stock projections. A series of performance indicators (measures) were developed to evaluate the impacts of various harvest policy options on the viability and sustainability of Pacific herring stocks in British Columbia. The performance indicators were developed through consultations with stakeholders throughout Pacific Region. Performance indicators utilized in the risk assessment were measured over a 15 year projection period (three generations) and included: the average spawning stock biomass (SSB), the average annual catch, the number of years of fishery closure, the proportion of the population consisting of individuals age 4 and older, the probability of SSB declining below a fixed threshold (the current cutoff level), the probability of the SSB declining below a floating cutoff level, the probability of a 50% decline in abundance within three generations, and the probability of the SSB increasing to the biomass generating the maximum sustainable yield (B_MSY) in three generations. The performance indicators were compared for a suite of proportional threshold harvest policies of which the current policy is one possible example. Five scenarios were designed for projection simulations to investigate the sensitivity of performance indicators to structural assumptions in the model (stock recruitment function and variable versus constant natural mortality), length of closure when biomass falls below a cutoff threshold, and average marine survival. Performance indicators were broadly similar across all five herring stocks in each scenario. The existing herring harvest policy remains precautionary for all stocks, particularly in the current environment of reduced survival. The risk assessment framework appears to be robust and should be applicable across a broad range of species in support of other OBFM initiatives.

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