Research Document - 2009/059

Evaluation of Benchmarks for Conservation Units in Canada's Wild Salmon Policy: Technical Documentation

By Carrie A. Holt

Abstract

Canada's Wild Salmon Policy requires that quantifiable metrics of biological status and benchmarks along those metrics that delineate Red, Amber, and Green zones of status be established for all Conservation Units. Although candidate benchmarks have been identified from the scientific literature and previous management experience, they have not been evaluated in terms of their ability to meet the goals of the Wild Salmon Policy. Using a simulation model, we evaluated lower benchmarks on two classes of indicators (abundances and fishing mortality relative to productivity) on two performance metrics: the probability of extirpation over the long term and the probability of recovery to a target. This technical document provides the methodology and results for those analyses and is a companion document to the overall synthesis paper "Indicators of Status and Benchmarks for Conservation Units in Canada's Wild Salmon Policy" (Holt et al. 2009). We considered the "worst-case" management scenario, with management decisions corresponding to CUs being depleted to the lower benchmark each year. Although unrealistic, this assumption provides an upper limit on the probability of extirpation and lower limit on the probability of recovery. We found that for metrics of spawner abundances, the lower benchmark, spawner abundance that will result in recovery to S_MSY (spawner abundance at maximum sustained yield) in one generation under equilibrium conditions (S_gen), was associated with a relatively low probability of extirpation (probability <25%) over 100 years (for populations with equilibrium abundances > 15 000 spawners) and high probability of recovery to S_MSY within three generations (probability >75%) when uncertainties in all major components of the fishery system were accounted for. Furthermore, the probability of extirpation for S_gen was more robust to variability in stock productivity compared with benchmarks calculated from proportions of S_MSY. For metrics of fishing mortality, the lower benchmark F_MSY (fishing mortality at maximum sustained yield) was associated with a relatively low probability of extirpation (probability <25%) over 100 years for populations with equilibrium abundances > 30 000 spawners, and high probability of recovery to S_MSY within three generations (probability >75%), and its performance was more robust to variability in stock productivity than other benchmarks on fishing mortality derived from the scientific literature. Based on those results and a risk classification scheme developed in DFO's Fishery Decision-Making Framework Incorporating the Precautionary Approach (2009), we suggest deriving lower benchmarks on spawner abundances from S_gen and lower benchmarks on fishing mortality from F_MSY. Further work will be required to identify specific risk tolerances of stakeholders in order to better inform the choice of lower benchmarks.

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