Evaluating Benchmarks of Population Status for Pacific Salmon

Abstract

Canada is developing an indicator approach for assessing the biological status of conservation units of Pacific salmon Oncorhynchus spp. under its Wild Salmon Policy that is based, in part, on the abundance of adult fish. Two benchmarks will be used to place populations in one of three abundance categories. The lower benchmark is proposed to be at a level that allows for a substantial buffer between it and the abundance that would result in a population's being assessed as at risk of extirpation based on quantitative criteria used by the Committee on the Status of Endangered Wildlife in Canada. Using Monte Carlo simulation, we evaluated eight candidate lower benchmarks calculated from parameters of the stock–recruit relationship against two criteria, the probability of extirpation over 100 years and the probability of recovery to spawner abundances that result in the maximum sustainable yield in one or three generations. For modeled populations of moderate size (unfished equilibrium abundances >25,000) and moderate productivity (∼four adult recruits produced per spawner at low spawner abundances), all benchmarks protected populations from extirpation when harvest restrictions were imposed at the lower benchmark. For small or unproductive populations, none of the benchmarks was adequate to prevent populations from being at risk of extirpation. However, those benchmarks that covaried with the productivity parameter of the stock–recruitment relationship performed better and are preferred to those that were derived mainly from estimates of habitat capacity.

Received May 31, 2010; accepted February 4, 2011

ACKNOWLEDGMENTS

Thanks to A. Cass, B. Holtby, B. Riddell, R. Peterman, and M. Folkes for input on earlier evaluations of benchmarks for Canada's WSP. The manuscript was greatly improved by comments from three anonymous reviewers. This research was funded by DFO.