Research Document - 2006/070

Fluctuations and declines in fish populations in the context of species at risk

By Shelton, P.A.

Abstract

Fish populations fluctuate naturally but severe declines and collapses resulting from overfishing or other causes that are outside the normal range of variation can be cause for concern. In a species at risk context, it would be useful to be able to recognize when a decline is outside the bounds of natural fluctuation expected given the life history characteristics of the population. In this study decline criteria used by the Committee on the Status of Endangered Wildlife in Canada are reviewed. A population with the life-history characteristics of a circa 1960s northern cod is simulated to describe what might be considered natural fluctuations. Simulations include examining fluctuations in an unfished population, a population fished at F_msy and 2x F_msy, a population fished down to MSY and a population fished down to a very low level. Different population metrics and measures of decline are compared. Steepness in the stock-recruit function, age at first maturity and body growth rate are varied to determine the effect of these life history characteristics on natural population fluctuations. The effect of random and autocorrelated error around a Beverton-Holt stock-recruitment curve is examined. Some tentative conclusions are drawn regarding fluctuations and declines in a species at risk context for a population similar to that being modeled. While a 90% decline in spawner biomass over 15 years is probably cause for considerable concern irrespective of the initial size of the population, declines of 70 to 80% can more commonly occur under a variety of conditions, not all of which would be indicative of extinction risk. An alternative approach based on extent of decline is briefly considered.

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