Research Document - 2005/095

A Biologically-based Escapement Goal for Cowichan River Fall Chinook Salmon (Oncorhynchus tshawytscha)

By Tompkins, A., B. Riddell, and D.A. Nagtegaal

Abstract

This assessment incorporates spawner recruit data available to 2004 and provides a revised biologically-based escapement goal for Cowichan River fall Chinook, a naturally-spawning population in the lower Strait of Georgia.

Based on the Ricker stock recruit model, excluding the 1981-1984 and 1986-1987 brood years, with the survival rate to age 2 as a covariate (log transformed) the biologically-based escapement goal for adult fall Chinook in the Cowichan River was estimated to be 6,514 (90% CI = 4159, 14962). The associated maximum sustainable exploitation rate at S_msy was estimated to be 0.69 (90% CI = 0.52, 0.80).

We recommend that a management plan be established to investigate production potential from escapements exceeding this point and to explore the effect of enhancement on wild stock productivity.

This assessment indicates that productivity and marine survival of the naturally-spawning population has continued to decline while the proportion of hatchery fish in the natural spawning population has increased substantially. At the same time ocean fishery exploitation rates have increased to 70%. Present population sizes are an immediate conservation concern. In 2004, natural escapement was 2226 adult Chinook (2721 total escapement), a similar level only experienced during 1986 -1987 (previous conservation concerns) when survival rates were three to eight times higher.

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