Research Document 1998/92

A risk assessment for Thompson River coho salmon

By M. Bradford

Abstract

Coho salmon populations of the Thompson River drainage have been declining at a rate of 50-70% per generation since 1988. Of a sample of 34 extant populations in 1988, no fish were observed in 11 of them (32%) in 1997. To identify the causes of these declines, freshwater production was modelled using reconstructed smolt abundances. The results of this analysis showed that: (1) interior populations are likely less productive than coastal populations, (2) Thompson coho populations are currently in the linear portions of their smolt-spawner relations, (3) populations are well below levels required to fully seed freshwater habitats. Simulations showed that the declines in Thompson coho are due to a roughly equal effect of declining marine survivals and fishing at inappropriately high rates in recent years. Under current ocean conditions Thompson coho will continue to decline in the absence of fishing and the rate of decline will increase with fishing. Because of the current poor population status, and because the risk of extinction increases exponentially with decreasing population size, it is recommended that no fishing mortality be imposed on these populations.

Complete document (PDF, 1,114 KB)

Accessibility Notice

This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.