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COSEWIC assessment and status report on the coho salmon (Interior Fraser population) in Canada

Summary of Status Report

Coho salmon are an important species, contributing to catches along the Pacific coast of North America. However, coho numbers are declining throughout much of their range, particularly in the northwestern United States and southern BC. This report focuses on coho salmon from the interior Fraser River of British Columbia. Coho in this region originated from populations that survived glaciation in Columbia River refugia. Since coho are now extinct in the upper Columbia, interior Fraser River coho are genetically distinct from other surviving coho salmon.

Coho salmon from the interior Fraser River constitute an Evolutionarily Significant Unit. The total population is comprised of at least five subpopulations (North Thompson, South Thompson, lower Thompson/Nicola, Fraser canyon, and upper Fraser). Genetic exchange among streams within subpopulations is much greater than among subpopulations. There is a concern that if the total population becomes too fragmented, genetic exchange within the total population may be insufficient to be assured of long-term survival.

The time series of reliable abundance estimates is 25 years duration for coho from the North and South Thompson drainages, 16 years from the lower Thompson/Nicola, and only 3 years for the Fraser canyon and upper Fraser tributaries. We have less confidence in the lower Thompson/Nicola time series than we do for the North and South Thompson. Spawner numbers in the North and South Thompson watersheds peaked in the mid-1980’s, declined until about 1996, and have been stable or increasing since then. On average, North and South Thompson coho declined in numbers by ~60% during the 10-year period from 1990-2000. There were four years (1991, 1995, 1997, and 1998) when productivity was so low that some populations may not have been able to replace themselves, even if fishing mortality had been zero. Although spawner numbers in 1999 and 2000 exceeded parental escapements, numbers were still critically low. Three consecutive strong returns are necessary to have confidence in any improvement in status.

The recent size of the total interior Fraser coho population was estimated by averaging spawner estimates for each subpopulation (area) during 1998-2000. Slightly more than half of recent estimates of the total population of 24 000 occur within the North and South Thompson watersheds. Natural spawning is thought to be responsible for producing most of the fish escaping to the interior Fraser in recent years (~20 000 of 24 000 total) although in the lower Thompson/Nicola area, hatchery-origin fish outnumber wild coho. There is no evidence that the extent of occurrence has changed, although spawners were seen in fewer streams as populations declined.

Overfishing, changing marine conditions, and habitat perturbations all contributed to declines in numbers of coho salmon in the interior Fraser. Excessive fishing resulted when harvest rates were not reduced quickly in response to climate-driven reductions in marine survival. Coho declines were often related to the intensity of human disturbance in the watershed. Fishing pressures have been reduced dramatically the last several years, and this combined with an apparent stabilization in marine survivals resulted in improved returns.

The outlook for interior Fraser coho is highly uncertain and depends on impacts due to fishing, habitat perturbations, and climate related changes in survival. A return to higher survivals experienced until 1997, combined with continued low fishing pressures and no additional habitat impacts, would produce rapid increases in escapements and rebuilding. In contrast, if survivals return to low levels such as those recorded in 1998, spawner numbers will decrease, eventually resulting in extinction. Since there is no consensus about future marine survivals, an extremely cautious approach to fisheries and habitat management will be required to ensure the long-term viability of populations of coho salmon from the interior Fraser River watershed of BC.