Research Document - 2001/083

2001 Assessment of Stock Status for Coho Salmon From the Interior Fraser River

By Irvine, J.R., Parken, C.K, Chen, D.G., Candy, J., Ming, T., Supernault, J., Shaw, W., and Bailey, R.E.

Abstract

The extreme fishery management measures undertaken in BC since 1998 to conserve coho appear to have stopped the declining trend for interior Fraser coho populations. We evaluated the impacts of continued restrictions in salmon harvest on the status of coho salmon of the interior Fraser River, including the Thompson drainage in 2000. Fishery exploitations in 2000 were the lowest on record, ~3.4% in total, of which half was in British Columbia. Fishery exploitations the last two years were low enough that spawner numbers generally exceeded brood escapements. Productivity measured in recruits per spawner has improved and populations are now above replacement levels.

A mark-recapture program that used fishwheels in the Fraser Canyon as marking platforms provided an independent estimate of spawner numbers in the interior Fraser watershed, as well as useful information on stock composition. Results indicated that spawner surveys may be missing significant numbers of coho, particularly in non-Thompson streams. Additional survey work is required to verify the distribution of coho in the upper Fraser watershed, determine abundance, and collect baseline genetic samples.

We updated our information on the population structure of interior Fraser coho, and present evidence that indicates that major drainage basins (e.g. North and South Thompson) may need to be considered as separate Conservation Units. We discuss reference points for various coho populations and present several values calculated using data from North Thompson coho. The mean of two minimum reference points was 5.2 female spawners per kilometer of accessible habitat. We presume that a limit reference point for North Thompson coho would be greater than or equal to this value. Since coho escapements in the North Thompson watershed have been near but generally below this mean reference point the past four years, we conclude that the viability of these fish remains at risk. This finding, combined with the short-term forecast for Thompson coho of continued poor survivals, leads to our recommendation that a cautious approach to fisheries management needs to remain in place in order to allow these populations the opportunity to rebuild.

The major recommendations from this paper are:

1. More extensive baseline coverage of interior Fraser coho, especially in non-Thompson tributaries upstream of the Fraser-Thompson confluence are required to aid in the delineation of populations and Conservation Units, and provide more precise estimates of the distribution and numbers of interior Fraser coho in catches.
2. Rates of genetic exchange between generations and among populations need to be determined.
3. To enable more effective fisheries management, coho encounter and DNA based stock composition information should be used to develop a model of coho marine distribution and migratory timing.
4. Although benefits can be seen from the extreme fishery management measures taken in recent years, these measures should remain in place to permit populations the opportunity to rebuild.

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