Research Document - 1999/028
Stock Assessment of Thompson River/Upper Fraser River Coho Salmon.
By J. R. Irvine, K. Wilson, B. Rosenberger and R. Cook
Abstract
Significant declines in the numbers of coho salmon returning to many Thompson River/upper Fraser streams have occurred in recent years. Spawning escapements to unenhanced streams in the South Thompson were at moderate levels from 1975 through 1983. For the next six years, returns were higher, but since then there has been a decline. The proportion of South Thompson streams where no spawning coho were observed during annual inspections also increased during this period. Escapements to two enhanced South Thompson rivers (Eagle and Salmon) followed a similar pattern to unenhanced streams although the rate of decline the last nine years was greater. Returns to unenhanced North Thompson streams also followed a similar pattern although the magnitude of temporal variations was less. A period of modest returns from the mid-1970's to 1983 proceeded about 5yrs of relatively good returns. Since 1989, returns have been relatively poor. Time series of escapement data for the lower Thompson/upper Fraser were of shorter duration and greater uncertainty than for the North and South Thompson. Escapements during the four most recent years were generally lower than in previous years.
A recent preponderance of males in the spawning escapements of some Thompson streams, combined with a suggestion that there may be declines in sizes of fish at return, and a tendency for smaller females to be less fecund are also causes for concern. Each of these factors reduces the reproductive potential of the population.
An analysis of coded-wire tag data did not show any appreciable differences in the marine tag recovery patterns among South Thompson, North Thompson, and Lower Thompson/upper Fraser stocks. Although some minor differences in the marine recovery patterns of Thompson and lower Fraser coho were seen, opportunities harvest of these two stock groups separately appear to be rare. Possible exceptions are late September-October fisheries in Georgia Strait North, and coho fisheries within the Fraser River after the end of October.
Fishery managers need to be aware of potential impacts of fisheries in Puget Sound (Washington) on Thompson coho in years when conservation concerns result in reduced catches in Canadian waters. For marine waters in 1997, although sample sizes were extremely small, approximately as many CWT'ed Thompson coho were reported caught by American fishers as by Canadian fishers.
The authors conclude that at current low marine survivals, coho populations within the North and South Thompson drainages may decline even without fishing mortality. Finally, the authors warn that selective mark fisheries, unmonitored fisheries, and incidental mortality will compromise our ability to evaluate the success of measures to conserve Thompson River/Upper Fraser coho.
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