Research Document - 1999/128

1999 Assessment of Thompson River / Upper Fraser River Coho Salmon.

By J.R. Irvine, R.E. Bailey, M.J. Bradford, R.K. Kadowaki and W.S. Shaw

Abstract

We evaluated the effects of the 1998 fishing season on the status of upper Fraser/Thompson coho populations. Restrictions in Canadian fishery mortalities during 1998 lowered the overall exploitation rate to about 7% (~2% in Canada) although estimates are not precise. As a result of expanded efforts to estimate spawner escapements in 1998, it was determined that coho in the upper Fraser/Thompson were more abundant than previously was thought. However, while there was a slight improvement in escapements in 1998 over 1997, for much of the aggregate, returns in 1998 were less than for the brood year. Coho spawners were not seen in 27% of streams that had coho spawners observed in them 3 generations earlier. We conclude that the status of South and North Thompson coho populations remains poor. The productivity of this portion of the aggregate has declined in the last 10 years to the point where the population is only barely able to sustain itself (even in itself in the absence of fishing). Our understanding of the status of lower Thompson and non-Thompson/upper Fraser coho is weak.

Major recommendations from this paper are:

1. Since some coho populations within the North and South Thompson drainages remain at risk of extinction, fishing mortality should continue to be minimised to conserve these populations.
2. We relied upon the use of DNA technology to estimate numbers of Thompson coho in marine fisheries. To reduce our uncertainty around estimates of fishery mortalities, we recommend that the use of DNA to identify stocks of concern in fisheries be critically reviewed.
3. Freshwater habitat degradation has occurred throughout the Thompson/upper Fraser. The degree of habitat change needs to be quantified for coho bearing streams and the relationship between these changes and freshwater survival examined in detail.
4. Expanded escapement surveys in 1998 relative to previous years identified significant populations of coho that we had not been aware of. Some of these populations were in non-Thompson Fraser tributaries. Because it is not yet possible to reliably separate these fish in the marine environment from Thompson fish, estimates of Thompson fishery mortalities requires knowledge of the numbers of coho in non-Thompson as well as in Thompson drainages. We recommend that expanded escapement surveys be maintained.
5. Our review of escapement survey methodology revealed numerous inconsistencies among systems and years. Methodologies need to become more standardized, and commentaries on methodologies should be available in order to reliably use escapement data in salmon stock assessments.

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