Seafood Watch

Seafood Watch® Wild Pacific Salmon Report October 8, 2010
relative to many other salmon runs, genetic diversity in coho is largely influenced by genetic
drift (Olsen et al. 2003). This combination of small population sizes and the low degree of
genetic overlap between coho stocks makes coho runs particularly vulnerable to anthropogenic
damages. Olsen, Miller et al. (2003) comment: “Activities or conditions that cause declines in
population abundance are more likely to have strong negative impacts for coho salmon than for
species in which genetic variation is distributed over a broader geographic scale (e.g., chum
salmon). Coho salmon are probably more susceptible to extirpation, less likely to be augmented
or ‘rescued’ by other populations through straying (gene flow), and the loss of populations
means loss of significant amounts of overall genetic variability.” Indeed, the National Marine
Fisheries Service confirms that coho populations have been declining in streams throughout
Oregon, Washington, and California, with the southernmost and easternmost stocks in the worst
condition. Over the last hundred years, nearly all of the naturally reproducing populations of
coho salmon are believed to have been extirpated from Columbia River tributaries (NMFS
1999).
As with Chinook salmon, roughly half of the natural range of coho in North America is found in
Alaska. The freshwater habitat of Alaskan coho stocks has been substantially less impacted than
spawning habitat in the contiguous U.S., and appears capable of supporting healthy populations
(Figure 1.4).
Figure 1.4. North American coho salmon range (Brownell 1999). 9
9 While coho are extinct in the interior Columbia, they historically did not extend very far up the Snake River; as
such the map misrepresents the species’ original range.
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