Seafood Watch

Seafood Watch® Wild Pacific Salmon Report October 8, 2010
2001). Generally, the timing of returns of ocean-type Chinook varies between late summer and
the winter months, though in some river systems the fish may return for much of the year (PFMC
2004a).
The seasonal variation in salmon spawning is important as the same river system can support
several runs of salmon. The 2004 PFMC Environmental Assessment notes that “not all runs
types are equally abundant. In Oregon and Washington, spring (March through May) and fall
(August through November) Chinook runs are most common; a few stocks run in summer (May
through July). In California there are also late fall and winter runs (December through July) in
the Sacramento River. A late fall run has also been reported from the Eel River” (PFMC 2004a).
The fact that Pacific salmon are semelparous reduces the stability of population sizes. Because
there is no overlap in the breeding population between years, one study (Waples 2002)
demonstrated both theoretically and empirically that variable semelparous populations with
overlapping year classes can reduce the effective population size in Pacific salmon. With respect
to Chinook in particular, the author estimated a reduction of 40-60% in the effective population
size of one particular Snake River Chinook salmon population due to variability in run sizes
between years (Waples 2002).
Habitat
Human settlement along stream and river corridors during the past 150 years has
altered the fundamental processes that created the habitat conditions under which
salmonid populations have evolved and adapted over centuries. (Larsen et al.
2004b)
Over the past two centuries, habitat loss has become a major issue for Chinook and other
salmonid species. Damming, logging, pollution, and hatchery fish have all put undue pressure on
many wild Chinook stocks. As a consequence, the Pacific Fishery Management Council notes
that “wild Chinook populations have disappeared from large areas where they used to flourish,
and several evolutionarily significant units (ESUs) have been listed or proposed for listing as at
risk for extinction under the Endangered Species Act” (PFMC 2004b). This is particularly true in
the southern half of the Chinook salmon range, as graphically depicted in Figure 1.1.
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