Degree of Threat: A : Very threatened throughout its range communities directly exploited or their composition and structure irreversibly threatened by man-made forces, including exotic species
Comments: Initial cause of decline was adverse effects of logging, mining, irrigation withdrawals, and overfishing, then construction of hydroelectric dams blocked migrations and resulted in high mortality of smolts in turbines (Nehlsen et al. 1991, Williams et al. 1992). Spawning runs continue to be threatened by construction of dams and degradation of natural environment. (Two dams on the Elwha River (Olympic Peninsula, Washington), which have blocked salmon runs for many decades, soon may be removed [Williams et al. 1992].) Extinctions or large declines of some local native populations in recent decades may have been masked by releases of non-native hatchery stock (Williams et al. 1992).
Juveniles incur high mortality as they migrate through today's river systems and out to sea. Sources of mortality include hydroelectric turbines, mechanical bypass facilities (including transporation by barge or truck), and predation by non-native fishes. Gas bubble trauma (GBT)associated with total dissolved gas supersaturation (TDGS) at spillways also causes mortality and detrimental sublethal effects, but passage of juveniles through spillways may be the least damaging of the routes for juvenile passage at dams (Backman et al. 2002). In the Columbia River basin, adults were rarely observed with GBT, despite high TDGS levels (Backman and Evans 2002).
Concurrent with the construction of fish hatcheries, bacterial kidney disease became prevalent and may now play a significant role in mortality.
A change in climate, beginning around 1977, led to poor ocean survival.
See also threats comments for individual populations (ESUs).