Recovery and management options for spring/summer chinook salmon in the Columbia River Basin

Construction of four dams on the lower Snake River (in northwestern United States) between 1961 and 1975 altered salmon spawning habitat, elevated smo lt and adult migration mortality, and contributed to severe declines of Sna ke River salmon populations. By applying a matrix model to long-term popula tion data, we found that (i) dam passage improvements have dramatically mit igated direct mortality associated with dams; (ii) even if main stem surviv al were elevated to 100%, Snake River spring/summer chinook salmon (Oncorhy nchus tshawytscha) would probably continue to decline toward extinction; an d (iii) modest reductions in first-year mortality or estuarine mortality wo uld reverse current population declines.