contribute to the productivity of anadromous fish populations. NOAA (National Oceanographic and Atmospheric Administration) Fisheries (formerly the National Marine Fisheries Service, or NMFS), the federal fishery agency responsible for the recovery of anadromous salmonid populations listed pursuant to the Endangered Species Act, embraces these strategies and calls for their continued improvement and use in fostering salmon recovery (NMFS, 2000). Even so, it is not known whether these actions alone can reverse or stall long-term declines in salmon populations. Much of the research identified in the 2000 Biological Opinion from the NMFS focuses on improving the implementation of these strategies and gaining a clearer understanding of the outcomes of management actions that are often confounded by environmental complexities. Furthermore, conditions in tributaries and in estuarine and marine habitats have pronounced effects on salmon productivity, as do harvest and hatchery programs. Large salmon returns in 2001 to 2003, for example, were viewed by many scientists as a function of favorable ocean conditions (NPCC, 2003), but ecological and biological complexities inhibit perfect understanding of cause and effect in such events. In any event, a 100-year snapshot of Columbia River salmon portrays long-term declines and provides a backdrop against which short-term events should be evaluated. This chapter reviews environmental variables that affect Columbia River salmon and examines competing hypotheses and models constructed to explain the relative importance of these variables.

COLUMBIA RIVER SALMON

Three species of anadromous salmonids commonly migrate through the middle and upper reaches (above Bonneville Dam) of the Columbia and Snake rivers in the State of Washington: Chinook (Oncorhynchus tshawytscha), steelhead (Oncorhynchus mykiss), and sockeye (Oncorhynchus nerka) all commonly migrate to spawning destinations well upstream from Bonneville Dam. Remnant wild and hatchery populations of coho salmon (O. kisutch) are also found in select locales in the upper Columbia basin. All these species have some population units that are listed as endangered or threatened under the Endangered Species Act (see Table 1-1). Additionally, chum salmon (O. keta), which



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