Relationship of Smolt-to-Adult Return Rates to Productivity and Implications for Population Recovery

Success of any hydrosystem mitigation strategy for Pacific salmon in the Columbia River Basin will require achievement of smolt-to-adult survival return rates (SARs) sufficient to meet recovery and rebuilding objectives, in combination with a program to maintain or achieve adequate survival in other life stages.  The Northwest Power and Conservation Council (Council) adopted a goal of achieving SARs in the 2%-6% range for salmon and steelhead. The Council’s strategy is to identify effects of ocean conditions on these populations and use this information to evaluate and adjust inland restoration actions. Long-term measures of population productivity (adult returns per spawner) were strongly associated with SARs for populations of stream-type Chinook salmon in the Snake River and mid-Columbia regions. Recent SARs for listed Snake River salmon, estimated using PIT tags, have remained well below the Council’s goal.  The strong associations between SARs and river conditions indicate that hydrosystem mortality increases with the number of powerhouse passages and decreases with the speed of out-migration.  A promising conservation approach would be to explore management experiments that evaluate these relationships by increasing managed spill levels at the dams during the spring migration period.