Balancing Ocean and Freshwater Survival Rates When Managing Anadromous Fish Passage

Salmon conservation efforts typically assume that expediting smolt movements into the ocean and mimicking historic conditions will improve overall survival in dammed systems.  Management agencies are therefore encouraged to reduce the duration of the freshwater migration phase by increasing migration rates by approaches such as increasing spill, transport, or reservoir drawdown.  Telemetry results for Snake River Spring Chinook smolts allow us to compare survival rates in four successive habitats in 2006-08: Hydrosystem (8 dam FCRPS), Estuary, River Plume, and Coastal Ocean.  Relative to the Hydrosystem, daily survival rates were usually lowest in the Estuary and Plume, while Coastal Ocean survival rates were slightly lower than the Hydrosystem.  Critically, marine survival rates never exceeded hydrosystem survival rates.  Because management actions reducing freshwater residence time increase saltwater residence time by equivalent amounts, accelerating the movement of smolts out of the hydrosystem may thus be counterproductive but costly, because they simply change the location where smolts die.  Whether hydrosystem actions enhance salmon conservation thus depends upon whether ocean survival rates are higher or lower than in freshwater.  There may be significant opportunities to increase both salmon conservation and power generation in years of poor ocean conditions by slowing freshwater migration rates.