Abstract:
Salmon conservation efforts typically assume that expediting smolt movements into the ocean to mimic historic flow conditions will improve overall survival in dammed river systems. Management agencies are therefore encouraged to reduce the duration of the freshwater migration phase by increasing migration rates using approaches such as increasing spill, transport, or reservoir drawdown and, in extreme cases, dam breaching. Telemetry results for Snake River Spring Chinook smolts in the Columbia River (western North America) allow us to compare survival rates in four successive habitats in 2006-08: Hydropower system (8 dams), 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. It is also possible to compare the survival rates of various species of acoustically tagged salmon reaching the mouth of the Columbia River with those making a similar migration down the undammed Fraser River in Canada. We will compare these survival values and show how they are similar and where they are different.
Overall, because management actions reducing freshwater residence time increase saltwater residence time by equivalent amounts, accelerating the movement of smolts out of the hydrosystem may be counterproductive and be economically costly, because such actions may simply change the location where smolts die, not improve survival as intended. Whether a chosen hydrosystem action actually enhances salmon conservation thus depends upon whether ocean survival rates are higher or lower than in freshwater. Using the data collected over the past decade using rigorously designed telemetry systems, my company has been able to develop a useful database of comparative survival rates demonstrating that there may be significant opportunity to increase both salmon conservation and power generation in years of poor ocean conditions by slowing freshwater migration rates, because a net survival benefit may accrue from remaining in freshwater for longer periods. Simply put, freshwater losses may be lower than those experienced in the ocean
