Downstream movement of fall Chinook salmon juveniles in the lower Snake River reservoirs during winter and early spring

We conducted a 3-year radiotelemetry study in the lower Snake River to (1) determine whether juvenile fall Chinook salmon Oncorhynchus tshawytscha pass dams during winter, when bypass systems and structures designed to prevent mortality are not operated; (2) determine whether downstream movement rate varies annually, seasonally, and from reservoir to reservoir; and (3) identify some of the factors that contribute to annual, seasonal, and spatial variation in downstream movement rate. Fall Chinook salmon juveniles moved downstream up to 169 km and at a sufficiently fast rate (7.5 km/d) such that large percentages (up to 93%) of the fish passed one or more dams during the winter. Mean downstream movement rate varied annually (9.2–11.3 km/d), increased from winter (7.5 km/d) to spring (16.4 km/d), and increased (from 6.9 to 16.8 km/d) as fish moved downstream from reservoir to reservoir. Fish condition factor at tagging explained some of the annual variation in downstream movement rate, whereas water particle velocity and temperature explained portions of the seasonal variation. An increase in migrational disposition as fish moved downstream helped to explain the spatial variation. The potential cost of winter movement might be reduced survival due to turbine passage at a time when the bypass systems and spillway passage structures are not operated. Efforts to understand and increase passage survival of winter migrants in large impoundments might help to rehabilitate some imperiled anadromous salmonid populations.