Difficulties in estimating survival for adult chinook salmon in the Columbia and Snake rivers

We reviewed current methods used to estimate survival of adult chinook salm on (Oncorhynchus tshawytscha) as they migrate upstream past hydroelectric p rojects in the Columbia and Snake rivers, evaluated known and unaccounted-f or loss factors, and assessed how adult survival estimates could be improve d. Dam counts and associated passage conversion rates do not always provide accurate estimates of adult survival between hydroelectric projects. Expan sion techniques for reconstructing run size and harvest rates also contribu te to variability in estimates of run size and potential loss between hydro electric projects. Use of passage conversion rates to estimate in-river sur vival of adult spring chinook salmon had less uncertainty than for estimate s of other runs. Fixed-run cui off dates for migration timing result in a h igh uncertainty for monitoring relative numbers of summer chinook salmon. W e also found it difficult to reconstruct run size to spawning areas or to e stimate interdam survival for fall chinook in lower Snake River darns becau se of straying and high incidence (e.g., up to 40% at some projects) of fal lback behavior. In-river survival estimates of adult chinook salmon would b e improved by factoring adult fallback percentages into passage estimates, combining spring and summer runs for accounting purposes, methods.