Effects of Encountering Juvenile Bypass Systems on Migration and Smolt-to-Adult Survival Rates

Smolt collection or bypass systems are currently in place at seven of the eight dams on the lower Snake and Columbia rivers.  Several studies have found that smolt survival estimates through bypass systems are nearly as high as through spillways, but few studies have examined the effects of bypass systems on migration delay, post hydro system delayed mortality and overall smolt-to-adult survival rates (SARs).  If smolts that migrate through single or multiple bypass systems experience migration delay, delayed mortality or reduction in SARs, then the direct survival rates estimated for smolts using bypass systems (e.g., performance standard testing approaches) may not be providing an entirely holistic view of bypass systems and their ability to help meet recovery goals.  Models developed for Snake River yearling Chinook salmon and steelhead that encounter bypass systems in route to the ocean examined the magnitude of migration delay associated with multiple bypass experiences.  Furthermore, models examined the degree to which the number of bypass experiences helped characterize variation in SARs and whether this explanatory factor significantly reduced SARs for the total number of bypass experiences at upriver dams.