Juvenile Salmonid Survival through the Wanapum Development during Emergency Reservoir Drawdown Conditions

Survival studies on juvenile migrants were conducted in the Wanapum Development to determine if the emergency reservoir drawdown resulted in low juvenile downstream migration survival. An 80% reduction in flow through the fish bypass occurred and there were concerns that this level of flow reduction could affect the bypass’s fish passage efficiency and passage route survival rate for out-migrating smolts. To address these concerns, 1,720 steelhead (Oncorhynchus mykiss) and 1,716 yearling Chinook salmon (O. tshawytscha) smolts were surgically implanted with JSATS tags and released at the upper end of the reservoir. Steelhead and yearling Chinook traveled a median of 20.7 and 41.5 h, respectively, to Wanapum Dam. The proportion of fish that passed the bypass was decreased for both species, passage proportions at the bypass decreased by 67% for steelhead and 30% for yearling Chinook. Survival of juvenile steelhead and yearling Chinook through the Wanapum Development was 92.9% (SE 1.4%) and 94.5% (SE 1.3%), respectively. Relative survival by passage route were not significantly different for steelhead that passed through the spillway and bypass, but was 5% less for steelhead that passed through the powerhouse; relative survival between passage routes was not significantly different for yearling Chinook between all routes.