Detection of PIT-Tagged Subyearling Chinook Salmon at a Snake River Dam: Implications for Summer Flow Augmentation

Abstract
Rearing subyearling chinook salmon Oncorhynchus tshawytscha (≥60 mm in fork length) were captured in the Snake River and tagged with passive integrated transponders to provide an index of their survival to Lower Granite Dam, the first of eight dams encountered by seaward migrants. Water was released from reservoirs upstream of Lower Granite Dam to augment summer flows and thereby increase subyearling chinook salmon survival. Mean summer flow and maximum summer water temperature in Lower Granite Reservoir were highly correlated (N = 4; r = −0.999). Acknowledging this correlation, we conducted two separate least-squares regressions using detection rate as the dependent variable. Detection rate at Lower Granite Dam was positively related to mean summer flow (N = 4; r2 = 0.993; P = 0.003) and negatively related to maximum summer water temperature (N = 4; r2 = 0.984; P = 0.008). Summer flow augmentation increased flow and decreased water temperature in Lower Granite Reservoir especially in low-flow years. Our results support summer flow augmentation as a beneficial interim recovery measure for enhancing survival of subyearling chinook salmon in the Snake River. Additional research should include replicate within-year releases of PIT-tagged subyearlings as well as studies of fish guidance efficiency.

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