Do Bypass Systems Protect Juvenile Salmonids at Dams?
Fish Passage Policy and Technology: Proceedings of a Symposium
American Fisheries Society
Bypass systems at dams on the Columbia and Snake Rivers divert large numbers of juvenile salmonids (Oncorhynchus spp) away from turbine intakes and downstream into tailrace areas. However, few rigorous assessments of comparative survival have been made between bypassed fish and those passing through turbines. A study was initiated at Bonneville Dam in 1987 to provide definitive information regarding passage survival. Results to date show bypassed fish had lower survival than fish passing the dam through turbines or spillways. Fish exiting the bypass system had elevated plasma concentrations of cortisol (an index of stress), but physical injuries were not apparent. One likely cause for decreased survival of bypassed fish was predation by northern squawfish (Ptychocheilus oregonensis) on fish exiting the bypass outlet. Study results indicated that bypass operational procedures, as well as location and physical conditions at the bypass outlet, favor high predation. Factors contributing to high predation include: (1) river-water velocity of less than 1.2 m/s at the bypass exit; (2) proximity of the bypass exit to predator sanctuary areas; (3) location of the bypass exit at a curved reach of river where the flow was directed towards shorelines; (4) poor dispersal of released fish, resulting in increased juvenile salmon density in the migration route; (5) disorientation of juvenile salmonids upon exiting the bypass outlet; and (6) continuous release at one site.