Effects of Decelerating and Accelerating Flows on Juvenile Salmonid Behavior

Migratory and resident fish species have evolved inherent flight responses to avoid potentially harmful situations. At many dams, fish screens or other structures have been installed to guide fish away from turbines or attract them to routes that will result in higher survival. Avoidance responses of fish to rapidly decelerating and accelerating flows at these structures have been repeatedly observed and can result in ineffective fish guidance. By using controlled flume experiments, we analyzed the avoidance behavior of actively migrating spring Chinook salmon Oncorhynchus tshawytscha smolts in relation to flow decelerations and accelerations. As smolts drifted into areas with decreasing velocities, they actively swam into the current; the larger was the change in water velocity with distance (spatial velocity gradient [SVG]), the faster was the swimming speed exhibited by smolts. Under accelerating flows, the response velocity varied significantly with flow conditions, but the median SVG at which smolts displayed an avoidance response was similar over all flows tested. For both decelerating and accelerating flows, the avoidance response occurred at an SVG of approximately 1 cm.s(-1).cm(-1). We suggest that this threshold is in part fixed by the energetically optimum swimming speed of the fish (similar to 1 body length/s).