Development of successful fish passage structures for downstream migrants requires knowledge of their behavioural response to accelerating flow

Fish have evolved intrinsic flight responses, allowing pre-emptive avoidance of potentially threatening situations. To direct downstream migrant fish away from deleterious conditions at darns and other barriers, mechanical devices such as travelling screens and fish bypass systems are often installed. However, field observations suggest that if these structures create areas of rapidly accelerating flow, they do not effectively guide the fish. We studied the avoidance behaviour of actively migrating fail Chinook salmon (Oncorhynchus tshawytscha) smolts in controlled experiments of low-, medium-, and high-flow accelerations. We Measured the response velocity (V-R) and the velocity gradient (VG) over body length. Although V-R varied significantly with flow conditions and increased with increasing water temperature, the median VG at the instant at which smolts displayed,in avoidance response was similar over the range of accelerating flows tested. Results from this study present the first quantitative information about the avoidance behaviour of fish to flow acceleration and should provide data needed to help engineers and biologists develop effective systems to alleviate anthropogenically altered flow regimes. Furthermore, the devised experimental setup provides a Valuable means to test the effects of accelerating flow oil my downstream migrant fish species.