Fish have evolved intrinsic flight responses, allowing pre-emptive avoidance of potentially threatening situations. To direct downstream migrant fish away from deleterious conditions at dams 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 fall Chinook salmon ( Oncorhynchus tshawytscha ) smolts in controlled experiments of low-, medium-, and high-flow accelerations. We measured the response velocity (VR) and the velocity gradient (VG) over body length. Although VR varied significantly with flow conditions and increased with increasing water temperature, the median VG at the instant at which smolts displayed an 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 on any downstream migrant fish species.

We assessed fine-scale behavioral responses of the smolts of four Pacific salmonid species to open and constricted channels in a flume. Natural migrants encountered two geometrically similar parallel channels with different hydraulic conditions representing constricted and open treatments. Observation of route selection under alternate discharge scenarios provided evidence of behavioral choice by smolts. As expected, the majority of smolts passed through the open channel in a ratio consistent with flow. After controlling for the influence of flow, both initial channel selection and subsequent channel rejection was higher for the constricted channel; rejection was probably due to fish detecting an area of rapidly accelerating flow. The majority of smolts traveled downstream headfirst and faster than the mean midcolumn water velocity. Those that faced the flow passed at a slower rate and tended to select the open treatment. The few yearling Chinook salmon Oncorhynchus tshawytscha and coho salmon O. kisutch smolts that did not pass through the treatment channels, but held position within the flume, were larger than their conspecifics that passed downstream. Large steelhead (anadromous rainbow trout O. mykiss) smolts and subyearling Chinook salmon were more likely to pass the constricted channel than smaller fish. These results suggest that efforts to effectively guide fish with diversion structures will require understanding how the structures alter the local hydraulic environment and, thus, influence fish 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 (∼1 body length/s).