Swimming behavior of juvenile silver carp near the separation zone of a channel confluence

Abstract Knowledge of locomotion of fish near river confluences is important for prediction of fish distribution in a river network. The flow separation zone near the confluence of a river network is a favorite habitat and feeding place for silver carp, which is one of the four major species of Chinese carp and usually provides positive rheotaxis to water flow. In the current study, a series of laboratory experiments were done to determine the behavioral responses of juvenile silver carp to the hydrodynamic forces near the separation zone of a channel confluence. The locomotion and trajectory of juvenile silver carp were recorded by infrared thermal imaging, while the flow velocity field near the separation zone was measured by a Particle Image Velocimetry (PIV) system. A total of 60 juvenile silver carp were released near the separation zone among which 40 carp swam in the upstream direction. Amongst them, 24 carp swam to the tributary and the remaining 16 swam into the main channel. Almost all these 24 carp travelled initially along the boundary of the separation zone near the corner, where flow shear was strongest, and then swam to the tributary. Instead of avoiding zones of strong vorticity, they chose and followed a trajectory along which the flow vorticity was large. On encountering these vortical flows, they increased the tail-beat frequency and decreased the tail-beat amplitude to maintain body stability. These observations provide important knowledge on locomotion of fish near river confluences and are beneficial for the fish habitat protection.