Abstract: Clarifying the response mechanism of fish swimming to surrounding flow environment is important for ecological protection and bionic application of fishes. Using high speed video technology and ANSYS FLUENT numerical simulation software, we monitored the swimming behaviors of red crucian carp in a sudden expansion flow and investigated the trajectory and kinematic pattern of fish in flow with different hydraulic parameters. By analyzing the variation of body profiles during swimming, the kinematic parameters (tail beat frequency, head swing angle, flapping amplitude, wavelength and wave speed of body deformation) were measured and the relationships between the kinematic parameters and the flow parameters were analyzed. The results showed that red crucian carp exploited two typical migration trajectories in the flow field, namely, directly backtracking upstream along the mainstream and meandering upstream around the backflow. When backtracking along the mainstream directly, the motion of fish was similar to that in the uniform flow field, in which the kinematic parameters such as swimming speed, tail beat frequency, head swing angle, tail flap amplitude increased with increasing local flow velocity. When meandering around the reflux area, the motion of fish was different from that in uniform flow, with the kinematic parameters varying greatly as its position changing in the flow field. Swing frequency was the single kinematic parameter showing a significant correlation with local flow velocity. There were obvious correlations between head swing angle and tail swing amplitude, deformation wavelength and motion phase difference under this mode. Our results deepen the understanding of swimming mechanism of fish in complex flow fields and provide basic support for bionic application and ecological protection of fishes.

Key words: red crucian carp, sudden expansion flow field, swimming behavior, kinematic characteristics