Understanding the hydrodynamic principles underlying fish swimming efficiency is critical for optimizing fishway design and improving habitat restoration. Wake morphology and energetics of continuously swimming crucian carp were examined across flow velocities in a Brett-type swimming tunnel and analyzed using the velocity gradient method. High-speed video captured swimming behavior and two-dimensional particle image velocimetry were used to visualize the flow field generated by the fish. The critical swimming speed (Ucrit) of crucian carp was 0.85 m/s and wake vortices were observed across flow velocity gradients ranging from 0.15 to 1.2 m/s. The wake beat frequency and vorticity of the wake increasing linearly with flow velocity. Hydrodynamic efficiency of the wake vortex energy ranged from 62 to 84%. Significant changes in tailbeat amplitude, head beat amplitude, and stride length were observed near (70–88%) Ucrit (0.60–0.75 m/s), when the dimensionless Strouhal number (St) indicates efficient swimming. Thus, we conclude that 70%-88% of Ucrit is the threshold range for efficient swimming and this provides practical guidance for setting flow velocities in fishways and habitat restoration projects that will enhance fish survival.
Hou et al. (Fri,) studied this question.