Swimming kinematics of Aristichthys nobilis in the wake of vertical bluff bodies with different cross-sectional shapes

The interaction between fish and complex flow environments has drawn attention; however, the underlying mechanisms remain poorly understood. This study investigated the swimming behavior of Aristichthys nobilis (A.N.) in the wake regions formed behind rigid vertical bluff bodies with four different cross-sectional shapes, under flow velocities ranging from 0.1 to 0.7 m/s. Key fish swimming kinematic parameters, including swimming speed, tail-beat frequency, amplitude, and angle, were quantified. The results showed that A.N. preferentially performed Kármán gaits in the wakes of bluff bodies with circular and rhombic cross sections. In these cases, the occurrence probabilities of the Kármán gait ranged from 72% to 55%, significantly higher than those observed for square or rectangular cross sections. Moreover, at lower flow velocities (0.1–0.3 m/s), the predominant swimming mode was gliding, characterized by lower swimming speed and tail-beat frequency, while at higher velocities, A.N. tended to switch to the Kármán gait, likely to enhance swimming efficiency.