Effect of an obstacle on swimming kinematics in juvenile silver carp, Hypophthalmichthys molitrix

Weirs and dams affect the hydraulics and geomorphology of natural rivers, greatly modifying fish habitat, in some cases, creating barriers that prevent fish swimming upstream. Typical engineering methods to mitigate dam effects on movements of fish include fishway construction and riverbed reconfiguration. Among these measures, it is common to use obstacles to increase bed roughness and create microhabitat that slows flow velocity and helps fish ascend fishways. Obstacles create a complex flow environment in fishway. Understanding swimming behavior in complex flow environment is critical to improving the effectiveness of fishways. A stationary obstacle within a fish passage can be either beneficial or detrimental to fish depending on the direction and magnitude of flow velocity and the ability of the fish to maintain stability. Flow velocity in relation to energy expenditure is critical when considering the effect of obstacles on locomotory behavior. We also discovered how silver carp (Hypophthalmichthys molitrix) alter their locomotory behavior in response to turbulent flow caused by varying flow velocities around obstacles. Silver carp is an important native commercial species in China. Silver carp tend to inhabit fast-flowing streams with structurally complex flow environments. No quantitative kinematic studies have described the locomotory behavior of silver carp in unsteady flow. In this study, the swimming kinematics of juvenile silver carp holding station and moving forward under obstacle conditions were analyzed at different flow velocities (1, 3 and 5 BL/s) (body length per second). Metrics used to quantify kinematics were tail-beat frequency, tail-beat amplitude, ground swimming speed, swimming acceleration, and ground stride. Compared to free stream swimming at flow velocity of 3 BL/s, silver carp holding station behind the obstacle showed a significant decrease in tail-beat frequency and significant increase in tail-beat amplitude; silver carp moving forward behind the obstacle showed a significant decrease in tail-beat frequency, ground swimming speed, swimming acceleration and ground stride, and significant increase in tail-beat amplitude. This result showed that the obstacle help fish to save energy at 3 BL/s. In the obstacle-placed currents, tail-beat frequency of the holding station behavior increased with flow velocity. Tail-beat frequency, tail-beat amplitude and swim acceleration of moving forward behavior increased with flow velocity, while ground stride decreased with flow velocity. At flow velocity of 3 BL/s, we found that silver carp held a preferential position from 5-30 cm downstream of an obstacle where fish might adopt Karmen gait, and residence time accounted for 92.5%. At flow velocity of 1 BL/s and 5 BL/s, silver carp did not hold an evident preferential position in the flume. Silver carp often swam close to the wall, and there was no obvious position preference. Our results indicate that a certain combination of obstacles and flow velocity will create flow refuge for fish. The results may provide important reference for fishway design.