Study on the swimming behavior of minnows under different obstacles

Riverine fish often encounter complex hydrodynamic environments containing turbulence, wake structures, and physical obstacles during migration and routine movement. These flow conditions can significantly influence swimming performance, energy expenditure, and behavioral strategies. Understanding how fish respond to different obstacle-induced flow fields is essential for ecological studies and for improving the design of fish passage facilities. However, quantitative evidence on how obstacle geometry affects locomotor behavior and energetic efficiency remains limited. This study aimed to investigate the adaptive responses of Opsariichthys bidens to different obstacle shapes under controlled flow conditions, focusing on swimming behavior, critical swimming speed, and energy-saving mechanisms. The objective was to clarify how hydrodynamic structures generated by different obstacles influence fish movement efficiency and to provide guidance for fishway optimization. Experiments were conducted in a closed-circuit swimming performance flume. Three obstacle geometries—square column, circular column, and semicircular column—were installed to simulate different hydrodynamic disturbances. The critical swimming speed was measured for each condition. Behavioral parameters were analyzed, including tail-beat frequency, tail-beat amplitude, body lateral acceleration, maximum turning angle, and the proportion of hydrodynamic sheltering behavior. Fish residence time in wake regions was also quantified. Flow velocity was standardized at 5.08 body lengths per second (BL/s) for behavioral comparisons. Results showed significant differences in relative critical swimming speed among obstacle types. Under square column conditions, the critical swimming speed was (8.18±0.96) BL/s, which was significantly higher than that observed under circular column (6.35±1.47) BL/s and semicircular column (7.27±0.70) BL/s conditions. No significant difference was detected between circular, semicircular, and control (uniform flow) treatments. Behavioral analysis indicated that hydrodynamic sheltering behavior was most frequently observed under square column conditions. Fish consistently spent the highest proportion of time in the bow wake region upstream of all obstacle types. At 5.08 BL/s, tail-beat frequency under square columns was significantly lower than that under circular and semicircular columns, whereas tail-beat amplitude and lateral body acceleration were significantly higher. In addition, circular column conditions induced the largest maximum turning angle, significantly greater than those observed under semicircular and square configurations, while no significant difference was found between square and semicircular obstacles. These results demonstrate that obstacle geometry strongly influences swimming kinematics and energy utilization strategies in O. bidens. Square obstacles generate more stable and predictable wake structures, allowing fish to exploit hydrodynamic sheltering more efficiently and reduce locomotor energy costs during upstream movement. In conclusion, square column obstacles provide more favorable hydrodynamic conditions for energy-efficient swimming compared with circular and semicircular forms. These findings highlight the importance of obstacle shape in regulating fish behavior and energetics and provide valuable empirical evidence for optimizing fishway design. The results suggest that engineering fish passage structures with appropriately shaped obstacles may improve passage efficiency and reduce energetic burdens, thereby supporting river connectivity and freshwater biodiversity conservation.