Abstract: The complex flow environment is formed by the obstacles at the bottom of natural river channels. The behavior of fish with migration demand in response to the complex flow environment is important for fish to complete life cycle upstream or downstream through obstacles. The critical swimming capability of juvenile bighead carp (Aristichthys nobilis) was tested under different obstacle types in a closed tank by increasing velocity method. The results showed that there was no significant difference in critical swimming capability of juvenile bighead carp among free stream, cylinder, and semi-cylinder conditions (P>0.05), but that of juvenile bighead carp under square-cylinder decreased significantly (P<0.05). Affected by the complex flow field downstream of the obstacle, the   juvenile bighead carp showed three characteristic swimming postures, which could be divided into three position areas. To analyze the causes of the decline of swimming ability under the squarecylinder, the time percentage of the juvenile bighead carp in the three position areas under different flow velocities was counted, and the corresponding swimming kinematic indices were extracted, including tail-beat frequency, tail-beat amplitude, maximum head lateral acceleration, maximum body lateral acceleration, body wave speed, body wavelength, and maximum head angle speed. The results showed that the juvenile bighead carp in the near cylinder area (6-26 cm downstream of the obstacle; area A) had the longest residence time, with a time percentage of 63.1%; the second was in the middle area (26-46 cm downstream of the obstacle; area B), with a time percentage of 29.1%; the far cylinder area (46-66 cm downstream of the obstacle; area C) had the lowest residence time, with a time percentage of 7.8%. The time percentage distribution of juvenile bighead carp in the three position areas downstream of the square-cylinder was significantly different under different flow velocities. At the flow velocity of 5.24 BL·s^-1, the residence time of juvenile bighead carp in area A was as high as 30.3%. By comparing the swimming kinematic indexes of juvenile bighead carp in three position areas downstream of the square-cylinder, it was found that tail-beat frequency and body wave speed of juvenile bighead carp in area A were significantly lower than those in area B and C (P<0.05), but tail-beat amplitude, maximum head lateral acceleration, maximum body lateral acceleration and maximum head angle speed were significantly higher than those in area B and C (P<0.05). From the perspective of swimming strategy and swimming kinematic, we found that the special combination of square-cylinder and velocity formed a complex flow pattern behind the cylinder, which may increase swimming energy consumption and reduce the swimming ability of juvenile bighead carp. This study can provide a reference for the design of bottom barrier when migratory fish pass the obstacle upstream and downstream.

Key words: square-cylinder, flow velocity, critical swimming speed, swimming kinematics.