Effects of flow velocity on swimming behavior of Lateolabrax maculatus juvenile with different population sizes

To investigate the behavioral characteristics to flow velocity, we measured the swimming behaviors of juvenile Lateolabrax maculatus of different individuals (1, 2, 4, 6 and 8) at various flow velocities (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 BL·s⁻¹; BL: Body length) by using swimming ability test tank. When the flow velocity reached 4.5 BL·s⁻¹, the group movement speed (Gc) of fish schooling was significantly lower than that of single fish (p<0.05). With an increase in the flow velocity, the inter-individual distance (IID) (Reflects the cohesion of fish swarm) first decreased and then increased in each group, and the minimum values were observed when the flow velocity reached 3.5–4.5 BL·s⁻¹. Besides, the flow velocity corresponding to the above IID minimum value increased with the expansion of fish population. The trend of swimming speed synchrony (S_V), reflecting the coordination of fish population, was completely opposite with that of IID, and the flow velocity corresponding to the maximum S_V value also increased with the fish schooling expansion. When the flow velocity reached 1.0 BL·s⁻¹, the tail beat frequency (TBF) of single fish was significantly higher than that of other groups at the same flow velocity (p<0.05). With the increase of flow velocity, the proportion of diamond lattice displayed in fish schooling first decreased and then increased, which was opposite to the phalanx lattice pattern. However, the pattern of diamond lattice was always dominant in each group at different flow velocities. In conclusion, with the increase of flow velocity, the cohesion and coordination of L. maculatus schooling first increased and then decreased. The expansion of the fish population is conducive to improving its cohesion and coordination, and the fish mainly adopts adiamond lattice pattern to reduce swimming energy consumption.