Scale-Dependent Microhabitat Choice of an Asian Minnow (Schizothorax grahami) in Heterogeneous Rock-Array Flows

Fish can perceive and respond to local flow structures when selecting microhabitats, yet how scale-dependent hydrodynamic heterogeneity shapes behavioral habitat choice remains poorly understood. In this study, a multi-scale rock-array flow field was constructed using flume experiments, and the volitional swimming behavior of Schizothorax grahami was observed. Hydrodynamic parameters including velocity magnitude (Umag), turbulent kinetic energy (TKE), and kinetic energy gradient (KEG) were analyzed. The results indicated that fish distribution changed with rock size: at a rock-element diameter of 0.12 m, fish were dispersed without forming stable high-utilization core areas; at 0.24 m, fish aggregated in the lee-side regions behind the rock elements; and at 0.50 m, fish shifted toward the upstream faces. The hydrodynamic variables significantly associated with fish microhabitat selection varied with rock-array scale: in the 0.24 m rock arrays, KEG differed significantly between high- and low-use units under all flow conditions; in the 0.50 m rock arrays, Umag and TKE showed consistent and significant between-group differences across flow conditions. Moreover, cross-scale overlap analysis revealed that the utilization range of velocity magnitude had the highest overlap, followed by TKE, with KEG exhibiting the lowest overlap. Collectively, these findings support a hierarchical interpretation of fish microhabitat selection characterized by “velocity rigidity–turbulence elasticity,” providing a potential perspective for understanding differences in the hydrodynamic variables associated with fish microhabitat selection across studies. These results indicate that both mean velocity constraints and local turbulence-creating structures should be considered in river habitat restoration and roughness element configuration.