Mechanism of Fish Habitat Selection Based on Hydrodynamic Indicators Under Multiple Topographic Conditions for Ecological Restoration Requirements

The establishment of hydropower projects has altered the flow regime and structure of fish habitats. Various habitat restoration measures based on natural river morphology have been implemented, but the effects of habitat restoration and the hydrodynamic mechanisms of fish preferred topography remain unclear. We constructed diverse near-natural microtopographic habitats and monitored test fish behavior in real time using a Radio Frequency Identification (RFID) system with Passive Integrated Transponder (PIT) tags. Results demonstrate that structurally distinct microtopographies differentially attract fish. Among four selected hydrodynamic indicators, flow velocity and vorticity most strongly influenced <i>Onychostoma sima</i> (<i>O. sima</i>). The optimal vorticity for fish habitats ranges between 1.75 and 10.85/s, with suitable flow velocities exceeding 0.545 m/s. Using Habitat Suitability Index (HSI) and Classification and Regression Tree (CART) methodologies, we revealed the hydrodynamic mechanism underlying habitat selection. Validation via the Receiver Operating Characteristic (ROC) method confirmed that the hydrodynamic-based habitat selection model effectively predicted suitable habitats for most test fish. This study elucidates linkages between fish activity and hydrodynamic indicators in complex terrain, identifying habitat utilization patterns under variable flow and topographic conditions. Our findings provide theoretical support for ecological operation and habitat management in hydropower-affected rivers.