Coupling of CFD and Telemetry to Characterize Velocity-Based Behavioral Rules for Downstream-Migrating Salmon Smolt in a Large River Reach

Downstream migration of salmonid smolts through regulated rivers remains a major ecological and engineering challenge, with high mortality and delay rates despite mitigation measures like bypasses and guidance systems. This study integrates Computational Fluid Dynamics (CFD) with fish telemetry to analyze how salmon smolts respond to local hydraulic conditions in a real riverine environment. By coupling detailed CFD flow models with two-dimensional smolt track data from a hydropower facility in northern Sweden, we identified behavioral tendencies linked to specific flow velocities. The analysis of fish movement patterns indicates a general tendency to follow the main current during migration, with occasional variations influenced by initial velocity and local flow conditions. This behaviorally informed CFD–telemetry approach provides a method for identifying behavioral patterns based on velocities and demonstrates its potential to improve fish passage models, supporting more ecologically effective hydropower design. This study highlights the need for broader datasets to fully capture smolt behavior and to develop standardized, transferable modeling frameworks for fish–flow interactions.