Flow velocity and boundary effects on fish interaction.

How do environmental cues shape the coordination rules underlying collective motion in fish shoals? This question is crucial as freshwater migratory fish are globally declining due to river fragmentation, and collective motion is known to influence the effectiveness of fish pass solutions. However, experimental data on individual interactions and the effects of solid boundaries and hydrodynamic conditions remain limited. Adopting a reductionist approach - using fish pairs as the minimal shoal's unit - we examined how mean flow velocity and boundary proximity affect interaction dynamics in a riverine species. Towards this end, we tracked pairs of Italian riffle dace (Telestes muticellus) across three bulk flow velocities (2, 4 and 7 BL/s) and four regions in a confined open channel flow. We found shoaling time to be flow-invariant; however, fish shifted from side-by-side to in-line formations as flow increased, especially near walls. Correlation analysis revealed stronger velocity coupling at short distances (< 6 BL) that remain largely stable across flow velocities. Response times differed by coordination direction: longitudinal responses (~ 0.5 s) were flow-invariant and symmetrical between lead and rear fish, while lateral responses primarily involved the rear fish reacting to the front one, and accelerated with increasing flow. Longitudinal coordination emerged only near side walls and in the central region, whereas lateral coupling peaked in central areas and strengthened with increasing flow. These findings reveal how physical context shapes coordination in riverine fish and provide empirical insights for fish pass design and fish movement modelling.