Impact of anthropized river hydrodynamic conditions on the downstream migration of Atlantic salmon smolts.

Knowledge gaps persist regarding the influence of river hydrodynamics (e.g., flow velocity, depth, temperature, pressure) on the seaward migration of young Atlantic salmon (smolts). These gaps are reflected in rivers by the reduction of safe migration routes for salmons caused by the artificial flow alteration from hydraulic structures such as dams. The situation has long contributed to the disappearance of Atlantic salmon in many rivers and hinders their sustainable reintroduction. To better understand the hydrodynamic factors influencing smolt migration speeds, this study examines the downstream migrations of 491 hatchery smolts over three years (2017, 2021, and 2023) characterized by contrasting hydrological conditions and distinct flow patterns. The study covers 82 km of the Meuse River in Belgium, where smolts crossed six reaches delimited by movable weirs. Smolt trajectories were tracked using acoustic telemetry from 12 detection sites. A one-dimensional hydrodynamic model calculated the water velocities encountered by smolts, while water temperature and the diurnal pattern were also monitored. The results confirm the positive correlation between flow velocities and smolt migration speeds suggested in the literature. Flow velocities below one body length per second (approximately 0.15 m/s) disorient smolts. However, smolts slowed at relatively high velocities, as shown by the negative correlation between flow velocities and relative migration speeds. Additionally, migration speed increased with distance travelled and daytime but decreased with water temperature. These findings pave the way for the implementation of more science-based environmental flow conditions in human-altered rivers during the migration season of smolts.