Human-induced river fragmentation is a major threat to migratory fish species. Restoringriver connectivity requires the construction of fish passage solutions, such as fishways forupstream and downstream migration. While many studies focussed on the upstream migra-tion of diadromous fishes, and especially of adult Atlantic salmons (Salmosalar), we analyzejuvenile behaviour under different hydraulic conditions at reservoir to improve the develop-ment of effective bypass systems for downstream passage of salmon smolt.Based on coupling three-dimensional (3D) computational fluid dynamics (CFD) simulationsto smolt positions tracked by two-dimensional (2D) telemetry, the present study aimed toexplore smolt behaviour in relation to hydraulic cues. More specifically, we explored howhydraulic conditions influence fish behaviour and how fish navigate depending on thisbehaviour. In 2017, 23 smolts were tracked in the reservoir of Poutes (Allier River, France),associated with different turbine discharge rates. 3D CFD simulations were performed andvalidated against field measurements in the reservoir upstream of the hydropower plant.The study of fish displacements in relation to flow conditions provided new insights withthe use of thrust force, swimming orientation and direction as means to precisely character-ize smolt behaviour, which can help in the design of downstream migration passage facili-ties. At Poutes dam, flow velocity, flow acceleration and turbulent kinetic energy are verylow and therefore can lead to fish disorientation. However, results underlined that having aminimum flow velocity of 20cm/s in reservoirs is sufficient to prevent delay and allowfish navigation;