Located on the Allier River (France), the largest upstream tributary of the Loire River which is one of the longest river system in Europe in which Atlantic salmon (Salmo salar) spawning migration occurs, the dam of Poutès was the scene of conflicts for decades. A technical solution, result of the stakeholders ‘conciliation, will lead to an important reconfiguration of the current dam to meet with the ecological and sedimentary continuity requirements. Part of the scientific monitoring of this innovative project, a 2D acoustic telemetry experiment with JSATS AMT (Juvenile Salmon Acoustic Telemetry System Acoustic Micro Transmitter-Lotek) was conducted in 2014 and 2015 at the Poutès dam. The first objective was to test this new technology which would allow us to validate the future bypass efficiency thanks to the study of smolts ‘precise trajectories approaching the dam. The second objective was to study the behaviour of wild smolts in an early phase of the downstream migration at almost 900 km from the ocean. In fact, recent studies using rotary screw traps upstream of Poutès highlighted early migrations of fishes which were not silver-skinned like typical smolts. The behaviour of these “early fishes” and specifically their behaviour close to the bypass has never been studied, the former studies having always used farmed fishes at a later period. Results suggest that this technology is powerful and can be very useful to study the attractivity of bypasses. Moreover, the behaviour of wild smolts was investigated and showed interesting patterns.