Evidence of late migrant smolts of Atlantic salmon (Salmo salar) in the Loire-Allier System, France

Smolts of Atlantic salmon Salmo salar of the Loire-Allier system have to perform an exceptional distance of 900 km to reach the ocean from the upstream spawning grounds, while being subjected to connectivity disruption induced by the presence of dams as well as fluctuating climatic conditions. In order to successfully perform their marine entry, smolts should reach the estuary within a limited suitable period. This study concerns smolt emigration dates from upper Allier, and arrival dates in the Estuary, as well as estimation by radio tracking of downstream migration speed. Comparison of these data revealed that a significant fraction of smolts from upper Allier leave breeding areas too late for successful marine entry. This delay may be partly induced by dam retention. We emphasize the importance of the clearing of the Pontes dam planed in 2015 and recommend to pursue and extend the current survey with additional monitoring stations along the migration axis, in order to study the fate of late migrants. These studies have major implications for the restoration of the threatened population of the Loire salmon.