Reservoirs and Dams Impact the Migration Success of Silver Eels. A Telemetry Approach on the Frémur River, France

River management is known to have severely fragmented river continuity. The steep decline of temperate eels is caused by a set of factors including dams that reduce accessibility to growth habitats and induce turbine mortality. We studied effects of dams and reservoir on downstream migration behaviour of silver European eels. We used an acoustic telemetry design on the Frémur catchment, France, comprising 5 hydrophones set along the river from upstream the reservoir to downstream two major dams used for freshwater facilities. Despite exceptionally favourable environmental conditions, only one third of these silver eels managed to move downstream the river. Migration speed was much lower in the Reservoir than in non-obstructed rivers (0.097km.day-1 vs. 1.91km.day-1). The main dam (14m high) was the major obstacle to downstream migration: eels were only able to pass over the crest when the reservoir was full. Four different downstream migration behaviours occurred. They were not related to biological traits, suggesting an individual flexibility of silver eels. This study provides useful information to manage eel populations in such water basins that are very likely to be applied to all reservoirs and dams, which are widespread through the distribution range of European eels.