The European eel (Anguilla anguilla) is a migratory fish that lives in rivers and breeds at sea. Eels are born in the Sargasso Sea - in the North Atlantic Ocean - before embarking on a long journey to European freshwater. After travelling thousands of kilometres, they return to rivers, where they live in freshwater for up to several decades. During their stay in freshwater, eels undergo transformations, passing through various stages of development, from glass eel to yellow eel to silver eel. This last phase is the signal that their journey back to the Sargasso Sea is imminent, where they will reproduce and die.

However, European eel populations have declined sharply due to habitat loss, pollution, overfishing and obstacles such as dams that hinder migration. The European eel is now classified as critically endangered worldwide. Numerous conservation measures, such as fishing restrictions, habitat restoration and the dismantling of dams, are being put in place to preserve this emblematic species.

The European eel is recolonising the Sélune

The European eel has been present on the Sélune and neighbouring rivers for centuries. But its migration was hampered by the presence of dams, particularly the Vezins dam. As with the Atlantic salmon, a number of monitoring tools have been put in place to study the movements of eels on the Sélune and their life history (acoustic camera, eDNA monitoring, scientific electrofishing and fishing during the downstream migration).

Every year, fishing is carried out during the eels' downstream migration, the period when eels of reproductive age leave the river for the sea. In 2022, the total biomass of eels present in the Sélune valley was around 1.3 kg per 100 m², and 281 eels at the silver stage were counted during the downstream migration. This represents a significant eel population, bearing in mind that this population was confined to downstream of the dams. In 2023, almost a year after the removal of the last dam on the Sélune, monitoring carried out by scientific electrofishing and environmental DNA measurement shows that European eels are recolonising the entire catchment. 87 eels have been identified in many sectors upstream of the old dams. This very rapid recolonisation shows that eels can react quickly to changing conditions to take advantage of the new habitats now available.