The sea lamprey (Petromyzon marinus) is born in freshwater, then migrates to the sea to feed. After spending 1 to 2 years in the ocean, sea lampreys return to the river to breed in summer. They dig nests to lay their eggs, after which they die. The larvae, called ammocoetes, emerge in autumn and spend many years filtering organic particles from the sediment. They then leave the fresh water of rivers to feed at sea.

Sea lampreys recolonise the Sélune river

Sea lamprey migration was also hampered by the presence of dams on the Sélune. Because of the presence of nests, the presence of sea lampreys during the breeding season is particularly noticeable. Monitoring of these spawning grounds is carried out every year and, prior to 2023, showed that many lampreys were circulating and breeding in the lower reaches of the Sélune valley and on two tributaries of the Sélune (Oir and Beuvron) located downstream of the dams. In the summer of 2023, following the removal of the last dam on the Sélune, numerous lamprey spawning grounds (nests where they lay their eggs) were counted upstream of the old dams. The lampreys have moved very high up the Sélune catchment. The ammocoetes from these nests will be monitored over the coming years to understand how the lamprey population is establishing itself in these new habitats.