The spawning grounds of the sea lamprey Petromyzon marinus are located within the perimeter of the Sélune Observatory. As this species is migratory, its colonisation of the Sélune river and its tributaries will probably be influenced by the removal of the Sélune dams. The spawning grounds are characterised by a depression accompanied by a sediment ejection dome immediately downstream. The stones returned to the dome are lighter, making the nests easily visible and identifiable for up to several weeks after spawning.