Lamprey in difficulty

Since time immemorial, the sea lamprey has been swimming upriver to spawn and die. A migratory species that lives alternately in fresh and salt water, it spends more than 70 percent of its life as a juvenile (ammocoete) buried in the substrate of the sandy-silty areas of watercourses. What is it doing there, hidden away like that? It is growing and awaiting the metamorphosis that will prepare it for life in the sea. Not strictly speaking a fish, the sea lamprey is a primitive vertebrate lacking jaws and paired fins, but equipped with a disc-shaped, suction-cup-like mouth for tightly latching on to the flanks of fish. Adopting a parasitic lifestyle at sea, it grows – over a period of between one and three years – in coastal areas before returning to rivers.

Population numbers in freefall

In the Loire catchment, particularly in the Vienne-Creuse sub-basin, a large population of sea lamprey had re-established itself following the removal of the Maison-Rouge dam (in Indre-et-Loire) in 1998. With a total of more than 90,000 individuals observed at video-counting stations in this sub-basin in 2007, the Loire basin was home to one of the largest populations of sea lampreys in Europe.
Over the past eight years, their numbers have collapsed as a result of chronic water shortages and undoubtedly a decline in marine survival rates, which appears to be affecting all French, Spanish and Portuguese populations. With 18 individuals recorded in 2024 at video-counting stations in the Loire basin, there is an urgent need for action, starting with the elimination or reduction of all anthropogenic mortality factors affecting lampreys: fishing, barriers to migration in both directions, water abstraction from rivers which exacerbates hydrological deficits, etc. These are all measures that may help to preserve this endangered species.

Marion Legrand
Programme Manager for the Loire Basin Migratory Fish Dashboard at Logrami (which stands for Loire Grands Migrateurs)