The Atlantic salmon (Salmo salar) is an amphihalin migratory fish that is born in a river, reaches the sea to feed and grow for 1 to 2 years, before returning to the river to reproduce and die. Depending on the individual, this migration takes place in spring and summer. Salmon thus leave the marine environment to complete their life cycle in the fresh water of rivers. Before finding partners and areas suitable for reproduction (spawning grounds), salmon rest in the deep, cool areas of the rivers until autumn, when they start to swim upstream again. In our latitudes, salmon reproduce between late November and early January.

Salmon equipped to track their movements in the Sélune valley

Since 2019, as part of a research project into the use of habitats by Atlantic salmon, salmon spawners of breeding age have been fitted with radio transmitters as they ascend the river in summer. Details of this research project and its initial results are available here.

The atlantic salmon recolonises the Sélune valley

Salmon spend the summer in deep areas downstream of the catchment. There, they wait for increases in flow in the autumn before making their final migration upstream, sometimes in tributaries, in search of shallower habitats with more turbulent flows where they reproduce. Until now, their migration was limited to the downstream part of the catchment, as dams were impassable. They therefore bred mainly in the downstream part of the Sélune and on tributaries such as the Oir and Beuvron, or even chose to return to neighbouring rivers in the bay.

Since 2022 and the start of the dismantling work at La-Roche-Qui-Boit, some salmon have begun to explore the upstream part of the Sélune catchment. Among the 18 salmon fitted with transmitters in 2022, two spawners were detected upstream of the old dams. One of them remained at Lapenty, more than 20 km upstream of La-Roche-Qui-Boit, throughout the breeding season. The data recorded by the transmitter showed intense activity at the end of December 2022, which could correspond to breeding activities.

Other monitoring tools are being used on the Sélune to understand and track the return of the Atlantic salmon: acoustic camera, eDNA monitoring, scientific electrofishing. These tools show that Atlantic salmon were present in downstream areas before the dams were removed.

Each year, before the dams were removed, a population of 200 to 700 adult salmon was assessed downstream of the dams. In 2023, young salmon (parr) born in the spring of 2023 and therefore resulting from reproduction in the winter of 2022-2023 were found upstream of the old dams. This is a small number of individuals, as 2022 was marked by a low abundance of salmon spawners throughout France and severe drought and high water temperatures. Nevertheless, this means that the blockage previously identified by the presence of the dams has been lifted and salmon now have the opportunity to recolonise the upper reaches of the Sélune catchment and reproduce there. The coming years, may they offer favourable climatic conditions for salmon, will enable us to monitor their recolonisation more closely.

The salmon of the Sélune River: a population at the scale of the Mont Saint-Michel bay

Salmon have been absent upstream of dams for more than a century. How can we explain their return?

The salmon found on the Sélune actually belong to a population that is distributed across several rivers in the Bay of Mont Saint-Michel: the Sée, the Couesnon and the tributaries downstream from the Sélune, such as the Oir and the Beuvron. Although salmon are able to recognise their native river and go there to reproduce (to find out more: Bretagne grands migrateurs page, in French), some individuals have a more erratic and exploratory behaviour, enabling them to colonise new habitats. Thus, with the disappearance of the dams, the upper reaches of the Sélune are being naturally recolonised by "exploratory" salmon from the waterways of Mont Saint-Michel Bay.