Salmon equipped to track their movements in the Sélune valley

Salmon are amphihaline migratory fish that are born in rivers, reach 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 from spring to summer. They 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), the salmon rest in the deep, cool areas of the rivers until autumn, when they start to swim upstream again. In our latitudes, the salmon breeding season takes place between late November and early January.

Equipping salmon?

Salmon spawners - those old enough to reproduce - are caught when they return to the river in summer. The idea is to fit these salmon with radio transmitters to track their movements and find out in which rivers they will reproduce, using a telemetry monitoring protocol. The equipped salmon are tracked using mobile or fixed radio receivers distributed along the river.

The salmon are caught in the Sélune estuary, at 'Gué de l'épine', at night. A barrage net is set up across the river to direct the salmon towards a carrelet (a square-shaped fishing net). The scientific teams wait and follow the movement of the fish using an acoustic camera placed in front of the net. As soon as a salmon is spotted by the camera on the net, it is lifted off. The captured salmon is then anaesthetised and gently handled by the scientists to measure it, weigh it and take samples (DNA, scales). The salmon is then fitted with a radio transmitter that records the fish's acceleration, pressure (used to estimate depth) and temperature. The transmitter is inserted directly into the fish's stomach, because a salmon that returns to the river to reproduce no longer feeds. The presence of the transmitter in its stomach is therefore of little or no inconvenience to the fish.

After equipping the salmon, the scientific team waits for the effect of the anaesthetic to wear off before releasing the salmon over the trap and allowing it to continue its ascent. Every year since 2019, around thirty spawners have been caught and fitted with transmitters. In 2022, a particularly hot and dry year, there were just 18. This year, 2023, 27 salmon have been fitted with transmitters and are currently being monitored in the Sélune.

After spawning, or sometimes as early as the summer or autumn, when fish die prematurely, meticulous work is carried out to recover the transmitters. Some transmitters are recovered from the water, others from the carcasses of fish that have died after spawning, or thanks to the cooperation of fishermen who have caught an equipped salmon.

Where do the salmon go on the Selune?

After returning to the estuary of the Sélune, the salmon spend the summer at Ducey and in the lower reaches of the catchment. Here, they wait for increases in flow in autumn to begin the final migration to the spawning grounds further upstream, and must conserve their strength. They then head up the tributaries, or turn around and head up another river in the Mont Saint-Michel bay, in search of partners and spawning grounds to reproduce. Until now, these movements have been limited on the Sélune. The salmon were confined to the downstream part of the catchment area because the dams were impassable. They mainly spawned on the Oir and Beuvron rivers, or even in neighbouring rivers in the bay.

Since 2022 and the start of work to dismantle the Roche-Qui-Boit, some salmon have begun to explore the upstream part of the Sélune catchment. Among the 18 salmon equipped in 2022, two spawners have been 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 show intense activity at the end of December, which could correspond to breeding activity.

The Observatory's monitoring of potential spawning grounds, which will be carried out in September and October 2023, will tell us whether or not any salmon spawned upstream of the dams in the winter of 2022-2023.

The team coordinated by Émilien Lasne is currently monitoring the 27 salmon spawners equipped this year as they return to the river. This telemetric monitoring, coupled with other monitoring tools, will enable us to understand how, how fast and where salmon move and reproduce in the Sélune valley.