What are the risks of invasion by exotic species?

The signal crayfish

The signal crayfish (Pacifastacus leniusculus) has been present in the Sélune catchment for several years. In total, 6 species of crayfish are present in the Sélune valley, including the native white-clawed crayfish. This article deals only with the signal crayfish, whose presence in the catchment is in direct competition with that of the white-clawed crayfish.

The distribution of the signal crayfish is monitored every year throughout the catchment to determine its dispersal. Monitoring is carried out using creel trapping and environmental DNA analysis on water samples. Until now, the spread of this exotic crayfish, which originated in North America and carries the "crayfish plague", has been hampered by the presence of dams.

With the removal of the dams, the signal crayfish colonisation front on the main course of the Sélune has moved downstream. In 2022, this front was located at the former Vezins dam. Signal crayfish have also returned to several tributaries. In autumn 2023, no crayfish were caught in the Roche Qui Boit reservoir, downstream of the Vezins station, but there is a high risk that they will spread to the edge of the estuary and throughout the catchment area.

However, this dispersal could be slowed by the return of the European eel. The eel, which is expected to return to the upper reaches of the catchment following the removal of dams, is a natural predator of crayfish. Its presence should regulate crayfish populations. Monitoring by the Sélune observatory will enable this hypothesis to be verified over the coming years.

Catfish

Little information is available on the current distribution and abundance of catfish (Silurus glanis) in the Sélune. However, the evidence collected in recent years all points in the same direction.

• Catfish, which represented almost 50% of the biomass of the Vezins reservoir emptied in 2018 (i.e. 6.5 tonnes) and of the La-Roche-Qui-Boit reservoir emptied in 2022 (i.e. 2.2 tonnes), are widely observed in the lower reaches of the Sélune.
• Reported as early as 2014 in the hydroacoustic logs recorded by our camera at Ducey, the presence of catfish on the site seems to have increased since 2019, with individuals of a size never before seen on the site, up to 150 cm.
• Since 2019, numerous adult catfish measuring between 80 and 150 cm have been caught by fishermen in the lower reaches of the Sélune.
• Nearly thirty young catfish, most of them juveniles of the year (or 0+), have been caught during scientific fishing.

The catfish is a major predator of aquatic animals. This species is not classed as invasive or harmful. However, its presence in the Sélune is a potentially major hindrance to the return of migratory fish and the stability of the ecosystem.

Invasive plant species

The presence of exotic plant species is low in the Sélune compared with other rivers: only 29 species identified in 2019, i.e. 8% of the total number of species present. What's more, their spread is declining in several sectors, controlled by the native vegetation in place, which is taking over. The risk of invasion by exotic plant species is therefore low for the time being. On the other hand, successive interventions on the plant communities being restored (clearing, mowing) may rekindle their establishment and spread, to the detriment of the native species.