The signal crayfish (Pacifastacus leniusculus) was introduced several decades ago and has since become established upstream in the Selune catchment. The spread of this invasive crayfish, which carries the "crayfish plague", was hampered by the presence of dams. With the removal of dams and the disappearance of reservoirs, new potential habitats are emerging. The removal of the dams represents a risk of propagation of this invasive exotic species, which can disrupt the ecosystem of the Sélune.

The progress and abundance of the signal crayfish are monitored every year throughout the basin. To do this, we carry out two types of monitoring:

• Creel trapping at several stations to characterise the crayfish population.
• Analysis of environmental DNA present in the water. This non-intrusive monitoring tool can be used to detect the presence of species in an environment by identifying DNA fragments released on a simple water sample. In this case, it is used to target the presence of signal crayfish and aphanomycosis (crayfish plague) in the watercourse.