2.1 Sampling
Sampling was carried out between 2015 and 2019, mainly in summer months, between May and October, focusing on rare species and under-sampled locations to cover most of the main French watersheds. The water filtration was performed using a Vampire Sampler (Bürkle GmbH), a 0.45 µm VigiDNA filtration capsule (Spygen), and disposable sterile tubing for each sample. The water was filtered for about 30 minutes or until the filter of the capsule was completely saturated. The volume of filtered water varied according to the turbidity and was estimated to be about 30 liters when the turbidity was low. The filter capsule was emptied by injecting air, then filled with 80 mL of CL1 buffer (Spygen), and stored at room temperature until DNA extraction. Two eDNA samples were collected per site.