Résumé
Passive environmental DNA sampling (PEDS) is a new method that is increasingly used to inventory aquatic biodiversity through environmental DNA (eDNA). In contrast to active filtration (AF) to obtain eDNA, which can require batteries and electronics to actively pump water, PEDS involves simply exposing a material to water to passively collect eDNA particles from the environment. As the duration of sampling takes place over several days, this could detect species missed during active filtration. However, its potential to spatially and temporally detect mobile fauna has been little investigated to date. To explore this, this study evaluated its potential to monitor a migratory fish, the first study of this kind. After a preliminary experiment in a fish pass equipped with cameras to validate our workflow, we monitored in the field the upstream migration of an anadromous fish, the twaite shad Alosa agone. The field study was carried out across 25 km of the Aude River in the south of France over a period of seven weeks (February to April 2025). This monitoring period was chosen based on historical direct observations to cover an initial phase without twaite shad in the river, followed by the first adult arrivals, and subsequent confirmed observations in the spawning area. Using a metabarcoding approach with 12S rRNA MiFish-U primers, we successfully detected twaite shad both with PEDS (22%) and AF (10%), finding no significant difference between the methods. At the ecological level, no clear upstream migration pattern could be characterized as no significant effects of environmental covariates (temperature, water flow) were found on twaite shad detection. Nonetheless, these results are encouraging for PEDS monitoring of fish species both spatially and temporally, especially considering that migration of this fish was incredibly low in the study area in 2025 (3 spawning events recorded in 2025 compared to a mean of 314 per year between 2021 and 2024). Moreover, our occupancy model indicates that the sampling design could be further optimized for future monitoring by increasing the number of PEDS replicates (to up to 11). The low cost, ease of deployment and effectiveness of passive samplers (for mobile species, in turbid waters, etc.) make these tools an additional asset for eDNA biodiversity surveys in a variety of environments.