Environmental DNA as a non-invasive sampling tool to detect the spawning distribution of European anadromous shads (Alosa spp.).

Populations of the European shads Alosa alosa (Linnaeus, 1758) and Alosa fallax Lacépède, 1800 (Alosa spp.) are protected under legislation because of their vulnerability to human disturbances. In particular, river impoundments block their upstream migration, preventing access to spawning areas. Knowledge on the spatial extent of their spawning is important for informing conservation and river management plans. Determining the spatial extent of Alosa spp. spawning is challenging. They enter rivers over a 2-3-month period and the species potentially migrate different distances upstream. Capture and handling can be problematic, spawning events generally occur at night, and kick-sampling for eggs is limited to shallow water. Assessing their spatial extent of spawning could, however, incorporate non-invasive sampling tools, such as environmental DNA (eDNA). An eDNA assay for Alosa spp. was successfully developed, based on the cytochrome c oxidase subunit I gene segment and quantitative polymerase chain reaction (qPCR). Application in spring 2017 to the River Teme (River Severn catchment, western England) revealed high sensitivity in both laboratory and field trials. Field data indicated Alosa spp. spawning between May and June, with migrants mainly restricted to areas downstream of the final impoundment. eDNA can thus be used as a non-invasive sampling tool to determine the freshwater distribution of these fishes in Europe, enhancing their conservation at local and regional scales.