Environmental DNA reveals the temporal and spatial extent of spawning migrations of European shad in a highly fragmented river basin

Artificial barriers on lowland rivers impede the spawning migrations of anadromous fishes, preventing access to historical areas. In cryptic European shads Alosa alosa and fallax (‘shad’ hereafter), this has resulted in population declines across their range. Conservation programmes aim facilitate passage migrators over these so require baseline information spatial temporal extent current migrations. Here, a shad-specific environmental DNA (eDNA) assay was used quantify shad River Severn basin, western England. This basin is characterized by presence multiple lower catchment. 2017, eDNA piloted Teme, an important tributary, then applied 2018 2019 basin. all years, detected between mid-May mid-June, with maximum distribution being early June when upstream weirs that were generally considered as impassable. 2018, included detection above most weir main required individual fish have passed six weirs. Although inhibit shad, showed some highly vagile individuals might be able ascend migrate considerable distances upstream. suggests efforts increase permeability could result relatively high numbers migrating reaching These results demonstrate also range, further spawning, including fragmented those where are believed spawn only occasionally rarely observed.