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

Abstract

1. Anthropogenic barriers on lowland rivers impede the spawning migrations of anadromous fishes, preventing access to historical spawning areas. In the cryptic European shads Alosa alosa and Alosa fallax (‘shad’ hereafter), this has resulted in population declines across their range. Conservation programmes aim to facilitate the passage of migrators over these barriers and so require baseline knowledge on spatial and temporal extent of current migrations. 2. Here, a shad-specific environmental DNA (eDNA) assay was used to quantify the spatial extent of shad spawning migrations in the River Severn basin, Western England. This basin is characterised by the presence of multiple anthropogenic barriers in the lower catchment. In 2017, the eDNA assay was piloted in the River Teme, an important shad spawning tributary, and then applied in 2018 and 2019 across the lower Severn basin. 3. In all years, shad DNA was detected between mid-May and mid-June, with the maximum spatial extent of shad distribution being in early June when shad eDNA was detected upstream of weirs that were generally considered as impassable. In 2018, this included the detection of shad above the most upstream weir on the main River Severn that required individual fish to have passed six weirs. 4. Although anthropogenic barriers inhibit the spawning migrations of shad, this eDNA assay revealed some highly vagile individuals might be able to ascend these barriers and migrate considerable distances upstream. This suggests that efforts to increase the permeability of these barriers could result in relatively high numbers of migrating shad reaching upstream spawning areas. These results demonstrate that this eDNA assay could be also utilised across their range, outside the study system, to further quantify the spatial extent of their spawning, including in highly fragmented rivers and those where shad are believed to only spawn occasionally and are rarely observed.