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

Abstract

1. Artificial 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 information on the 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 characterized by the presence of multiple 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 barriers inhibit the spawning migrations of shad, this eDNA assay showed that 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 also be used across their range, to further quantify the spatial extent of their spawning, including in highly fragmented rivers and those where shad are believed to spawn only occasionally and are rarely observed.

Fishery resource monitoring of the East China Sea via environmental DNA approach: a case study using black sea bream (Acanthopagrus schlegelii)

“…In the present context, the data obtained from quantitative eDNA monitoring methods remain questionable due to several complex environmental factors (Hinlo et al, 2018;Tillotson et al, 2018;Antognazza et al, 2021). Thus, despite the relatively high eDNA detection rates in this study, information on the spatial resolution of these detections (distribution and biomass) is inconclusive.…”

Using environmental DNA method to clarify the distribution of Pampus echinogaster in the East China Sea

“…These features confer eDNA with more potential for development in large lotic water scenarios. Continuous improvement in current eDNA monitoring methods is needed (Goldberg et al, 2013;Jane et al, 2015;Hansen et al, 2018;Hinlo et al, 2018;Tillotson et al, 2018;Shogren et al, 2019;Duda et al, 2020;Spence et al, 2020;Antognazza et al, 2021). Nevertheless, given the disadvantages of traditional survey methods, eDNA remains a new and improved survey tool that can be used for large-scale surveys at an early stage of research.…”

Hybridization and genetic population structure of Alosa population in the United Kingdom

“…Given the persistence of both A. alosa and A. fallax in British waters, including A. alosa genes in areas where the pure species is currently absent, increasing river connectivity through modifying these engineered structures should potentially enable the re-establishment of the spatial segregation of the species' spawning areas. For example, on the River Severn, a major restoration project (Unlocking the Severn) is underway to reopen the entire river to migrating shad through weir modifications, including fish pass installations (Antognazza et al, 2019(Antognazza et al, , 2021. Despite the high proportion of Alosa spp.…”