Abstract
Abstract 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. 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. 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. 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.
Highlights
The artificial modification of lowland rivers has resulted in profound impacts on biodiversity, with dams and weirs that regulate river flows interrupting longitudinal connectivity
The samples collected in 2018 and 2019 confirmed the ability of the environmental DNA (eDNA) assay to detect the presence of migrating shad in the rivers, as reported for the shad spawning period in 2017 (Antognazza et al, 2019)
In 2017, the eDNA assay was successfully piloted in the River Teme, an important shad spawning tributary, and showed that shad were able to pass the weirs on the lower river, with shad eggs and their DNA detected in upstream areas (Antognazza et al, 2019)
Summary
The artificial modification of lowland rivers has resulted in profound impacts on biodiversity, with dams and weirs that regulate river flows interrupting longitudinal connectivity This inhibits the upstream migrations of anadromous fish, affecting the sustainability of their populations (Dudgeon et al, 2006; Rolls et al, 2014). Mapping the extent of the spawning migrations of anadromous fishes traditionally relies on methods such as visual or telemetry observations of migration or evidence of their spawning, such as the visual identification of redds or sampling of eggs (Antognazza et al, 2019) These methods can, require considerable effort and might not be feasible under certain river conditions, such as during high flows (Radinger et al, 2019). The development of environmental DNA (eDNA) detection techniques has provided methods that can be rapidly deployed and provide high spatial resolution of spawning distributions, including in unfavourable conditions (e.g. Deiner et al, 2016; Klymus, Marshall & Stepien, 2017; Maruyama et al, 2018; Tillotson et al, 2018; Wilcox et al, 2018; Itakura et al, 2019)
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