Identifying migration hotspots of the potamodromous fish Opsariichthys uncirostris in Lake Biwa tributaries using environmental DNA and visual counts during its reproductive season

Abstract Migration is vital for the continuation of a species. In this study, we explored migration hotspots of the vulnerable Hasu fish in Lake Biwa tributaries using two complementary methods: environmental DNA (eDNA) and visual counts. The study encompassed the known range of Hasu around Lake Biwa tributaries during its reproductive season. Monthly water sampling and visual inspection was conducted, from May to September, in 32 Class A tributaries—at the river mouth and within the river channel. Hasu eDNA was extracted from water samples and quantified using real‐time polymerase chain reaction. Environmental factors were also assessed on‐site, and their effects on eDNA and visual count trends evaluated using linear models and Akaike information criterion. eDNA was detected at sites where the fish were both observed and not observed. A zero‐hurdle model revealed positive correlation between eDNA copies and visual counts of migrating Hasu, with pH having a reducing effect on the relationship ( p < 0.05). Analysis of Hasu eDNA copies and visual count trends, with environmental factors as explanatory variables, indicates that Hasu is likely to be found in rivers that are wide and deep enough to accommodate migrating individuals, have fast‐flowing currents, and sandy‐gravel substrates during reproductive migration. Such rivers are mostly located on the western side of the northern basin and include the Ado, Chinai, and Shiotsuo Rivers. These could be considered as Hasu migration hotspots and require protecting if the population of Hasu in Lake Biwa is to be recovered.