Summary Environmental DNA (eDNA) analysis for detecting the presence of aquatic and terrestrial organisms is an established method, and the eDNA concentration of a species can reflect its abundance/biomass at a site. However, attempts to estimate the abundance/biomass of aquatic species using eDNA concentrations in large stream and river ecosystems have received little attention. We determined the eDNA concentration and abundance/biomass of a stream fish, Plecoglossus altivelis, by conducting a snorkelling survey in the Saba River, Japan. Furthermore, we evaluated the relationship between eDNA concentrations and the estimated abundance/biomass of P. altivelis, and determined its spatial distribution within the river. Across the three seasons from spring to autumn, we found significant correlations between the eDNA concentration of P. altivelis and its abundance/biomass at study sites within the river. We detected the eDNA at the sites where we found only feeding traces on stones (where P. altivelis was not directly observed), but not at sites without feeding traces. Additionally, we tested the optimal number of qPCR replicates needed for the eDNA evaluation of P. altivelis abundance and biomass; only a small number of replicates was required when the eDNA concentration was high. Our findings suggest that eDNA analysis is a useful tool to estimate fish abundance/biomass as well as their spatial distribution in rivers.

The potential invasiveness of 28 freshwater fishes in northern Kyushu Island, Japan, was evaluated using the Fish Invasiveness Scoring Kit (FISK). The five co-authors scored the level of invasiveness for each species and calculated the total FISK scores; the maximum and minimum scores were then eliminated, and the mean of the remaining three scores was used as the final score for each species. The mean scores ranged from 11.0 (Hypomesus nipponensis) to 31.0 (Cyprinus carpio). The receiver operating characteristic curve indicated that the threshold value between fishes that present a high risk of invasion and the other species were 19.8.

Environmental DNA (eDNA) techniques utilizing DNA fragments from water have recently been developed to investigate the distribution and abundance/biomass of aquatic organisms. The eDNA technique is based on the analysis of DNA fragments in sampled water; thus, an unmanned aerial vehicle (UAV; drone) would be a useful way of collecting water for eDNA sampling, and may consequently allow us to extend eDNA surveys both spatially and temporally. Here, we developed a new method of water collection by using UAV with bleachable equipment, to avoid DNA contamination. To test the performance and contamination risk of UAV water sampling in eDNA surveys, we sampled water from a dam reservoir, detected eDNA from two fish species, and compared the water samples obtained by UAV with those obtained by boat. Additionally, we investigated contamination using blank samples. The results revealed that our UAV water sampling method performed similar to the boat sampling method. No positive signals were detected in the blank samples, including those used for UAV sampling, transportation, filtering, and PCR blanks. Our UAV method can be used to investigate species distributions using eDNA. Combinations of UAV technologies, including remote and thermal sensing, will enable efficient environmental monitoring in various waterbodies.

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