Environmental DNA Characterization of the Fish Species Composition in the Mukawa River and Adjacent Habitats

The diverse freshwater fish fauna of the Japanese archipelago is distributed among four main island landmasses, which include Hokkaido in the north, with many diadromous species. One relatively well-preserved river drainage along the southern coast of Central Hokkaido is the Mukawa River. Fish fauna surveys in the Mukawa River were mostly in downstream areas and the fish diversity is not well-documented among the upper, lower river, and coastal environments. Fish communities in the river, estuary, and sea were sampled using eDNA analysis to evaluate upstream and downstream species detections, and tidal and spatial detection variation near the river mouth. The number of species was higher at the river mouth and nearshore sites compared to the river and offshore. Fish detections reflected life history categories (freshwater resident, diadromous, brackish, or marine) and the environments. Similarity analysis showed that fish species compositions were divided into (1) upstream and midstream, (2) downstream and river mouth, (3) adjacent shore, and (4) offshore. Salmonid, cyprinid, loach, and sculpin species were detected in the river, compared to a mixture of species downstream and along the coast. This rapid assessment type study demonstrated that eDNA survey methodology would be effective for multiple river comparative surveys, seasonality studies, or evaluating possible effects of cross-river weirs or dams.