Overwintering Fish Community in Ice-Covered Environments in Hokkaido, Japan, Inferred From Environmental DNA

The overwintering ecology of fish in the seasonal ice zones (SIZs) remains largely unexplored owing to methodological limitations. Environmental DNA (eDNA) can reveal the distribution and diversity of fish species in various aquatic environments, thereby offering a possible solution to the methodological limitations of SIZ studies. Therefore, we aimed to detect the overwintering fish community in the ice-covered Saroma-ko Lagoon, located on the Okhotsk Sea coast of Hokkaido, and its inflow, using eDNA metabarcoding. eDNA extracted from under-ice seawater collected from the lagoon yielded 28 fish taxa, predominantly Clupea pallasii based on the relative DNA read abundance. Dissimilarity analysis suggested short-term temporal variations in eDNA composition in under-ice seawater, even at the same site. Nineteen fish taxa, predominantly Tribolodon brandtii and T. hakonensis, were detected in the eDNA extracted from under-ice river water. The high dissimilarity between eDNA results from under-ice seawater and river water suggested segregation of the overwintering community between the lagoon and river. Fish eDNA detected in meltwater from sea ice was assigned to five taxa, suggesting the entrainment of particulate matter containing fish eDNA during ice growth. The true species richness estimated based on eDNA results and discrepancies with historical reports suggest that sampling efforts need to be optimized for ice-covered environments to promote more comprehensive species detection. This study demonstrated the usefulness of using fish eDNA metabarcoding to study the ecology of overwintering fish under ice. Clarifying eDNA shedding patterns, persistence, and dispersal in under-ice environments would improve the reliability of this technique and expand its use in SIZs.