The Plecoglossus altivelis population, the most important target fishery species in rivers, is in decline. Previous studies have shown that environmental DNA (eDNA) can be used to monitor the P. altivelis spawning grounds．In this study, we sampled the eDNA in Shimanto River and Takatsu River to identify differences in the spawning season and spawning time, the relationship between water temperature and spawning season, and changes to the spawning ground during the spawning season. Our results showed that the spawning season is longer in Shimanto River than in Takatsu River because of the water temperature in December. Furthermore, the main spawning grounds change during the spawning season in both rivers, and the spawning times are different.
