Monitoring reproductive migration is essential for the conservation of anadromous species. Shishamo smelt (Spirinchus lanceolatus) is endemic to Hokkaido, the northernmost large island in Japan. S. lanceolatus is an anadromous species that is known to migrate into rivers for a very short period in early winter. While this species has a special value for local fisheries, the catch amount has drastically declined in the last few decades. Information about S. lanceolatus reproductive migration dynamics is limited, which prevents them from being efficiently managed as a resource. In this study, we used environmental DNA (eDNA) methods as a noninvasive molecular tool for estimating presence/absence and abundance/biomass of S. lanceolatus during their migration into rivers. We developed a species‐specific qPCR system for S. lanceolatus, examining (a) temporal variation in S. lanceolatus eDNA concentrations compared with catch data gathered by traditional methods and (b) variability of migratory patterns among river systems. In a core river for their spawning migration, we consistently detected S. lanceolatus eDNA throughout the spawning season, and the temporal distribution of eDNA concentration was consistent with that of the number of migrating S. lanceolatus estimated by catch survey data. In addition, we were able to detect S. lanceolatus eDNA even from rivers without any official record of their migration. Among rivers with eDNA detection, the relative eDNA concentrations varied, indicating that the population biomass differs largely among the river populations. Our study suggests that eDNA detection systems are useful for tracking reproductive migration of S. lanceolatus at fine spatio‐temporal scales.

Migratory organisms have their own life histories that efficiently link multiple ecosystems. Therefore, comprehensive understanding of migration ecologies of these organisms is essential for both species conservation and ecosystem management. However, monitoring migration at fine spatiotemporal scales, especially in open marine systems, often requires huge costs and effort. Recently, environmental DNA (eDNA) techniques that utilize DNA released from living organisms into their environment became available for monitoring wild animals without direct handling. In this study, we conducted an eDNA survey for understanding marine migration of an endemic fish species, Shishamo smelt (Spirinchus lanceolatus). We examined 1) seasonal habitat changes in coastal regions and 2) environmental factors potentially driving the migration of this species. The eDNA concentrations along a 100 km-long coastline exhibited spatiotemporal variation, suggesting that this species shifts their habitat away from nearshore areas between spring and summer. We also found a significantly negative association between the eDNA concentration and sea surface temperature. That finding suggests that the offshore migration of this species is associated with increased sea surface temperature. This study reveals new aspects of S. lanceolatus life history in coastal regions. Together with our previous eDNA study on the freshwater migration of S. lanceolatus, this study illustrates the potential of eDNA techniques for understanding the whole life history of this migratory species.