Estimation of tributary–mainstem migration of freshwater fishes by Sr isotope analysis of otoliths

The study of fish movement patterns in freshwater ecosystems remains a major focus of contemporary research. Small freshwater fish are known to migrate between tributaries and mainstems at different stages of their life history, helping to avoid local extinctions and maintain genetic diversity. However, fitting small fish species with telemetry equipment is difficult, making it challenging to track their detailed migration history via conventional methods. We evaluated the suitability of otolith isotope analysis for verifying the migration histories of small fish species inhabiting small tributaries. In this study, we compared the ⁸⁷Sr/⁸⁶Sr values in stream water and the otoliths of two fish species (Nipponocypris temminckii and Rhinogobius flumineus) inhabiting small streams. The ⁸⁷Sr/⁸⁶Sr ratios in the stream waters within the study area displayed considerable variation among tributaries. Our findings indicate that only N. temminckii has high dispersal ability, showing movement between tributaries or between tributaries and the mainstem in some locations. In contrast, such movement was not generally observed for R. flumineus. These results show the potential of ⁸⁷Sr/⁸⁶Sr as a tool for estimating the migration patterns of small fishes that inhabit small tributaries.