DATE2025.05.30 #Press Releases

Decoding the Life History of Ayu Gathered at Spawning Grounds

~ The birth timing of Ayu controls their subsequent destiny! Nagara River Research ~

Summary

Ayu spawners (known as ochi-ayu) are crucial not only for fishing and recreational angling but also for sustaining wild Ayu populations in the following year. Each autumn, during the spawning season, these parent fish instinctively migrate downstream from various locations across the wide river basin to reach spawning grounds. While this behavior has long been known, the exact areas within the river system where these fish grew and from where they originated had remained a mystery—until now.

In a collaborative study, Shigeya Nagayama, Project Assistant Professor at the Center for Co-Creation of Symbiosis Society and Environment, Gifu University (appointed as Associate Professor at the Faculty of Co-Creation Informatics, Nagano University as of April 2025), and Morihiro Harada, Director of the Center, worked together with Tamihisa Ohta, Lecturer at the Faculty of Science, University of Toyama, Ryoji Fujii from the Gifu Prefectural Fisheries Research Institute, and Tsuyoshi Iizuka, Associate Professor at the School of Science, The University of Tokyo. With support from Nagara River fishermen, they analyzed the otoliths (ear stones) of Ayu and successfully reconstructed the long-unresolved habitat use history of spawners within the river basin. Moreover, they discovered that the pattern of habitat use is related to the timing of hatching.

This research was published in Scientific Reports on May 28, 2025 (local time).

Figure: Isoscape (Left: ⁸⁷Sr/⁸⁶Sr map) and analysis results of Sr isotope ratios in otoliths (Right).
A schematic diagram in the lower right explains how to interpret the graph. By matching the Sr isotope ratios in the otoliths with the isoscape, the utilized habitat locations can be estimated.

Related links

Gifu University, Toyama University, Nagano University

Published Journals

Journal name	Scientific Reports
Title of paper	Habitat use and growth strategies of amphidromous fish "ayu" throughout a river system