Gifu University conducts study in Nagara River and successfully decodes life history of ayu fish gathered at spawning grounds

Ayu is an important fishery species representative of Japan and has been actively studied since aquaculture and fisheries research began in the Meiji era (1868-1912). However, ecological studies in actual rivers have been limited, making it a fish that remains mysterious despite being familiar. A well-known characteristic of ayu is that in the autumn, adult ayu (known as "ochi-ayu") travel down rivers and gather at downstream spawning grounds to spawn, but it was not understood where in the river basin they came from or in what numbers. Additionally, while it was known that the timing of hatching (autumn) affects the timing of river ascent (spring) and spawning (autumn), and that growth rates also differ, the relationship with habitat use-where in the river basin they grow-remained unclear.

Project Assistant Professor Shigeya Nagayama (at the time of the research, currently Associate Professor at the Faculty of Collaborative Information Science Establishment Preparatory Office, Nagano University) and Director Morihiro Harada at the Center for Environmental and Societal Sustainability, Gifu University conducted joint research with Lecturer Tamihisa Ohta in the Faculty of Science, Academic Assembly, University of Toyama, Mr. Ryouji Fujii of the Gifu Prefectural Research Institute for Fisheries and Aquatic Environments, and Associate Professor Tsuyoshi Iizuka at the Graduate School of Science, the University of Tokyo. By analyzing ayu otoliths obtained with the cooperation of Nagara River fishermen, they elucidated the long-standing mystery of habitat use history within the river basin by spawning adult ayu and discovered that these utilization patterns are related to the timing of hatching. Their study was published in Scientific Reports.

The research group captured 133 spawning adult ayu with the cooperation of fishermen at Kagashima in the lower reaches of the main Nagara River (47.4 kilometers from the sea) between September and December 2022. They extracted and sectioned otoliths, and measured strontium isotope ratios from the otolith core to the edge.

They cross-referenced these with the Nagara River system strontium isotope ratio map (isoscape) obtained from river water analysis. They also counted otolith rings (daily age determination) and estimated hatching dates and river ascent initiation dates. As a result, approximately 90% of the adult ayu were found to be of natural-ascending origin, with only about 10% of stock origin. Naturally ascending ayu contributed significantly to the spawning population. However, biased fishing pressure on stocked fish through tomozuri (the traditional ayu fishing method) may be one contributing factor. Naturally ascending adult ayu showed five habitat use patterns. Of the 116 naturally ascending individuals, approximately 84% grew in the upper and middle reaches of the main river, while the remainder utilized tributaries in the middle reaches. The upper and middle reaches of the main Nagara River were found to be important areas producing many spawning adults.

Ayu that hatched early in the previous autumn and ascended the river early in spring tended to be main river users, while those that hatched late and ascended late tended to be tributary users. In the Nagara River, early-hatched ayu preferentially used feeding grounds in the main river, while late-hatched ayu arriving later avoided the already-occupied main river and sought feeding grounds in tributaries.

In the Nagara River, it became clear that ayu growing in the upper and middle reaches of the main river, including the uppermost reaches (over 110 kilometers) farthest from the sea and spawning grounds, serve as important parent fish producing the next generation, highlighting the increased importance of a "main river accessible for upstream migration."

Additionally, it was revealed that late-hatched, late-ascending small ayu also seek growth habitats in tributaries, demonstrating the importance of river system networks that allow ascending to tributaries and the diversity of growth habitats provided by the presence of tributaries.

From these findings, it is clear that there is a strong need for river environment management that preserves river continuity and networks allowing migratory fish to freely ascend and descend, as well as maintaining good habitats for growth and survival. Additionally, optimal management of ochi-ayu fishing is necessary to avoid depleting the main river user type that comprises the majority of the spawning population.

The research group plans to expand their work in the future to elucidate the spatiotemporal dynamics and population maintenance mechanisms of ayu in the Ise Bay basin, including identifying the natal rivers (rivers where they hatched) of naturally ascending ayu entering from the sea to the three major rivers of the Kiso River system. They will also examine climate change impacts on ayu reproduction and recruitment by focusing on survival and mortality during the larval and juvenile stages from autumn hatching through winter marine life to spring river ascent, and the relationships with hatching timing and river/sea water temperatures.

Journal Information
Publication: Scientific Reports
Title: Habitat use and growth strategies of amphidromous fish "ayu" throughout a river system
DOI: 10.1038/s41598-025-02988-8