The life of ayu begins with eggs laid in the lower reaches of rivers in autumn. About two weeks later, the eggs hatch, and the larvae drift downstream to the sea. There, they spend the winter in coastal waters rich in plankton, growing to about 10 cm in length by spring.
In spring, ayu return to the rivers, swimming upstream with strength and determination. During the summer, they live in the upper reaches, feeding mainly on algae attached to rocks. Using their comb-like teeth, they scrape the surface of stones to eat. This behavior is also related to their territorial nature and is one of the distinctive characteristics of ayu.
By autumn, the fish grow to around 20 cm in length. Mature ayu move downstream to spawn in gravel beds, completing their life cycle. In this way, both river and marine environments are essential for the life of ayu. In some areas, such as Lake Biwa, there are also “landlocked ayu” that live in lakes instead of migrating to the sea.
Ayu that spend the winter in Osaka Bay migrate upstream in rivers such as the Yodo River in spring. However, structures like weirs and dams can sometimes prevent them from reaching upstream areas. To address this, fisheries cooperatives carry out stocking programs and develop fish passages that allow fish to move more easily, helping to create environments where ayu can migrate upstream.
In addition, research institutions in Osaka Prefecture study when and where ayu are found, and whether rivers and seas provide suitable habitats for them. These efforts contribute to resource conservation and improvements in aquatic environments. As an opportunity to raise awareness of these initiatives, a nationwide event promoting the importance of protecting rivers and seas—the 45th National Convention for Creating an Abundant Sea (“Naniwa no Umi Osaka”)—will be held in Osaka this autumn.
Through these efforts, it is expected that interest in protecting rich marine and river environments where ayu and other organisms can thrive will continue to grow.
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