Ayu (鮎), also known as sweetfish, is a freshwater fish widely found in Japanese rivers. It is named for its distinctive sweet aroma and delicious taste. Ayu can grow up to about 20-30 centimeters in length and usually has a silver-white body with yellow vertical stripes on its sides. The name "Ayu" is believed to originate from its beautiful appearance and its characteristic of completing its life cycle within one year, hence the term "year fish."

Ayu migrate upstream from spring to summer and grow in clear streams. In Japanese cuisine, Ayu caught in this season can be enjoyed in various ways. The most common method is salt grilling, which enhances the natural flavor of Ayu. Other popular dishes include Ayu simmered in soy sauce, tempura, and sushi. Particularly as a summer tradition, many people enjoy "Ayu no Tomozuri," where Ayu is salt-grilled and enjoyed by the riverside.

Moreover, Ayu is not only valued for its taste but also as an indicator of the river's health. Rivers with abundant Ayu populations typically have good natural environments, as Ayu thrive in clean water. Thus, Ayu is a symbol of both Japanese culinary culture and environmental conservation.