The ayu or sweetfish, Plecoglossus altivelis, is an amphidromous fish, the only species in the genus Plecoglossus and family Plecoglossidae. It is a relative of the smelts, and is placed in the order Osmeriformes. Native to the Palearctic ecozone, it occurs in rivers, lakes, and coastal waters of western Hokkaido in Japan southward to the Korean Peninsula, China, Taiwan, and Hong Kong.

The name “sweetfish” is due to the sweetness of its flesh. In reference to its typical one-year lifespan, it is also written as “year-fish”. The ayu is the prefectural fish of Gunma Prefecture and Gifu Prefecture.

An omnivore, the ayu feeds on algae, crustaceans, insects, sponges, and worms. They are also very territorial animals. The adults ascend from coastal waters into the lower reaches of rivers to spawn in the spring. The larvae descend to the sea immediately on hatching and winter there before returning to fresh water again in the spring. Most but not all individuals die after their first spawning.

Ayu is an edible fish, mostly consumed in East Asia. Its flesh has a distinctive, sweet flavour with “melon and cucumber aromas”. It is consequently highly prized as a food fish. The main methods for obtaining ayu are by means of fly fishing, by using a trap, and by fishing with a decoy which is known as ayu-no-tomozuri in Japan. The decoy is a living ayu placed on a hook, which swims when immersed into water. It provokes the territorial behavior of other ayu, which assault the “intruder” and get caught.

Japanese anglers also catch it using a traditional method, cormorant fishing (__ ukai). On the Nagara River where Japanese cormorants (Phalacrocorax capillatus) are used by the fishermen, the fishing season draws visitors from all over the world. The Japanese cormorants, known in Japanese as “sea-cormorant”, are domesticated birds trained for this purpose. The bird catches the ayu, stores it in its crop, and delivers it to the fishermen.

Ayu is also fished commercially, and captive juveniles are raised in aquaculture before being released into rivers for sport fishing.

A common method of preparing ayu and other small fish in Japan is skewering the fish in such a way as to form a wave, as if they are swimming upriver, and then roasting them over a fire or charcoal.