Ayu (アユ, 鮎, 年魚, 香魚) or sweetfish, Plecoglossus altivelis, is a species of fish. It is the only species in the genus Plecoglossus and family Plecoglossidae. It is a relative of the smelts and other fish in the order Osmeriformes.
Native to East Asia, it is distributed in the northwestern Pacific Ocean along the coast of Hokkaidō in Japan southward to the Korean Peninsula, China, Hong Kong and northern Vietnam. It is amphidromous, moving between coastal marine waters and freshwater lakes and rivers. A few landlocked populations also exist in lakes in Japan such as Biwa. It is an introduced species in Taiwan.
The name “sweetfish” was inspired by the sweetness of its flesh. In reference to its typical one-year lifespan, it is also written as 年魚 (“year-fish”). Some individuals live two to three years. The ayu is the prefectural fish of Gunma Prefecture and Gifu Prefecture.
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 fish 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 fishers also catch it using cormorant fishing. 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 umi-u (ウミウ, “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 to skewer it in such a way so that its body forms a wave, making it look as though it is swimming.