The ayu sweetfish (Plecoglossus altivelis), also known simply as ayu or sweetfish, is a species of bony fish in the family Plecoglossidae. It is the only species in the genus Plecoglossus and family Plecoglossidae, and is a relative of the smelts in the order Osmeriformes.
Taxonomy and Distribution
Native to East Asia, the ayu sweetfish 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. Landlocked populations also exist in lakes in Japan, such as Lake Biwa. Original wild populations in Taiwan became extinct in 1968 due to pollution, and present populations were reintroduced from Japan in the 1990s.
Subspecies
Two to three subspecies are recognized by some authors, while others do not distinguish subtaxa:
- P. a. altivelis (Temminck & Schlegel, 1846) — ayu, sweetfish
- P. a. chinensis Y. F. Wu & X. J. Shan, 2005 — Chinese ayu
- P. a. ryukyuensis M. Nishida, 1988 — Ryukyu ayu-fish (endangered)
Biology
The ayu is an omnivore, feeding on algae, crustaceans, insects, sponges, and worms. It scrapes algae off rocks using its saw-shaped teeth. Adults typically maintain a feeding territory, though the lake-restricted form is schooling.
Most populations are amphidromous: they breed in the lower part of rivers during autumn, laying eggs in small pits dug in gravel. Larvae are carried downstream to the sea, overwinter in coastal regions, and migrate back upstream in spring at about 6 cm in length, reaching 15–30 cm by summer. Some die after breeding at one year, while others may spawn up to three times. Freshwater-restricted populations typically live two to three years.
Maximum size is reported up to 70 cm, but a more typical maximum is about 30 cm. The freshwater form is considerably smaller than the amphidromous form.
Etymology and Common Names
The name "sweetfish" refers to the sweetness of its flesh. In reference to its typical one-year lifespan, it is written as 年魚 ("year-fish") in Japanese. It is also known as 香魚 (kōgyo, "fragrant fish"). The ayu is the prefectural fish of Gunma Prefecture and Gifu Prefecture.
Human Uses
The ayu is highly prized for its flavor in East Asian cuisine. Its flesh is described as distinctively sweet, with melon and cucumber aromas. Fishing methods include fly fishing, fish traps, and a decoy technique called ayu-no-tomozuri, which exploits the fish's territorial behavior. Ayu is also caught by traditional cormorant fishing (ukai) on the Nagara River, using Japanese cormorants (Phalacrocorax capillatus).
In Japanese cuisine, a common preparation is shioyaki — the fish is skewered in a wave-like "swimming" shape and grilled with salt over charcoal. Young ayu (waka-ayu) are often prepared as tempura. Ayu is also fished commercially, and captive juveniles are raised in aquaculture before being released into rivers for sport fishing.
Genetics and Sex Determination
The ayu genome is approximately 420–449 Mb in size. Research published in 2021 identified a Y-linked duplicate copy of the anti-Müllerian hormone type-II receptor gene (amhr2bY) as the sex-determining gene in ayu, which has an XX/XY sex-determination system. This gene is expressed in male-specific supporting cells surrounding germ cells in undifferentiated gonads.
Conservation Status
The IUCN Red List assesses Plecoglossus altivelis as Data Deficient (ver. 3.1, assessed 2010). Major threats include overfishing, river modifications such as dam construction, and soil erosion from deforestation. The subspecies P. a. ryukyuensis is listed as endangered.
