Latin name

Plecoglossus altivelis

Other names

Ayu, sweetfish.


Vertebrae: 60-63. Blind gutters 350-400. Has small leaf-shaped teeth loosely attached to the jaw by two ligaments. The body is slender and elongated. The head is large with a relatively large mouth. Scales are small. 

Features of fish fins

Dorsal rays: 0. Dorsal soft rays: 10-11. Anal soft rays: 14-15. Dorsal fin above pelvic fins.

Fish colouring

The back is dark green with a white belly. There is an orange patch behind the gill cover. All fins are bright yellow, the upper jaw is white, the lower jaw is green and the upper edge of the eyes is orange.


The range is restricted to the northwestern Pacific coast from Hokkaido to Vietnam. A few inland populations also inhabit lakes in Japan, such as Biwa.


Marine, freshwater, brackish, bottom dwelling, amphidromous, subtropical species. Depth range from 10 m. Movement between coastal marine waters and freshwater lakes.


Body length 30 cm, maximum length 70 cm. Maximum age: 3 years.


Inhabits the sea, spawns in rivers. They live in schools.

Food and feeding habits

It feeds mainly on algae, but also on plankton, invertebrate larvae, crustaceans and worms. 


During spawning, some fish migrate upstream and others spawn on the bank above the gravel. It spawns at night and digs a hole 10 cm deep. The eggs are about 1mm in size and remain on the sand or pebbles for 14-20 days before hatching. This fish is semi-fertile and will lay many eggs in a short period of time, releasing about 10,000 eggs each time. Sometimes females that want to spawn but can't will conserve their energy and move to deep standing water. These females wait until spring before travelling upstream with their young. After hatching, the larvae grow to 6mm and swim upstream to feed on plankton. The juveniles return to the river in the spring. When schools of Plecoglossus altivelis return to their river, they defend their territory and feed. Large females spawn once, while smaller females spawn twice at two-week intervals.


It has cult status in Japan and is one of the elements of traditional Japanese fishing, dating back to the Middle Ages. 

Relationship with a person

In East Asia, fish flesh is prized for its flavour. Fish are farmed artificially.

Phylum Chordata
Class Actinopterygii
Squad Osmeriformes
Family Plecoglossidae
Genus Plecoglossus
Species P. altivelis
Conservation status Data Deficient
Habitat Bottom
Life span, years 3
Maximum body weight, kg No information
Maximum length, cm 70
Sailing speed, m/s No information
Threat to people Edible
Way of eating Planktonophage

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Ayu sweetfish

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