Plecoglossus altivelis
FAMILY
Osmeridae
TAXONOMY
Salmo (Plecoglossus) altivelis Temminck and Schlegel, 1846, Japan.
OTHER COMMON NAMES
English: Ko-ayu, sweetfish; French: Ayu; Japanese: Koayu.
PHYSICAL CHARACTERISTICS
Length about 11.8 in (30 cm). Small, moderately deep bodied; dorsal fin medially placed; adipose fin present. Body covered in small scales. Olive dorsolaterally, white ventrally.
DISTRIBUTION
Coastal seas and rivers in Japan, China, Korea, and Taiwan.
HABITAT
Marine, brackish, and fresh waters. Amphidromous, demersal in coastal seas, estuaries, rivers, streams, and lakes to a depth of about 33 ft (10 m). Landlocked forms also exist. In rivers and lakes appears to prefer clear waters.
BEHAVIOR
Except for feeding and reproduction little is known regarding specific behavior patterns. In rivers, form territories, which they guard by attacking and nipping other ayu.
FEEDING ECOLOGY AND DIET
Larvae and juveniles feed primarily on small benthic crustaceans such as copepods and amphipods. Juveniles of the amphidromous form move into fresh waters in winter and spring, moving up into the higher reaches of rivers and streams. Adults use specially modified jaws and teeth to scrape algae from rocks. Adults have also been reported to eat small pebbles, although it seems likely that these were inadvertently ingested while grazing on algae. The few adults that survive spawning and return to the sea have been reported to feed on zoobenthos.
REPRODUCTIVE BIOLOGY
Sexual maturity usually attained at end of first year at about 7.9–11.8 in (20–30 cm). Spawns in freshwater during autumn, when adults move downstream to the spawning grounds. At night, excavates small pits in sand or gravel banks into which about 10,000 adhesive eggs (0.04 in/0.1 cm) are released. Eggs hatch 14–20 days later. Larger individuals spawn once, after which most die; smaller individuals have about a 50% chance of surviving to spawn again two weeks later.
CONSERVATION STATUS
Not listed by the IUCN.
SIGNIFICANCE TO HUMANS
Ayu form important commercial, aquaculture, recreational, and traditional fisheries. In Japan the commercial fishery and aquaculture accounted for 16,500 tons (15,000 metric tons) and 8,820 tons (8,000 metric tons), respectively, in 1979. Sport fishing in Japan involves baited hooks, flies, as well as a traditional method. In the traditional method, an ayu that has a small free-swinging treble hook fastened to it is attached to the line. Anglers swing the attached fish to ayu guarding territory, and when the resident fish nips the back of the "invader" it is caught on the free-swinging hook. In Japan the ayu forms the basis of the cormorant fishery, whereby cormorants are trained to dive for fish. This fishery is believed to date back at least 2,500 years, but is now for tourists.
