Ayu

Taxobox
name = Ayu, Sweetfish


image_width = 260px
status = NE
status_system = iucn3.1
regnum = Animalia
phylum = Chordata
superclassis = Osteichthyes
classis = Actinopterygii
subclassis = Neopterygii
infraclassis = Teleostei
superordo = Protacanthopterygii
ordo = Osmeriformes
subordo = Osmeroidei
superfamilia = Osmeroidea
familia = Plecoglossidae
familia_authority =
genus = "Plecoglossus"
genus_authority = Temminck & Schlegel, 1846Verify source|date=January 2008
species = "P. altivelis"
binomial = "Plecoglossus altivelis"
binomial_authority = (Temminck & Schlegel, 1846)Verify source|date=January 2008

The ayu (アユ, 鮎),(香魚) or sweetfish, "Plecoglossus altivelis", is an amphidromous fish, the only species in the genus "Plecoglossus" and in 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 Hokkaidō in Japan southward to the Korean Peninsula, China, and Taiwan.

The name "sweetfish" is due to the sweetness of its flesh. In reference to its typical one-year life span, it is also known as "nen-gyo" ("year-fish"). The ayu is Gunma Prefecture's prefectural fish.

ystematics

The Ryūkyū ayu ("Plecoglossus altivelis ryukyuensis") is a subspecies of ayu, endemic to the Ryukyu Islands: it is much smaller, with a maximum total length of only about 14 cm (5.5 in). IUCN classifies the Ryūkyū ayu as Endangered on the basis of declining and highly fluctuating population and limited and declining range and habitat, but cautions that the status is in need of reassessment. [WCMC (1996)]

The mitochondrial control region, usually held to be hypervariable and evolving rapidly, is peculiar in the ayu. For reasons unknown, the Ryūkyū population's control region differs from that of others far less than for example genes encoding for proteins. A consequence of this fact is that control region DNA sequences, commonly used in molecular phylogenetics, provide unreliable data for the ayu. In addition, identification of ayu to subspecies cannot be accurately performed using control region sequence data. [Takeshima "et al." (2005)]

Ecology and uses

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". [Gadsby (2004)] 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. This method has been criticized for its cruelty towards the animal.

Japanese anglers also catch it a traditional method, cormorant fishing (鵜飼 "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 "umi-u" (ウミウ, "sea-cormorant"), are domesticated birds trained for this purpose. The birds catch the ayu, store it in their crop, and deliver it to the fishermen. [ [http://www.gifu-rc.jp/ukai/u_main.html Cormorant Fishing "UKAI"] . Version of May, 2001. Retrieved 2008-JAN-30.]

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

Footnotes

References

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* (2004): The Chemistry of Fish. "Discover Magazine" November 2004: 50-57. [http://discovermagazine.com/2004/nov/chemistry-of-fish HTML fulltext]
* (2005): Unexpected Ceiling of Genetic Differentiation in the Control Region of the Mitochondrial DNA between Different Subspecies of the Ayu "Plecoglossus altivelis". "Zool. Sci." 22(4): 401–410. doi|10.2108/zsj.22.401 (HTML abstract)
*|year=1996|id=17595|title=Plecoglossus altivelis" ssp. "ryukyuensis|downloaded=31 January 2008