The ayu (??, ?),(??) 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 Hokkaid? 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.

Systematics

The Ry?ky? 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 Ry?ky? 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.^[1]

The mitochondrial control region, usually held to be hypervariable and evolving rapidly, is peculiar in the ayu. For reasons unknown, the Ry?ky? 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.^[2]

Ecology and uses

Ayu no sio yaki ?????
(Ayu grilled with salt)

Ayu being grilled with salt in Japan

Ayu is an edible fish, mostly consumed in East Asia. Its flesh has a distinctive, sweet flavour with "melon and cucumber aromas".^[3] 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 (?? 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 (???, "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.^[4]

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

• (2004): The Chemistry of Fish. Discover Magazine November 2004: 50-57. 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. (HTML abstract)

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