Rain trout - Rain

A fish of the Salmonidae family, Osteichthyes, genus Salvelinus. It is considered the basic type of Iwana found in mainland Japan, and is found in northern Japan, the Kuril Islands, Sakhalin, Kamchatka Peninsula, the Sea of ​​Okhotsk, and northern Korea. The distribution boundary with Iwana in Honshu is unclear, but it is thought to be found in rivers along the Sanriku coast on the Pacific coast and the coast of Akita Prefecture on the Sea of ​​Japan side. The back is greenish brown with a purple tinge, the ventral side is white or yellowish white, and small white spots are scattered from the back to the side of the body. Young fish have oval-shaped markings called par marks on the side of the body. In Hokkaido, mainly those in their third year of life turn silvery white, the ends of the dorsal and caudal fins turn black, and the circular spots on the sides of the body become noticeably larger. They go down to the sea in May and June and grow along the coast. They go up the river from around August to September, but do not mature, and many go down to the sea again in winter. They mature by repeatedly traveling back and forth between the river and the sea, and spawn in the flat shallows of the upper reaches in September and October. Large ones can reach over 60 centimeters in length. The Iwana-type fish found in the upper reaches are the subject of mountain stream fishing, but the large ones found downstream, which are mainly of the sea-run type, are considered a pest to salmon fry. In Hokkaido, the lake- and river-type fish are sometimes simply called Iwana or Ezo-iwana.

[Tatsuro Kubo]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

硬骨魚綱サケ科イワナ属の魚。日本本土のイワナの基本型とみなされており、北日本、千島列島、樺太(からふと)（サハリン）、カムチャツカ半島、オホーツク海方面から朝鮮北部にわたって分布する。本州のイワナとの分布境界は明確でないが、太平洋側では三陸沿岸、日本海側では秋田県沿岸の河川と推測される。背部は紫色を帯びた緑褐色、腹面は白または黄白色で背面から体側に小白斑(はくはん)が散在する。幼魚には体側に小判形のパーマークとよばれる斑紋がある。北海道ではおもに生後3年目のものが銀白化し、背びれ、尾びれの末部が黒化し、体側の円斑は目だって大きくなり、5～6月降海して沿岸で生育する。8～9月ごろから遡河(そか)するが成熟せず、冬季、ふたたび降海するものが多い。この川と海の往復を反復して成熟し、9～10月上流域の平瀬に産卵する。大形のものは60センチメートルを超す。上流域のイワナ型のものは渓流釣りの対象となるが、下流の降海型を主とする大形のものはサケの稚魚の害魚とされる。なお、北海道では湖沼型や河川型の魚を単にイワナおよびエゾイワナとよぶことがある。

［久保達郎］

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