Gazing into a slightly turbid stream, you find a huge shadow of a fish slowly maneuvering its way through. The fish calmly but lordly glares around while retaining strength in its lean body. Known as sea-run taimen, or “Itou” in Japanese, its appearance and behavior renders the fish the deserving title, the “King of Marshes.”

Sea-run taimen are the largest fresh water fish belonging to the genus Hucho of the family Salmonidae. The adults exceed 1 meter long with the weight reaching up to 45 kg. The largest record of sea-run taimen is 2.1 meters in length that was caught from a lower reach of the Tokachi River in 1937. With their huge bodies, taimen can even eat mice and snakes that fall into the river as well as other fish. Their ferocity and tough-looking features have earned Itou a Chinese character that consists of a combination of the radicals of fish and devil. This character obviously fits the image of the “Legendary Giant Fish.”

Sea-run taimen are considered relative to the fish in the genus Salvelinus. Compared with other salomonids, they look thinner because the body depth is relatively low in comparison with the body length. They are also distinguishable from other salmons in that they have a large mouth and different body marks, and the top side of their head is rather flat. Another notable characteristic is that they are able to live for as long as 15 to 20 years or more. Born in the upper reaches of a river, taimen gradually swim down to the lower reaches as they grow.

It is said that there are 5 species in the genus Hucho living in the Eurasian Continent, etc. Unlike the other 4 species that spend their entire life in a river without descending to the sea, only sea-run taimen live in the sea for a certain period of time as other salmonids do. Previous reports indicate that at least the taimen living in northern Hokkaido move their places of living to brackish water and coastal areas when they are around 3 years old, then they run up the river to spawn only during a short period in spring, and move back down the river after spawning. However, there are still many mysteries remaining about their life history such as: how many times during their lifespan they swim down to the sea, how many years or how many months they spend in the sea, and whether or not all the sea-run taimen migrate downstream to the sea. Meanwhile, there are some populations in dam lakes that are identified to have repeatedly reproduced without traveling to the sea.

Sea-run taimen are the only Japanese salmonid that spawn in spring. They start to run upstream altogether during an April-May period, when the snowmelt-swollen river starts to recede, and they prepare their spawning beds to lay eggs in the upper reaches of the river. Their spawning period is short: about 2 weeks in each river. The females lay 2,000 to 10,000 eggs in separate spawning beds made at the end of stream pools. This is the taimen’s wisdom of nature in order to prevent hatched juveniles from competing with each other for food in the same place. Sea-run taimen are also characterized by repeated spawning during their lifetime unlike salmon that die after their single spawning.

Juveniles grow between 2 and 3 centimeters long by the time they leave their spawning beds to start swimming during a July-August period. They first stay in shallow waters with a very low water velocity such as a stream pool on the river’s edge, catching the larvae of aquatic insects.
Soon, they expand their range to the center of the river current but they are by far slower in motion than Seema, or Yamame in Japanese that can swiftly catch aquatic insects. This is why juvenile taimen tend to stay around the pool end, downstream from where Seema stay, eating organic matter deposited on the river bed and flowing aquatic insects.

After spending about 2 years at the pool ends covered by plants and trees with almost no water flow, juveniles reach a length of about 15 centimeters. Around this time, they begin to eat small fish in addition to aquatic insects but once their body length exceeds over 30 centimeters, they are mostly ichthyophagous (fish-eating). As for their growing speed, sea-run taimen are much slower than other salmonids: It is said to take at least 15 years for them to grow up to 1 meter long. Therefore, they are also slow in reaching the maturity age: 4 to 6 years for male (Body length: about 40 cm) and 6 to 8 years for female (Body length: about 60 cm).