【Reading】たんすいぎょ

【English Name】Freshwater fish

Tansuigyo (freshwater fish)—a broadly inclusive term for fish whose living sphere is rivers, lakes, and ponds that contain almost no salt. According to data as of 2006, the existence of about twelve thousand species was confirmed, forming a large group amounting to nearly 43% of all fish.

Based on differences in adaptability to salt and in life history, they are broadly divided into three groups. The type that completes its entire life only within fresh water, represented by catfish and carp, is the primary freshwater fish. Species that travel back and forth between the sea and rivers, like sweetfish (ayu) and eel (unagi), are called diadromous fish, and examples of originally saltwater fish such as mullet (bora) and sea bass (suzuki) entering rivers from the estuary are treated as peripheral freshwater fish.

As sushi toppings, eel, sweetfish, and trout (masu) are representative, and among them eel is indispensable as a classic ingredient of Edomae sushi. Its hallmark is a rich sweetness of a different character from simmered conger eel (anago), and it remains firmly popular today. Like push-sushi (oshizushi) made with biwa trout (biwamasu) from Lake Biwa and the narezushi handed down in various regions, a sushi culture that makes use of river fish has been continuously passed down throughout Japan.

The proportion that freshwater areas occupy of the total volume of water on the planet falls below 0.01%. The phenomenon of this many species being densely packed in such scant waters is a theme that attracts high interest from the standpoint of biodiversity.