The sweetfish is an amphidromous fish and a relative of the smelts. Native to the Palearctic ecozone, they are found 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.