Tomiyo is a species of fish in the family Gasterosteidae, and the order Gasterosteiformes. It is characterized by 7 to 8 spines and about 30-lined scutes. The fish lives in freshwater, brackish water and marine water. In parts of Japan other than Hokkaido, it lives in freshwater environments, including low-temperature spring water and streams, because it prefers cold water (around 15C). Freshwater Tomiyo are about 5 to 6.5 cm long, and marine-water tomiyo are 8 to 9 cm long. The male constructs a spherical nest with waterweeds and guards the eggs laid there. They are vulnerable to changes in water quality and water shortages. In Ishikawa Prefecture, they are only found in Sagi-ike Pond in Sueyoshi, Shika Town, and in Hakusan City and Nomi City. Tomiyo are therefore categorized as a Class 1 endangered species in the Ishikawa Red Data Book. The southernmost natural distribution of tomiyo is said to be Ishikawa Prefecture. When the agricultural land improvement project started in Sueyoshi, in 2002, Sagi-ike Pond was converted into a biotope after having been neglected for over 30 years. The agricultural union “Tomiyo-no-sato,” which was established in 2004, is playing a key role in various activities, along with Horimatsu Elementary School, and senior citizens’, middle-aged men’s and women’s clubs. The members of these groups remove reeds and sediments in an effort to conserve the environment for the protection of tomiyo.