The Kuril archipelago is formed by the summits of an underwater mountain ridge. Today this chain of mountainous islands of volcanic origin stretches for almost 1200 km from north-east to south-west, from the Kamchatka Peninsula to Hokkaido Island. There are 39 active volcanoes on the archipelago. Kunashir Island (which means "Black Island" in the language of native Ainu people) is the southernmost of the islands of the Kuril Ridge. This island is oriented from north to south for 130 km, and its width varies from 9 to 30 km. About half of the island area is occupied by the Kurilski State Nature Reserve (zapovednik). Those areas are remarkable in terms of the great diversity of species and forms of salmonids, including various anadromous and freshwater char, salmon, trout, and taimen. The area of the reserve is 65,365 hectares, in addition to a 41,475 hectare buffer zone. Large marine coastal areas are included in the reserve. The Saratovka, Tyatina and Nochka spawning rivers, on the relatively gentle slopes of the Tyatya volcano, and the unique Peschanoe Lake, are protected salmon ecosystems and particularly important centers of salmonid biodiversity.
Populations of Sakhalin taimen and other salmon species have declined throughout the island. The principal threats to fisheries are in subsistence fishing with nets, illegal poaching, and pollution from mining operations. Additionally, there are planned fish hatcheries in the southern portion of the island, slated for construction starting in 2002. These may pose an as-yet-undetermined threat to salmon genetic diversity and water quality.