Itou (sea-run taimen) currently inhabit 11 river systems in Hokkaido, forming 12 populations. These 11 rivers can be classified into the following 3 categories:
- [1] Rivers in northern Hokkaido that support the largest number of the species (Teshio River, Sarufutsu River, etc)
- [2] Rivers in eastern Hokkaido where sea-run taimen still exist in some rivers while they have become extinct in others. (Bekanbeushi River, etc)
- [3] Dam lakes (reservoirs) and their inflow rivers where artificially-landlocked taiman populations remain. (Uryu River and Sorachi River, etc. in the Ishikari River system.)
Regarding rivers falling into the last two categories, environmental conditions in the taimen rivers in eastern Hokkaido ([2]) are further deteriorating whereas it is uncertain how much longer the artificially-landlocked taiman ([3]) can maintain their healthy populations in the closed environment.
Looking closely into the habitat of each population, we can learn that most populations inhabit rivers that have wetlands such as: Kushiro Marsh, Bekanbeushi Marsh, Lake Furen • Shunkunitai (slender sand reef lying between Lake Furen and the Okhotsk Sea), Sarobetsu Marsh, Uryunuma Marsh, and Sarufutsu/Mokeuni-numa Marsh, and lagoons unique to them in the lower river basins. This is a showcase of sea-run taimen dubbed as the “Fish of Marshes.” Conversely, however, taimen are extinct in rivers without wetlands due to expanded development in their well-drained river basins. Also, even some poor-drained rivers have lost their wetlands and lagoons due to channelization as explained in the “Spawning sites destroyed due to channelization” section. Sea-run taimen are thought to have disappeared one after another from such rivers.
