1Hokkaido Fish Hatchery, Mashike Research Branch, Hokkaido 077-0216, 2Faculty of Bio-Industry, Tokyo University of Agriculture, Hokkaido 099-2493, 3Hokkaido Fish Hatchery, Hokkaido 061-1433, Japan
In the Monbetsu River, a small branch of the Ishikari River basin, west Hokkaido, a brown trout population that was probably introduced 15 years ago is dominant. The population of brown trout was estimated to be 1.8 times greater than that of the native white-spotted charr. The brown trout occurred markedly in the mid- and downstream reaches. On the contrary, the white-spotted charr was predominant only in the upstream reaches above weirs in the river. The water temperature ranged from 5 to 16°C during the summer-autumn period at the lower reaches of the stream. In the mid- and downstream reaches of this stream, the non-native brown trout have likely replaced the native white-spotted charr over the last 15 years.