Longitudinal distribution and changes in the fish fauna of a mid-scale river, Shoro [Japan] river system, eastern Hokkaido, with notes on signal crayfish

The distribution of fishes in the Shoro River system, a mid-scale river in eastern Hokkaido, Japan, was assessed using electric fishing gear. Of the 8 species recorded during the summer of 2007, Siberian stone loach (Barbatula toni), white-spotted charr (Salvelinus leucomaenis), masu salmon (Oncorhynchus masou masou) and dace [Tribolodon sp(p).] were relatively abundant. Among of these four species, Siberian stone loach showed the most extensive distribution including above a large scale dam which provided a barrier to the fish migration. Only three other species were recorded below the dam, but not recorded above the dam. Two exotic species, rainbow trout (Oncorhynchus mykiss) and signal crayfish (Pacifastacus leniusculus) were also recorded, the former being relatively abundant above the dam and in the uppermost reach below the dam in the main stream, but rare in the tributaries. Signal crayfish appeared mainly in the tributary streams flowing into the lower reaches of the main stream. Neither species had been recorded from the Shoro River system in the 1970's. The distribution of the two native salmonids, masu salmon and white-spotted charr, overlapped broadly in both the main stream and lesser tributaries. In the uppermost reach below the dam, exotic rainbow trout were dominant salmonid, and native masu salmon being less common than downstream.