Effects of dams and weirs to the distribution of Japanese mitten crab in Imizu plain, Toyama

The distribution of Japanese mitten crab ( Eriocheir japonica (de Haan)) was researched in three river systems in the Imizu plain, central Toyama, and the effects of dams and weirs on the upstream movement of the crabs were examined. Mitten crabs could pass the dams and weirs when the dams or the stream banks below the dams had a gentle slope and a rough surface; sometimes, stems and leaves attached to the dams and slope faces helped crabs climb. Crabs did not pass the dams when such conditions were not present, even if the slopes were only about 1m high. It was confirmed that mitten crabs would reach the most upstream areas if there was no critical obstacle. In the studied rivers, the presence of many dams and weirs worsened the river environment.