Department of Applied Aquabiology, National Fisheries University, Shimonoseki, Yamaguchi 759-6595, Japan
To establish standards of appropriate dam height and the depth of pools to minimize damage to Japanese mitten crabs, Eriocheir japonica, when passing over dams during the downstream migration for reproduction, experiments to examine the effect of falling from various heights onto concrete or into a pool were conducted. For crabs of carapace width 5 cm, some ambulatory legs were lost or damaged when falling onto concrete from 1 m in height. For heights of more than 2 m, half of the test crabs that collided with the concrete hit their abdominal surface and some of them died. All animals that dropped more than 5 m onto concrete died. When a pool of depth 60 cm was present, no crabs were damaged by falling from even an 11 m height. In conclusion, even the largest crabs of carapace width 8 cm, can safely fall from a 10 m height when a pool depth of 50 cm or more is present.