Abstract
Microhabitat distribution of the Japanese mitten crab Eriocheir japonica was surveyed in May in the lower river reaches of Fukuoka Prefecture. Quadrat samples (50 cm X 50 cm) and environmental factors were measured at 24 and 36 points along the river course for nearly 50 m at 2 sites (Research 1), and at 75 points within 3 X 25 compartments separated by 1 m across and along the river course (Research 2). Carapace width (CW, mm) and crab density per stone relative to the stone coverage area (cm2) were investigated in October (Research 3). Crabs mostly with 4-10 mm CW exhibited aggregated distribution. In Research 2, crabs preferred a rapid environment (1-20 crabs / 0.25 m2) with shallow depth, high current velocity, and large grain size (5-15 cm deep, 40-70 cm / sec, and 4-50 mm [gravel] to 250-500 mm [large rock]) instead of a pool environment (0-5 crabs / 0.25 m2; 20-40 cm deep, 0-20 cm / sec, and < 0.125 mm [mud] to 0.125-1 mm [fine sand]). Crabs also preferred high stone coverage areas. However, high crab density (10-25 crabs / 0.25 m2) was found around the upper rapid and the lower pool boundaries, caused by positive rheotaxis and high locomotive activity of juvenile crabs. Thus, a natural river environment with repeated rapid-pool units is an important microhabitat for mitten crabs. There was no significant relationship between stone coverage area and CW, but crab density was positively correlated with stone coverage area. This suggested that gaps beneath large stones were utilized by a large number of crabs regardless of their size, and they opportunistically use the stones in the river. River-crossing structures such as weirs may facilitate aggregation of mitten crabs, but mitigation may be possible by increasing stone coverage in the pool area riverbeds.
