抄録
The mitten crab Eriocheir japonicus inhabits the river. In Ohbiso River, mature crabs with high gonad index [GI=(gonad weight) / (carapace width)^3×10^5>0.48] appear in November, presenting a wide abdomen (AI=abdomen width/abdomen length>0.88), moving to the brackish water region for mating and reproduction. Immature crabs have a narrow abdomen (AI<0.88) and remain in the fresh water region of the river.
