ECOLOGICAL-STUDIES ON THE JAPANESE MITTEN CRAB ERIOCHEIR-JAPONICUS DEHAAN .1. LONGITUDINAL DISTRIBUTION OF THE JAPANESE MITTEN CRAB IN THE KAMINOKAWA RIVER, KAGOSHIMA

The longitudinal distribution and process of migration of the Japanese mitten crab Eriocheir japonicus were investigated in the Kaminokawa River, Kagoshima Pref. The crabs were collected in crab baskets between September 1985 and August 1986. Crabs were distributed in the freshwater zone throughout most of the year, but were present only from September to January and from March to May in the brackish water zone. Adult females were present mainly from August to December in the freshwater zone, and from September to January in the brackish water zone. Puberty molting by females was considered to occur mainly in August. Active migration of crabs large enough to be caught with crab baskets was considered to occur mainly from September to December. Nearly all the year round, larger crabs were distributed in the upper reaches. In the case of females, the minimum size of adults and the maximum size of juveniles gradually increased further upstream. Therefore, it is presumed that crabs migrate upstream as they grow older. Crabs arriving at the upper reaches will grow larger or older during the juvenile stage. Consequently, juveniles of larger size in the upper reaches become larger adults.