Abstract
Actual achievements of the four fishways of the Nagara Rivermouth Barrage for a catadromous mitten crab, Eriocheir japonica (de Haan), were investigated based on the data of five-year monitoring by the Japanese Ministry of Construction on environmental changes after operation of the Barrage in 1995. Juveniles and adults of the crab under upstream migration were collected using mini-traps at upstream sites of each fishway and at downstream sites of each riverside of the Barrage in April and May for the five years. Significantly more juveniles migrated upstream through the brook-type fishway than the other fishways particularly in 1998, whereas the adults used the guide-flow fishways more frequently than the other fishways. The highest rate of juvenile upstream migration at the brook-type fishway showed its exceeding performance as a migration passage for small benthic animals. In spite of the remarkable amount of juveniles migrated upstream through the fishways at the barrage, the results of Nobori-ochi fishing at 46km and 57km from the rivermouth showed a significant reduction in the crab population. It is probable that the long distance of backwater area of the barrage may hinder the crab from safe migration.
