A Study on Evaluation of Fishways of the Nagara Estuary Barrage based on Habitat and Upstream Migration for Japanese Mitten Crab

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Actual achievements of the four fishways of the Nagara estuary barrage for fishes were investigated based on the data of five-years monitoring on environmental changes. It should be noted that temporal concentration of upstream migration was observed both hourly and daily in the Seseragi (brook-type) fishway. The peak of the upstream migration of fishes corresponded to the neap tide period. Under natural condition, the range of salt water extends upstream farther as the tide advances. Settlement of the Japanese mitten crab (Eriocheir japonica) was investigated in the Seseragi-fishway. The megalopa larvae settled and metamorphosed to crab stage in the upper tidal zone in the fishway. The youngest crabs were nearly two millimeters in carapace width. Settlement of megalopa larvae occurred mainly in early summer (June) and early winter (December). The majority of larvae that settled in winter did not survive and metamorphose to the crab stage due to low water temperatures. Thus, the growth … More and upstream migration of the Japanese mitten crab seemed to be strongly influenced by water temperature in the estuary. The suitable settlement position of megalopa larvae corresponded to the tip of salt water intrusion at neap tide. In this study, it is clarified that the Seseragi-fishway have a fulfill function to mitigate for decreased estuary zone that change aquatic organism phase from freshwater to seawater. Furthermore, the effects of saltwater intrusion characteristics on benthic animal communities and algae were investigated in the fishways of the Nagara estuary barrage under uniform substrate. As a result of the cluster analysis of the benthic animal communities, the fishway zone was divided into three parts ; the freshwater, the saltwater and the transitional section. The benthic animal communities and algae inhabited mainly in these section related to saltwater intrusion characteristics. The megalopa larvae of the Japanese mitten crab (Eriocheir japonica) settled and metamorphosed to crab stage in the upper tidal zone in the fishway. Settlement of megalopa larvae occurred mainly in early summer (June) and early winter (December). The majority of larvae that settled in winter did not survive and metamorphose to the crab stage due to low water temperatures. Thus, the growth and upstream migration of the Japanese mitten crab seemed to be strongly influenced by water temperature in the estuary. The suitable settlement position of megalopa larvae corresponded to the tip of salt water intrusion at neap tide. In this study, it is clarified that the Seseragi-fishway have a fulfill function to mitigate for decreased estuary zone that change aquatic insect and organism phase from freshwater to seawater. Less