upstream

Currently, catches of the Japanese eel, Anguilla japonica（Anguillidae） have decreased drastically compared with those reported previously. Habitat fragmentation in freshwater streams, mainly because of the presence of dams or sluices that block upstream access for juveniles and downstream access for adults, is thought to be a significant factor contributing to population declines, and hence declines in eel fisheries. Fish passageways may allow fast-swimming fish to pass by obstructions in the river but these are not well suited for juvenile eels. In Europe, the Eel-ladder, a form of fish passageway with a slope of consisting of grass, was developed to allow for A. anguilla and A. rostrata glass eels and elvers to bypass dams and other obstructions. In this study, we investigated the suitability of a brush-type Eel-ladder in an experimental stream to facilitate passage of A. japonica elvers. We determined three variables that influence upstream movement by elvers using this device: water volume, the angle of brush structure within the Eel-ladder, and the space between nylon supports in the Eel-ladder that was filled with brush. In our experiments, we manipulated these three variables and measured the number of glass eels that ascended to the top of the Eel-ladder during each manipulation. Our results showed that climbing success was greatest for the brush-filled space between the nylon supports, and we inferred that this variable would promote greater movement upstream by elvers. In contrast, the angle of the brush itself had no effect on movement success and the effect of water volume was limited. We propose that future experiments should test the climbing success rates of post-elvers（yellow eel） individuals using this experimental device and also the effects of brush Eel-ladders built to a larger scale for use in rivers.