Abstract
A fishway is a kind of facilities attached to a river-crossing structure to aid fishes to ascend it. The entrance of conventional pool-type fishway or fishslope is, however, located apart downstream from the facility, which often let fishes lose the entrance. A fan-shaped fishway and a terrace type fishway that is a modified form of the fan-shaped one are improved versions of pool-type fishway and fishslope whose entrances are widely opened in order for fishes to find them safely and surely. The effectiveness of their widely-opened entrance is known qualitatively by several surveys, whereas not known quantitatively. The aim of this paper is to confirm its superiority over the conventional ones by investigating the ascending routes of juvenile Ayu (Plecoglossus altivelis) at two terrace type fishways. The path of each fishway is spatially devided by sand bags into three routes, i.e. right side, center and left side and the number of ascending fish is counted by using video-cameras installed at the upper end of each route. As a result, 66.6% of acsended fish entered the fishways from sides. They might have lost the entrance, if the entrance were the conventional shape. The choice of route seemed to depend on location of the thalweg or gut. Ayus ascended only in daytime from noon to around 6p.m. in both fishways, which might be due to water temperature, light intensity and their periodic habit. More than 1,000 Ayus ascending the fishways were seen in each day during the investigation period, which would indicate that these terrace type fishways works properly.
