A rock ramp fishway (Fig. 1) is an artificial but nature-like fishway that simulates the conditions in natural rapids. It consists of boulders on a slope used as a method to various fishes migration and movement on low-head weirs in Japan. The turbulence and backwater created around the boulders decreases the flow velocity locally and allow fish to migrate to upstream sites. Low velocity flow behind the boulders provides resting areas for migrating fishes. Given that the swimming performance of fishes varies according to species, body size, and growth, the boulder arrangements should be designed to create a flow suitable to permit the target species to pass, especially if it is weak swimmer. The hydraulic conditions of the flow over the slope of the fishway are controlled by the “dimensions of the fish passage (incline and length of the longitudinal slope of ramp.)” , the “flow conditions (the water depth of the flow)”, and the “boulder arrangement (sizes, shapes, layout)”. Though scientific knowledge of the influence of hydraulics on fish are important for designing fishway, few researchers have examined rock arrangements on fishway. Boulder arrangements, including seven boulder shapes and 27 layouts, were tested in a 0.4 m-wide flume. The swimming performance of Char (Salvelinus richardson) and freshwater Sculpin (Cottus pollux) around boulders were observed by five ultra-high-speed cameras, and the appropriate arrangement of boulders for each species was considered. Both species was unable to maintain its swimming balance within the boundary layer, and a different arrangement of boulders is therefore required according to the particular swimming physiology of each species. |