New portable fishway design for existing trapezoidal weirs

The improvement of the weir without a fishway is required in order that diadromous fishes, shrimps, and crabs can migrate to their upstream and downstream habitats. This paper presents a proposal of a portable fishway to be set on the waterside of a trapezoidal weir and discusses the effect of the proposed fishway on the migration of diadromous aquatic animals in the trapezoidal weir. Field experiments concerning the proposed fishway were carried out at a trapezoidal weir having a slope of 26.5° (Vertical 1: Horizontal 2) and 2.0 m drop height. The field experiments revealed that the juvenile ayu fish (Plecoglossus altivelis) having a body length of 4.4–10 cm could migrate to their upstream habitats through the proposed portable fishway even if the slope of the fishway is 19°. About 120 000 juvenile ayu could migrate upstream via the fishway during 9 d. The migration route of juvenile ayu can be explained on the basis of the flow characteristics in the fishway. Measurements of the flow velocity and the air concentration distribution in the portable fishway support the conclusion that the juvenile ayu can easily migrate upstream through the fishway. Also, the proposed fishway could be helpful for the upstream migration of benthic fishes, shrimps, and crabs. Key words: fishway, migration, diadromous fish, gabion, drop structure, weir.