Abstract
To analyse the effects of small‐scale dams on fish communities, species diversity, population density, biomass, migration over dams and trophic relationships were investigated in the above‐ and below‐dam sections of three Japanese streams. The dams were 1·5–3·9 m high, constructed within 400 m of the sea, and had no impoundment or fishway. Fish communities were mainly composed of diadromous fishes. Of the 17 species sampled, only a few species of gobies, Rhinogobius spp. and Gymnogobius spp. were able to climb and migrate upstream over the dams. Consequently the number of species, and total density and biomass of fishes below the dams was high, but low above the dams. One of the most dominant fish below dams, Plecoglossus altivelis altivelis , predominantly fed on benthic algae, whereas other fishes preyed on benthic invertebrates. Trophic relationships above dams were very simple, the few species of gobies preying on chironomid larvae and other aquatic invertebrates.