Abstract
The grazing effects of a herbivorous fish, Plecoglossus altivelis (ayu), on the species composition of benthic algal communities were investigated in the Kiso River (Nagano prefecture). Algae were collected from the cobbles and boulders with and without grazing scars of ayu. Achnanthidium convergens predominated in the non-grazed algal communities, whereas Homoeothrix janthina in addition to A. convergens in the grazed algal communities. The biomass of H. janthina was greater in the grazed algal communities than in the non-grazed ones. There was no significant difference in the biomass of A. convergens between two kinds of communities. These results indicate that the grazing by ayu affects the algal species composition by increasing the biomass of H. janthina.
