Effects of a grazing fish, Plecoglossus altivelis (Osmeridae), on the taxonomic composition of freshwater benthic algal assemblages

The effects of grazing fish, Plecoglossus altivelis (ayu), on freshwater be nthic algal assemblages were investigated by experiments in outdoor water t anks with fish (2.2 indiv./m(2)) and without fish for 25 d. To simulate the condition of running water in each tank, a water current was generated wit h a water pump. In three replicate experiments, algal biomass and diatom bi ovolume were reduced in presence of the fish. The biovolume of the filament ous cyanophyte. Homoeothrix janthina increased in response to grazing, and finally predominated in presence of the fish. Observations of grazing scars by scanning electron microscopy verified the supposition that the basal ce lls of Homoeothrix janthina, which were the hormogones attached to the subs trate, had relatively higher resistance to fish grazing. The grazing resist ance of the basal cells would contribute to colonization of the filamentous cyanophytes under fish grazing. These results show that grazing fish regul ated algal assemblages dominated by filamentous cyanophytes through their f oraging activities.