Abstract
Interspecific relationships between ayu Plecoglossus altivelis and Japanese dace Tribolodon hakonensis were investigated experimentally in a semi-natural water course. Both species attacked conspecifics in territorial defense or dominance-subordination relationships. When both species encountered, ayu occasionally attacked Japanese dace, but were not attacked. Ayu fed predominantly upon algae on cobbles and their feeding activities were not affected by Japanese dace. Japanese dace were omnivorous, feeding upon both algae and terrestrial and aquatic invertebrates, but did not feed upon algae when ayu were present. On this occasion, Japanese dace predominantly used the area near the stream banks where ayu were not present. The growth rate of Japanese dace was not affected by ayu as a result of resouce partitioning between the two species, whereas ayu growth was facilitated when they coexisted with Japanese dace, probably because Japanese dace increased the algal standing crop by decreasing the number of aquatic invertebrates.
