Abstract
Dark chub, Zacco temmincki, and pale chub, Z. platypus, coexisting in two rivers near Kyoto City, exhibited both intra- and interspecific aggressive behaviour. Larger individuals more frequently utilized the region near the water surface and were more dominant in both intra- and interspecific aggressive encounters than smaller ones. Both were omnivorous, although Z. platypus fed upon algae more frequently. The two species used similar feeding areas wherein frequent aggressive interactions occurred. The frequency of intraspecific aggressive interactions was correlated with that of algae feeding by both species. Individuals of the two species commonly occupied overlapping home ranges, but on one occasion defended a territory against all other individuals.
