Pale chub Zacco platypus is an omnivorous fish, coexisting with dark chub Nipponocypris temminckii in the middle reaches of Japanese rivers. Previous studies have examined their feeding behavior, including algal, drift and surface feeding, and aggressive interactions. In particular, algal feeding of pale chub is known to decrease in the presence of algivorous ayu Plecoglossus altivelis. Such behavioral shifts may be common in fish communities, but have been little studied among coexisting cyprinid fishes. To determine whether or not feeding behavior of pale chub changed in the presence of dark chub, 8 individuals of the former were established in each of 6 current-influenced ponds. One week later, 8 dark chub individuals each were introduced into 3 (randomly chosen) of the 6 ponds. All fishes were observed during three 5-days periods. Pale chub exhibited predominantly algal and drift feeding, whereas dark chub showed mainly drift feeding. However, following dark chub introduction, pale chub increased algal feeding, but did not change the number of drift feeding acts. The growth rates of pale chub were consistent, regardless of the presence or absence of dark chub. The latter sometimes attacked pale chub, which subsequently moved to the water layer adjacent to the substrate and changed feeding behavior, thus allowing resource partitioning and continued coexistence of the two species.
