Prey Selectivity of Two Cyprinid Fishes in Ogochi Reservoir

In order to evaluate the predational impact on various zooplankton species, gut contents were examined on two dominant cyprinid fishes, Honmoroko Gnathopogon elongatus caerulescens and Oikawa Zacco platypus, in Ogochi Reservoir through warm water season. Crustacean plankton is usually dominant in the gut contents of both fishes and one of the largest crustacean prey species Daphnia galeata is exclusively dominant when this species is abundant. When Daphnia is scarce, Honmoroko preys mainly on other crustacean plankton such as copepodids of Cyclopidae and Diaphanosoma, but total ingestion is relatively low regardless of the abundance of these crustacean plankton. On the other hand, Oikawa preys mainly on terrestrial insects. The amount of ingestion for some other zooplankton species such as rotifers, Bosmina and Bosminopsis is considerably small in both fish species. These results suggest that the most vulnerable zooplankton to fish predation is Daphnia in this reservoir and other zooplankton may have more chance to coexist with Honmoroko and Oikawa because of the small size or escape ability.