Gastric evacuation in koayu Plecoglossus altivelis altivelis examined in aquaria

Abstract

The landlocked dwarf ayu (koayu) Plecoglossus altivelis altivelis in Lake Biwa is one of the most important predators of planktonic crustacea. As part of predation ecology of koayu, the process of gastric evacuation was examined by two kinds of aquarium experiments. In the first experiment, koayu was fed for 1 h and the stomach contents after feeding was enumerated. Stepwise multiple regression analysis of the number of prey in the stomach on eight independent variables (numbers of six kinds of behavior during feeding, food concentration, and predator mass) selected only the number of foraging attacks as the predictor variable. In the second experiment, koayu was starved for 2 h after 1 h feeding. Prey number in the stomach before starvation was calculated from the number of foraging attacks and was compared with the number actually enumerated after starvation. Almost all stomach contents were digested and evacuated during the 2-h starvation. The stomach contents of koayu were, hence, ingested less than 2 h before the moment of collection. The present results enabled the estimation of predation rate in the field from the mass of stomach contents.