Foraging patterns of juvenile walleye (Stizostedion vitreum) in a system consisting of a single predator and two prey species: Testing model predictions

We tested the hypothesis that the presence of a large number of energetically inferior prey (Daphnia pulex) in the environment with energetically superior prey (larval carp, Cyprinus carpio) interferes with foraging efficiency of juvenile walleye (Stizostedion vitreum). We monitored functional responses of juvenile walleye feeding on larval carp alone or in combination with D. pulex. When walleye were offered larval carp at increasing densities (10, 20, 50, and 100 individuals/30-L aquarium), they responded in accordance with a type-II functional response in 10-min trials. Walleye captured the maximum number of larval carp when offered a carp density of 20 individuals/aquarium without the daphnids. Further increase in density of larval carp had no effect on walleye foraging rate. The presence of D. pulex (900 individuals/aquarium) suppressed the efficiency of foraging walleye. The predator consistently captured fewer larval carp in all treatments when they were offered together with the daphnids even though walleye continued to select the more profitable larval carp. The negative effect of daphnids on the feeding rate is an example of environmental constraints interfering with optimal foraging models. In this study, walleye appeared to experience a confusion effect caused by the large number of inferior prey. Consequently, the predator could not gain as much energy per unit time when confronted with two prey types as it was when foraging on larval carp alone.