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Ontogenetic diet shifts of walleye: predicting consumption.
Galarowicz, Tracy1, 2, Wahl, David1, Herendeen, Robert1, 1 2
ABSTRACT- Young-of-year walleye (Stizostedion vitreum) undergo two ontogenetic diet shifts, switching from zooplankton to benthic invertebrates to fish. We examine how walleye size, prey type, and prey density influence consumption of zooplankton, benthic invertebrates, and fish. In single prey experiments, walleye of ten different sizes (20-150 mm) were offered either zooplankton, chironomid larvae, or fish over a range of prey densities. Zooplankton consumption increased with density; consumption also increased with length but then dropped sharply for 150 mm walleye. The number of chironomids consumed increased with size and prey density. Prey density but not walleye size influenced fish consumption. In multiple prey experiments, walleye were offered all three prey type simultaneously at varying densities. Walleye length was a critical factor determining the number of zooplankton, benthic invertebrates, and fish consumed. In general, fish density influenced consumption of all three types, benthic invertebrate density affected consumption of zooplankton and chironomids, and zooplankton density influenced foraging on zooplankton. At all sizes, walleye had the highest growth on fish. Using information from these experiments, we modeled consumption of zooplankton, benthic invertebrates, and fish by young-of-year walleye. We determined the relationship between walleye size and prey density for each prey type, and then fit these curves to our multiple prey data set incorporating the densities of the alternative prey available. Availability of prey at appropriate size is crucial to walleye growth and consequently survival.
KEY WORDS: Ontogenetic diet shifts, growth, walleye, Stizostedion