Foraging behavior, morphology, and life history variation determine the ontogeny of piscivory in two closely related predators

The ontogeny of piscivory is an important process during the early life history of many fishes, and why ontogenetic patterns of closely related species vary is unclear. We experimentally evaluated the importance of several factors that can determine the switch to piscivory in two predators with different dietary ontogenies: walleye (Sander vitreus), a specialist piscivore, and yellow perch (Perca flavescens), a dietary generalist. We conducted growth and prey selection experiments across several sizes of both predators using fish, zooplankton, and benthic invertebrates as prey. Walleye exhibited piscivorous feeding behavior throughout all size classes (20-80 mm), whereas yellow perch were generalist predators, showing negative to neutral selection for fish prey and lower growth than walleye when feeding on fish. Walleye foraged more efficiently than yellow perch on all prey types, in part because gape widths of walleye increased more quickly with size. Bioenergetic model simulations showed that walleye grew slower than yellow perch when the proportion of fish was low in diets, but walleye growth was faster than yellow perch on a diet dominated by fish. Feeding behavior, morphology, and life history patterns likely work in combination, allowing walleye to switch to piscivory earlier than yellow perch.