Young-of-year piscivores typically undergo ontogenetic diet shifts from planktivory to benthivory to piscivory. These shifts are often the result of changes in predator foraging abilities, but little is known about the influence of relative prey availability. As a result, we examined diet shifts across a range of sizes (20–150 mm) of a young-of-year piscivore, walleye (Sander vitreus), in feeding experiments in which zooplankton, benthic invertebrates, and fish were made available at different density combinations. Consumption of each prey type changed with walleye size and prey densities. Small juveniles (20 mm) selected zooplankton and fish, whereas larger walleye (40–100 mm) selected benthic invertebrates and fish. Relative prey densities influenced consumption; increased densities of more profitable prey types resulted in reduced consumption of less profitable prey. Walleye larger than 100 mm selected only fish. Foraging efficiencies also varied with size and prey types; small walleye (20 mm) were less likely to pursue benthic invertebrates and retain captured fish. These ontogenetic changes in foraging patterns are linked to prey profitability, have growth consequences for juvenile walleye, and have implications for understanding diet shifts of other juvenile fish.

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.

Young-of-the-year piscivores undergo ontogenetic diet shifts, but mechanisms influencing prey selection and implications for growth are unclear. We examined foraging and growth of 20- to 150-mm walleye (Sander vitreus) fed either zooplankton, benthic invertebrates, or fish over a range of prey densities in the laboratory. The number of each prey type consumed was influenced by walleye size and prey density. Walleye exhibited type II functional responses on each prey type; attack coefficients were constant across zooplankton and fish densities but decreased with benthic invertebrate densities. Handling time estimates were greater for fish than for other prey types but similar for zooplankton and benthos. Foraging efficiencies on zooplankton and benthic invertebrates increased with walleye size but were variable for fish prey. The smallest walleye size class (20 mm) had similar energy return (J·min–1) and growth (g·day–1) on zooplankton, benthic invertebrates, and fish. For larger walleye, both energy return and growth were highest on fish, intermediate on benthic invertebrates, and lowest on zooplankton. Diet shifts of juvenile piscivores and, consequently, growth can be explained by ontogenetic changes in foraging abilities and prey densities.

Physiological responses to environmental factors such as temperature can vary between stocks of the same species and may be linked to differences in latitude. To determine whether physiological differences exist among populations of young‐of‐year walleye Stizostedion vitreum as a function of geographic origin, we compared the metabolic rates, food consumption, relative growth, and conversion efficiency among walleyes from Arkansas, Missouri, Wisconsin, and Alberta, Canada, over a range of temperatures (5–25°C). Few or no differences were observed in metabolic rate (mg O2 · g−1 · h−1) among populations at the cooler temperatures, but walleyes from the Arkansas River, Arkansas, had higher rates than the northern populations at warmer temperatures. Both Arkansas populations also had greater food consumption rates (g · g−1 · d−1) than the northern populations at 25°C. However, growth (g · g−1 · d−1) was similar among stocks within each temperature. Our experiments indicate that physiological differences exist among walleye populations related to latitude. Walleye stocks are adapted to regional thermal conditions, and bioenergetic demands should be taken into account when managing native and introduced populations.