Influence of Predation Risk on the Overwinter Mortality and Energetic Relationships of Young-of-Year Walleyes

Abstract

We investigated three potential mechanisms—size-dependent overwinter energy loss, size-specific predation, and size-dependent predator-induced metabolic costs—to determine the role of body size and predators on the overwinter survival and energy reserves of young-of-the-year walleyes Stizostedion vitreum. Walleyes were reared in outdoor hatchery ponds in Westport, Ontario, for three overwinter periods (1997-1998, 1998-1999, 1999-2000) in the presence or absence of predators (burbot Lota lota or adult walleyes). Other young-of-the-year fishes were provided as potential prey items. We predicted that (1) smaller individuals of a young-of-the-year walleye cohort would have lower prewinter energy stocks and would deplete them at a higher rate; (2) no size-dependent overwinter survival would occur in walleyes reared without predators; and (3) physiological stress, in combination with size-selective predation on smaller individuals, would lead to size-dependent overwinter mortality in young of the year reared with predators. Prediction 1 was not supported by lipid, protein, or specific-energy content data; indeed, smaller walleyes actually gained energy over the winter, whereas larger walleyes lost energy. Prediction 2 was supported but prediction 3 was not, as no significant survival differences were observed between four size-class quartiles in any pond or year. However, young-of-the-year walleyes reared over winter in the presence of predators lost significantly more weight and energy and had significantly lower lipid concentrations at the end of winter than did those reared in the absence of predators. Although no differences in overwinter survivorship were detected between young-of-the-year walleyes reared in predator and predator-free ponds, the presence of significant predator-induced energetic costs in the absence of size-selective mortality suggests that the size-selective mortality observed in some natural walleye populations is the direct effect of the presence of predators.