Intraspecific Variation of Walleye (Sander vitreus) in Ontario, Canada and Lake Winnebago, WI: Ecological Consequences of Differences in Life History, Color and Morphology

Northern freshwater fish species in postglacial environments frequently show remarkable amounts of phenotypic variability that promote the formation of discrete subpopulations and intraspecific polymorphisms. Phenotypic divergence among subpopulations may result from different selection pressures affecting predator avoidance, reproduction, foraging and habitat use. Since intraspecific morphotypes can be related to adaptive differentiation in trophic ecology and often involve reproductive isolation, they provide unique systems to address the forces promoting evolutionary divergence.;Divergence in color, life history and ecomorphology was studied within and between two populations of walleye (Sander vitreus). First, blue and gold color forms were examined between two depauperate lakes in Ontario, Canada that differ in trophic habitats. Second, the walleye population in Lake Winnebago, Wisconsin was used to explore ecomorphological differences among walleye stocks that vary in the distance of upstream spawning migration.;Blue and gold color forms differ in growth rate and morphology which suggests the existence of a resource polymorphism tied to different foraging niches between color forms. The slower growth of blue walleye may be due to prey switching from fish to benthic invertebrates that are less energetically rewarding. Analyses of head and whole body morphology indicated that these color forms are morphologically adapted to efficiently utilize prey found in their respective habitats.;In Lake Winnebago, morphological divergence was detected between male and female walleye and between stocks that migrate short (Fox River) or long (Wolf River) distances upstream to spawn. The shape of females from both rivers allows them to carry more eggs. Wolf River walleye possess morphological adaptations that facilitate longer migrations to spawning grounds. Tagging studies confirmed that morphological variation can distinguish between these spawning stocks when they are intermixed and away from their natal rivers.;Lastly, a new secretory cell within the epidermis of walleye in Ontario was described that contains Sandercyanin, a blue protein. Sandercyanin was located disproportionately on the dorsal surface and its production varies seasonally. Furthermore, gold walleye produce more Sandercyanin than blue color forms throughout the year; this difference is highest in summer months. Potential mechanisms responsible for these differences and implications for conservation and management are discussed.