With climate warming, warm-water adapted fish predators have expanded their northern range boundaries, establishing in Ontario lakes that were previously too cold. Concurrently, abundances of cool-water adapted fishes have declined in northern lakes. Co-occurrence of Smallmouth Bass (warm-water) and Walleye (cool-water) is expected to become more common and is associated with reduced population abundance of both species. We test for evidence of competition and predation between resident Walleye and range-expanding Bass contributing to this pattern. We use fish morphology to examine niche overlap across ontogeny. We also examine differences in fish growth and population size-distributions in lakes where these species occur separately and together.
Abiotic gradients contribute to population differences making it necessary to control for environmental variation when comparing populations to investigate the impacts of biotic interactions. Morphological differences suggest that Walleye and Bass are more likely to compete as juveniles than as adults. Walleye in lakes with Bass appear to grow faster than those in lakes without Bass indicating selection for fast growth to escape gape-limited predation. Together, this evidence suggests that population demography is influenced by biotic interactions between predators during early life stages. Changes in size-distributions, however, were equivocal reflecting the complexity of understanding demographic trends in fishes and across regional scales.