an evolving threat to freshwater ecosystems is invasive species. the spiny water flea (bythotrephes) is an invasive zooplankter well known for its severe negative impacts on native freshwater zooplankton, but some knowledge gaps exist regarding their impacts to fishes, high-level trophic organisms, and food chains. changes in trophic positions at all levels of the ecosystem can affect contaminant concentrations in large organisms. in this study i used walleye (sander vitreus) total muscle mercury concentration ([mehg]) and size data, as well as stable isotope ratios and a bioenergetics model to evaluate the potential impacts of bythotrephes on the build up of toxic metals in fishes. i expected to see an increase in walleye mercury concentration and a systematic impact of bythotrephes on walleye growth efficiency. in my first data chapter (chapter 2) i found that the presence of bythotrephes was associated with an increase in small (<50cm) walleye [mehg] in quetico provincial park and in rainy lake. these results suggest that bythotrephes have a greater impact on hg accumulation in smaller fish and may alter walleye growth trajectories. [...]