In lake ecosystems, knowledge of the direct and indirect effects of apex predators and piscivory is essential to managing fisheries and maintaining water quality. To determine if population increases of the double-crested cormorant (Phalacrocorax auritics ) on boreal lakes are influencing food web structure and function via top-down effects, I used a combination of conventional diet and stable isotope analyses. Analysis of regurgitation samples collected from five colonies in the Lac La Biche area of north-central Alberta during 2003 and 2004 identified 13 different prey species in cormorant diets. The majority of fish consumed were small in size, less than 1.00 mm in length. Yellow perch was the most frequently captured species on all colonies and also comprised the largest proportion of biomass in regurgitation samples. Based on isotopic signatures and diet composition, birds nesting on small lakes were found to forage on the local nesting lake as well as on Lac La Biche; however, foraging appeared to be focused primarily on Lac La Biche. In large lakes, such as Lac La Biche, isotopic ratios of carbon and nitrogen yielded similar trophic levels for double-crested cormorants and predatory fish: walleye and northern pike.