Double-crested Cormorants (Phalacrcorax auritus) and other piscivorous birds are frequently implicated in having negative effects on commercial and sport fisheries despite the fact that several studies suggest that such effects may be infrequent. Increases in cormorant populations in North America over the last decades have fueled the debate further and population controls, official and unofficial, are in effect in several jurisdictions. Of importance to the management community is the need to obtain the best information on actual diets of cormorants throughout their annual cycle but particularly on the temperate breeding grounds where they frequently overlap with fisheries. Despite success in quantifying seabird diets, managers have yet to exploit the potential of the stable isotope approach to examine trophodynamics of cormorants where they can potentially interfere with fisheries. Here, the use of stable isotope methods (δ15N and δ13C) to quantitatively predict trophic interactions between cormorants and their prey are recommended to augment conventional dietary approaches. The stable isotope approach is particularly suited to situations where birds feed at a single, well-defined lake or waterbody. The approach can provide a quantitative estimate of fish consumption to species or species groups in cases where such groups are isotopically distinct. This paper is intended to provide the research and management communities with the necessary background to apply stable isotope methods to better assess diets of cormorants and other piscivorous waterbirds.