Fatty acids, stable isotopes, and regurgitate reveal diet differences between Lake Ontario and Lake Erie double-crested cormorants (Phalacrocorax auritus)

A detailed understanding of the possible confounding effects of natural diet is important for ecotoxicological research where exposures cannot be controlled. The opportunistic feeding of double-crested cormorants (Phalacrocorax auritus) may limit the ability to draw toxicological conclusions where food sources are potentially contaminated, such as in some sites across the Great Lakes. Using a variety of methods to study diet may allow for the detection of finer differences that would be missed without these complementary approaches. We used regurgitates, essential fatty acids (EFA), and 13C and 15N stable isotopes to understand cormorant diets at sites in Lake Ontario (Hamilton Harbour) and Lake Erie. Lake Ontario cormorant diets as estimated with regurgitates consisted almost entirely of alewife (Alosa pseudoharengus; 51–56%) and round goby (Neogobius melanostomus; 25–42%). A higher number of fish species were identified in Lake Erie cormorant regurgitate, but overall diets were dominated by round goby (70%). Fatty acid profiles and stable isotope mixing models indicated large differences between the diets of cormorants from Lake Ontario and Lake Erie but not between sexes. Polymethylene-interrupted fatty acids (PMI-FA) were useful as indicators of secondary contribution of mussels; mass fractions were much higher in cormorants from Lake Erie, indicating those cormorants consumed higher amounts of prey that feed on invasive zebra and quagga mussels (Dreissena spp.), especially round goby. All methods used to study diet give incomplete and biased results, but combining traditional and biochemical methods can help generate a more complete picture than has been attempted previously.