Evidence Double-Crested Cormorants Are Selective Predators: Analysis of 15 Years of Diet Data from Oneida Lake, NY

Cormorants have been reported to cause declines in some fish populations, but not others. This difference may be due to prey selection that is further dependent on the composition of the fish assemblage affected. We present 15 years of diet data collected from Oneida Lake, NY, where we have previously documented negative effects of cormorants on walleye and yellow perch. The fish community changed through the study period and this change was reflected in cormorant diets. Diets varied by season and year with emerald shiner, gizzard shad, Lepomis, logperch, walleye, and yellow perch having the highest overall relative importance. When age-0 gizzard shad were abundant they dominated cormorant diets in the fall. Consumption of emerald shiner and gizzard shad was positively related to species’ abundance, but no significant correlation between availability and consumption was found for other prey species. Positive selection was found for gizzard shad, walleye, white perch, and yellow perch during the 15 year period, as well as evidence of prey switching behavior by cormorants. We conclude that variation in fish recruitment influence cormorant food selection and stress the importance of continued diet monitoring with changing fish communities to reliably assess potential impacts on a fish community.