Analysis of Prey Selection by Double-Crested Cormorants: A 15-Year Diet Study in Oneida Lake, New York

Abstract

Piscivorous birds, in particular cormorants Phalacrocorax spp., have been reported to cause declines in some fish populations in both Europe and North America, but not in others. This difference may be due to prey selection by cormorants that is further dependent on the composition of the fish assemblage present. We present 15 years of diet data collected from Oneida Lake, New York, where we have previously documented negative effects of double-crested cormorants Phalacrocorax auritus on two fish populations valued by anglers: Walleye Sander vitreus and Yellow Perch Perca flavescens. The Oneida Lake fish community changed through the study period and this change was reflected in cormorant diet samples. Diet samples were variable based on season and year with Emerald Shiner Notropis atherinoides, Gizzard Shad Dorosoma cepedianum, Lepomis spp., Logperch Percina caprodes, Walleye, and Yellow Perch having the highest overall relative importance. In years when age-0 Gizzard Shad were abundant they dominated double-crested cormorant diets in the fall after the shad reached a length of 45 mm. Consumption of Emerald Shiner and Gizzard Shad was positively related to each species’ abundance, but no significant correlation between availability and consumption was found for Walleye, White Perch Morone americana, or Yellow Perch. Double-crested cormorants may be displaying prey-switching behavior and selecting for smaller, soft-rayed, prey species. We conclude that variation in fish recruitment influences double-crested cormorant food selection habits and highlight the importance of continued monitoring with changing fish communities to reliably assess potential impacts of cormorants on a fish community over time.

Received April 24, 2012; accepted October 31, 2012

ACKNOWLEDGMENTS

Funding and support for this project was provided by New York State Department of Environmental Conservation (New York Federal Aid in Sport Fish Restoration Program); U.S. Department of Agriculture, Animal and Plant Health Inspection Service, Wildlife Services; U.S. Geological Survey, New York Cooperative Fish and Wildlife Research Unit; Oneida Lake Fish Cultural Station; The Jack H. Berryman Institute; and by a grant from the U.S. Fish and Wildlife Service, Sport Fish Restoration. We thank the following individuals for their invaluable contributions to this effort: John Forney, Connie Adams, Milo Richmond, Tom Brooking, Scott Krueger, Ken Preusser, Justin Gansowski, Martin Lowney, Travis DeVault, Rich Chipman, Carl Cranker, Elizabeth Cranker, Bridger and Kelly Thompson, and the host of students and researchers who have contributed to long-term fish and waterbird monitoring efforts at the Cornell Biological Field Station. We also thank Milo Richmond, Richard Beamish, and three anonymous reviewers for their comments, which improved the manuscript. Mention of trade names or commercial products in this manuscript does not imply endorsement or recommendation for use by the U.S. Government. This is contribution number 290 from the Cornell Biological Field Station and is also a product of the New York Cooperative Fish and Wildlife Research Unit.