Increased numbers and productivity of Double-crested Cormorants, Phalacrocorax auritus, on Lake Ontario

Abstract

From 1974 to 1982 double-crested cormorants (Phalacrocorax auritus) nesting on Lake Ontario increased from approximately 22 to 770 pairs, showing an average annual rate of increase of 56%. During the same time, productivity on Canadian colonies increased from zero to 1.3-1.4 young/pair. We believe the increased population size and productivity are the result of decreased contaminant levels. Population growth since 1974 can be explained by enhanced reproductive success and early first breeding, except in 1976, 1978 and 1979 when immigration must have occurred.