Double-Crested Cormorant Feeding in Multiple Lake Environments: Intrinsic Markers Reveal Several Prey Sources and Frequent Site Switching by Breeding Birds

Abstract

Conflicts between piscivorous cormorants (Phalacrocorax spp.) and humans over fisheries resources occur worldwide. To determine the level of cormorant impacts on fisheries, fish biomass removals are estimated but typically assume only one source of prey near the roost or breeding colony. Cormorants can fly long distances (>30 km) to forage, possibly resulting in fish removal being spread out over several areas within large lakes, or among other bodies of water. I examined the diet and feeding locations of double-crested cormorants (Phalacrocorax auritus; hereafter cormorants) breeding in a multiple lake environment in north-central Saskatchewan, Canada. A majority of their diet was composed of non-sport and non-commercial fish species. Yellow perch (36-196 mm) were a common prey item for cormorants, making up 30-55% of fish biomass consumed, and were therefore used as a model species to determine sources of cormorant prey. Comparison of carbon (δ13C) and nitrogen (δ15N) stable isotopes values in yellow perch collected by cormorants and those from known locations revealed several prey sources (different lakes and areas within lakes) and frequent, large-scale switching of feeding locations on a daily and seasonal basis. These findings were also substantiated by cormorant surveys throughout the study area using transect counts, and examination of flight directions to and from a major breeding colony. Prey from areas well-removed from the breeding colony lake (up to 30 km away) were an important part of cormorant diet, representing 70% of fish fed to nestlings in 2010 during the early chick rearing stage. Cormorants began to feed closer to the breeding colony during the late chick rearing stage. Linear discriminant analysis revealed classification accuracy of known location yellow perch to range from 69% to 86%, suggesting that stable isotopes of carbon and nitrogen performed well as intrinsic markers of fish source in my study area. My research clearly shows that cormorant consumption of fish happens at a variety of locations, negating the value of the traditional approach of estimating biomass removal from the breeding colony lake as the guideline for making fisheries management decisions. Knowing where prey fish come from and estimating relative proportions taken from various sites will refine biomass removal estimates to help managers better understand potential interactions between cormorants and fisheries. In addition, my research shows that cormorants make decisions about foraging and feeding locations that are independent of breeding colony site selection; i.e., they often use sites well removed from the breeding colony. Factors that influence cormorant foraging locations need to be more thoroughly identified to advance our understanding of their ecology, and to aid fisheries management.