Double-crested cormorant predation on northern pike in the Eastern Basin of Lake Ontario and the Upper St. Lawrence River (Phalacrocorax auritus, Exox lucius, Esox masquinongy)

In the Eastern Basin of Lake Ontario and the Upper St. Lawrence River, double-crested cormorant (Phalacrocorax auritus) predation on northern pike was evaluated by the analysis of their diet based on fish otoliths recovered from cormorant pellets, a cormorant time-activity study and bioenergetic modeling to predict consumption. Esocid otoliths, from a known species reference collection for northern pike (Esox lucius ) (N = 20) and muskellunge (Esox masquinongy) (N = 20) were shown to have morphometric differences that could be used to identify otoliths removed from fresh fish samples (p = 0.008). Species identity of unknown esocid otoliths recovered from cormorant pellets could not be classified as northern pike or muskellunge due to otolith erosion. Based on northern pike and muskellunge abundance ratios (66:1) in the Lake Ontario-St. Lawrence River, it was assumed that esocid otoliths recovered from cormorant pellets were northern pike. Predicted fish size and age from cormorant pellets indicated that primarily age 1–2 fish (N = 200) were consumed and the maximum size of northern pike was 475 mm (SD = 56.5). However, erosion of cormorant pellet collected otoliths likely produced underestimations of northern pike size and age in the diet. The time-activity study during nesting indicated a high degree of variability in behaviors between individual birds for the pre-chick and chick period. The estimates of daily energy expenditures were 1118.6 kJ/day for the pre-chick period and I365 kJ/day during the chick period in 2001. Based on bioenergetic modeling results, cormorants consumed 12.1 metric tons and 79,500 northern pike. Overall, this study indicated that a cormorant's daily energy expenditure is important for determining predictions of fish consumption.