The invasive round goby (Neogobius melanostomus) in the diet of nestling double-crested cormorants (Phalacrocorax auritus) in Hamilton Harbour, Lake Ontario

Populations of the invasive round goby (Neogobius melanostomus) have expanded dramatically since their discovery in the Laurentian Great Lakes in 1990. The abundance of these fish and possible competitive displacement of native species from aquatic food webs suggest that they will become an important prey item for predatory birds and sport fish. To date, there is very little information on the predation of round gobies by piscivorous birds in the Great Lakes. We used an abdominal palpation technique to stimulate regurgitation by double-crested cormorant (Phalacrocorax auritus) chicks in ground nests from colonies in Hamilton Harbour, western Lake Ontario, during the 2002 nesting season. We collected and identified all fish species present in regurgitated boluses. For tree-nesting birds, we collected fish that were regurgitated and fell to the ground as a result of targeted disturbance of particular nest trees. At all locations and times, alewife (Alosa pseudoharengus) was the most abundant fish species present by a large margin. The second most abundant species was the round goby, which was present in the regurgitated stomach contents of chicks in a total of 18 percent of nests surveyed, and made up 1.8 to 11 percent of all individual fish specimens identified. Our results show that the round goby is already an important food item for breeding cormorants in Hamilton Harbour, despite relatively recent establishment of goby populations in western Lake Ontario. Fish species of sport or commercial interest were detected in our samples with extremely low frequency (< 0.1%).