Colonial waterbirds are an important component of Great Lakes ecosystems. In order to investigate the role these birds play in an aquatic food web, bioenergetics models, using allometric equations, were applied to breeding double-crested cormorants (Phalacrocorax auritus) and their offspring in the Beaver Archipelago, northern Lake Michigan. These models were parameterized using detailed information collected during the 2000 and 2001 breeding seasons, as well as literature values. Each breeding season was divided into stages in the models to reflect changes in cormorant diet and population size documented in the study area. The models estimated the total prey biomass consumed as 1,445 metric tonnes (mt) of prey in 2000, and 1,585 mt of prey in 2001. Each year, the majority of the prey biomass was alewife (Alosa pseudoharengus), with these fish comprising a greater percentage of prey biomass consumed in 2001. An increase in cormorant reproductive success in 2001 may be linked to this increase in alewife biomass consumed; the breeding bird population size, however, declined in 2001 as compared to 2000. The other prey items, which are not considered to be species of commercial or sport value, were also important contributors to bird diet and consumption of these species did tend to vary from year-to-year. Overall, the application of bioenergetics models allows for greater understanding of the role of cormorants as predators and as energy links in the system.
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Vol. 34 • No. 1