Interactions of Double-crested Cormorants (Phalacrocorax auritus) and fish in a constricted shallow basin near a cormorant nesting site (Young Island) in Lake Champlain, Vermont were studied. Much of the basin was dominated by submerged aquatic vegetation (SAV), which led to the investigation of how fish residing among SAV could affect cormorant foraging. Data were collected on foraging flocks, individual birds and their stomach contents, the fish community, and SAV from May to September. Fish were most abundant in August and September; they did not migrate from the basin during the warmest time periods or the greatest densities of SAV. Although many fish species (N=28) were collected, Yellow Perch (Perca flavescens) numbers comprised almost half of the fish community and were the major prey, based on numbers, in cormorant stomachs. Double-crested Cormorants regularly foraged in the basin until early August when they began foraging in more distant areas. The shift occurred soon after the peak in SAV biomass and height, which covered essentially the entire area previously frequented by foraging birds. The results indicate that cormorants expand their foraging range when prey become less available. Thus, dense SAV can indirectly contribute to the expansion of Double-crested Comorants, adding complexity to strategies for managing cormorant numbers throughout the lake.
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Vol. 32 • No. 3