The breeding population of Double-crested Cormorants (Phalacrocorax auritus) increased in the Beaver Archipelago of northern Lake Michigan, while simultaneously, a Smallmouth Bass (Micropterus dolomieui) population in the region declined. However, the role that cormorants played in this decline has remained uncertain. During summer 2003, VHF radiotelemetry and rafting locations were used to determine whether birds foraged primarily in bass habitat. The foraging activities of ten breeding cormorants were monitored by radiotelemetry from both land and water on a daily basis, weather permitting, throughout the breeding season. In addition, cormorant foraging raft locations were documented by boat survey throughout the breeding season. Radiotelemetry indicated that cormorants typically foraged 2.5 km away from their colonies, at the northeastern end of Beaver Island. This area overlaps with the area determined from rafting locations; however the latter were centered further south. These data allow for better estimation of foraging patch use by archipelago cormorants and indicate that birds are not typically concentrating their foraging in Smallmouth Bass habitat. Cormorants forage in open water in areas with dramatic changes in depth, and these locations agree with diet data that indicates that Alewife (Alosa pseudoharengus) are important prey. Data gathered has led to a better understanding of cormorant foraging patterns in the study area, and indicate that if cormorant foraging areas remain spatially separate from bass habitat, the probability of birds directly impacting these fish is low.
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Vol. 29 • No. 4