Double-crested cormorants (Phalacrocorax auritus) of Leech Lake, Minnesota: Temporal variation of diets and assessment of differential prey selection in adults

In 1998, after 160 y without documented nesting, Double-crested Cormorants (Phalacrocorax auritus) recolonized Leech Lake, Minnesota. Competition for nesting space with the state threatened Common Tern (Sterna hirundo), and concern regarding potential walleye (Sander vitreus) and yellow perch (Perca flavescens) population declines prompted culling of cormorants and research into their diet. Stomach contents of adult and subadult cormorants were collected in 2004, 2005, and 2006 (186, 356 and 348, respectively) and from cormorant chick regurgitants in 2005 and 2006 (151 and 114, respectively). Perca flavescens (>70% composition and biomass) and shiner species (Notropis sp.) (>4% composition and biomass) were the main components of subadult and adult cormorant diets in 2004 and 2005 and the diet of chicks in 2005. However in 2006, whitefish species (Coregonus sp.) was the majority of percent biomass consumed by subadults and adults (43.4%) and a large proportion of percent biomass consumed by chicks (17.8%). In 2006 percent biomass of Corengonus sp. was different from other years for adults and subadults (P < 0.001) and chicks (P < 0.01) and may have been a product of a warming event die-off in late summer. All eight of the select common prey species varied (P < 0.01) in percent biomass between at least one of the ten delineated periods within 2004, 2005, or 2006. Differential prey selection by adults for self-feeding and provisioning for chicks was detected in Coregonus sp. (P < 0.01). While the diet of Leech Lake cormorants mainly consists of small abundant fish (P. flavescens and Notropis sp.), cormorant diet can vary within and among years. This study represents the first description of the diet of cormorants from a midwestern location other than the Great Lakes and reinforces the importance of site specific diet assessment, as the particular abundant prey species differs between locations.