When and Where to Control a Top Predator for Benefits of the Prey: Spatial and Temporal Comparisons of Diets of Double-Crested Cormorants on Lake Champlain

Increasing numbers of cormorants on Lake Champlain have caused concerns related to potential impacts on the yellow perch population. However, with the establishment of alewife in 2003, cormorant foraging may have changed. We examined cormorant diets from four areas to assess past, current, and potential impacts of cormorants on the changing fish community. During the 2001-2002 and 2008-2009 breeding seasons, we observed spatial and temporal differences in cormorant diets. Yellow perch dominated diet composition during 2001-2002 at Young Island during all reproductive periods. Four Brothers Islands diet composition in 2002 varied with reproductive period. Post-alewife period, alewife were predominant in diets at Four Brothers and the South, with yellow perch comprising a high proportion of diets at Young Island. MANOVA results confirmed differences among sites, reproductive period, and the interaction of these factors when describing diet compositions for the post-alewife years. PCA results denoted a general shift in cormorant diets after alewife. Our study demonstrated that the diet of piscivorous birds may shift with a new forage species and may vary significantly within a single large water body. Accordingly, efforts to manage piscivorous birds with the intent to decrease mortality of specific fish species should be site specific when possible.