Cooperative Fish and Wildlife Research Units Program: Vermont
Education, Research and Technical Assistance for Managing Our Natural Resources

Cormorants are a recent addition to the avifauna of Lake Champlain and have since become a concern to wildlife managers. They first nested on Young Island, Vermont, in 1981; they expanded to Four Brothers Islands, New York, by 1984, and peaked at > 20,000 birds in 1999. Population increases were associated with destruction of vegetation and displacement of other birds from nesting colonies. In 1999, Vermont Department of Fish and Wildlife and U.S.D.A. Wildlife Services began spraying cormorant eggs with corn oil to prevent hatching and limit recruitment on Young Island. By 2001, this colony decreased in size by 58% and the Four Brothers colony increased by 78%, making it the largest colony on Lake Champlain. In other locations, cormorant populations are managed to limit negative effects on fish populations. In 2001, we began studies to 1) determine if oiling eggs increases dispersal away from Young Island, 2) estimate the quantity of fish consumed by cormorants, and 3) develop tools to identify optimal strategies for cormorant management based upon knowledge of their population dynamics. To determine how cormorants respond to egg oiling, we implemented an experiment on Young Island from 2001-2004. The experiment included oiling eggs in some areas of the cormorant colony and allowing cormorants to reproduce in other areas. We individually marked birds and tracked their movements between Young and Four Brothers Islands. We found that oiling eggs increased the proportion of cormorants that dispersed away from Young Island by 0.03. Predation of eggs by gulls, which occurs when eggs are oiled during daytime, increased dispersal by 0.20 when frequent day-time oilings were applied. Cormorant colonies on Lake Champlain interact through dispersal of individuals, and management actions increased dispersal rates. We constructed an energetics model for cormorants from Young and Four Brothers Islands to estimate fish consumption for 2001-2003. The model estimated energy requirements by combining data on duration of time cormorants spent in various activities with energy requirements for these activities. Cormorants from Young Island rested more and traveled shorter distances to foraging locations than cormorants from Four Brothers Islands. We estimated that fish consumption ranged from 320,000 – 416,000 kg of fish for Young Island and 877,000-1,066,000 kg on Four Brothers Islands. Differences in fish consumption were due to differences in energy demands and population sizes between colonies. An unanticipated consequence to management on Young Island was changes in patterns of fish consumption by cormorants across Lake Champlain. We also developed a tool that wildlife managers can use to identify optimal strategies for cormorant management on Lake Champlain. This tool was based on a population model for cormorants, which was parameterized from an analysis of observations of marked cormorants from 2001-2006. We projected population changes expected when individual colonies were managed. These projections were then incorporated into a decision analysis to identify optimal management strategies for a given objective and a set of alternative management actions. Optimal management strategies to reduce sizes of cormorant populations included management at multiple colonies.