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October 25, 2011

GREEN BAY - The nearly quarter-century population growth of double-crested cormorants in the Wisconsin waters of Green Bay and Lake Michigan may have ended. Numbers had been on the rise since 1986, but in 2011 the population on islands where control efforts were conducted showed an 18 percent decline from the 2009 peak of 15,227 nests. Management efforts resulted in 2011 nest numbers of 12,534.

"I anticipate that this year's removal of around 2,500 adult birds coupled with ongoing egg-oiling efforts will result in future reductions in the cormorant population, and bring us closer to the goals set for the managed islands," said Tammie Paoli, a fish biologist with the Department of Natural Resources in Peshtigo.

Cormorant numbers are estimated by nest surveys completed every few years on approximately 10 islands in Green Bay and Lake Michigan/Door County as a cooperative effort with Wisconsin DNR, U.S. Fish and Wildlife Service, and U.S. Department of Agriculture, Animal and Plant Health Inspection Service/Wildlife Services.

Cormorants on Green Bay island
Cormorant nesting colony on Cat Island, near the mouth of the Fox River, Green Bay.
WDNR Photo

Cormorants, native to North America and the Great Lakes, were scarce in the 1950s through the 1970s. Their populations had declined as a result of habitat loss and the use of DDT, which contributed to eggshell thinning and cross-bill deformities. They were listed as a state endangered species in 1972, but numbers rebounded reaching problem levels on the islands in this century.

Some research suggests cormorant predation negatively affects yellow perch abundance in southern Green Bay. Fisheries biologists collect and analyze survey and creel data annually to investigate the impacts of double-crested cormorant management on fish populations. This information is used to determine future management actions and strategies.

DNR fisheries surveys documented strong year classes of yellow perch during the last eight years. Still, the adult perch population has not rebounded as expected.

Declines in Green Bay brown trout harvest also coincide with increasing cormorant numbers. This prompted DNR to modify stocking strategies for brown trout to try reducing post-stocking mortality. Fisheries biologists are hopeful a combination of fewer cormorants, more forage fish such as alewives in recent years, and adjustments to stocking strategies will result in improved harvest numbers for brown trout.

Additional background information about the cormorant's history in Northeast Wisconsin can be found in a February 2008 Wisconsin Naturual Resources magazine story Cormorant conundrum.

FOR MORE INFORMATION CONTACT: Tammie Paoli, Fish Biologist, Peshtigo (715) 582-5052 or Trish Ossmann, Public Affairs Mgr., NE Region, Green Bay (920) 662-5122

Last Revised: Tuesday, October 25, 2011

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