Field Notes: Cormorant Conflicts
Double-crested cormorants are becoming a casualty of their own success. Nationwide, the birds have grown to an estimated population of 2 million. While this number is likely only a fraction of the historical North American population, it is big enough to cause competition between some anglers and cormorants, which can consume up to a pound of fish per day.
In May agents from the U.S. Department of Agriculture began culling cormorants on Leech Lake, one of the state's most popular walleye fisheries. The action came after an environmental assessment on the impact of cormorant control. Federal sharpshooters removed 2,200 birds during the first month of the operation.
The move reflects a nationwide trend to decrease protection on the once uncommon bird, which includes Minnesota in its natural range. Only three nesting colonies were documented in Minnesota in 1925, when the species was extensively hunted as a nuisance because of its perceived impact on local fisheries. Last summer a cooperative study by the University of Minnesota and DNR Nongame Wildlife Program found 38 active nesting colonies in Minnesota and estimated a statewide population of about 16,000 nesting pairs.
The largest colony in the state, Leech Lake in northern Cass County, had more than 2,500 nesting pairs in 2004. Biologists with the Leech Lake Band of Ojibwe counted just 73 cormorant nests on the lake in 1998, when the breeding colony was first established. The explosion in cormorant numbers has caused anglers and resort owners on Leech Lake to blame the birds for the lake's declining yellow perch and walleye fisheries.
With escalation of conflict between birds and people and increasing biological evidence that the Leech Lake fisheries were declining, the DNR, U.S. Fish and Wildlife Service, Leech Lake Band of Ojibwe, and the U.S. Department of Agriculture Wildlife Services program teamed up to examine the potential environmental impacts of cormorant control.
A 1972 amendment to the Migratory Bird Treaty Act entrusted the USFWS with protecting cormorants from the open season predator control that had decimated their numbers in the early part of the century.
As cormorant populations rebounded and human-wildlife conflicts mounted, the USFWS in 2003 granted the USDA Wildlife Services program, 24 states, and federally recognized tribes, including Minnesota and the Leech Lake Band, the power to control local cormorant populations when such agencies could document harm to other natural resources. As part of the step-down in federal protection, agencies that take actions to control cormorants are required to assess the potential impacts of control methods and to report population reductions annually to the USFWS.
"There is fairly compelling evidence that the cormorants on Leech Lake are responsible for the decline in walleye and perch numbers," said Henry Drewes, DNR regional fisheries manager. DNR data shows small walleye and mid-sized perch numbers have dramatically declined in the lake's main basin, where most cormorant foraging occurs. Meanwhile, walleye and perch numbers are normal in the lake's western bays, where cormorants are rarely seen. "Reducing the size of the cormorant colony is critical to our four-part plan to improve the Leech Lake sport fishery. We are also protecting walleye brood stock through reduced bag limits and size restrictions, stocking marked fry for research purposes, and working to preserve important habitat in the Leech Lake watershed."
The Leech Lake Band of Ojibwe has primary jurisdiction over the Leech Lake cormorant colony because it owns Little Pelican Island, where the birds nest. The site is also home to a long-standing colony of common terns, a state- and tribally listed threatened species.
Tribal biologist Steve Mortensen has monitored the tern colony since 1992 ("Uncommon Terns of Leech Lake," Nov.-Dec. 1993). In recent years he has watched the burgeoning cormorant population edge ring-billed gull nests closer to tern nests, causing terns to abandon their nests.
Some scientists recommended doing diet studies before attempting to control the birds. But according to Larry Jacobson, a third-generation owner of Leech Lake's Hiawatha Beach Resort, local businesses could not afford to wait for more research. "If we don't remove cormorants, more resorts are going to be out of business. It's as simple as that," Jacobson said. "We don't have to bring the number [of cormorants] to zero, but these birds have to be kept in balance--like anything else in nature."
Some scientists, such as University of Minnesota waterbird researcher Francesca Cuthbert, have disagreed with the level of cormorant control work. Major bird organizations, such as Minnesota Audubon and the Minnesota Ornithologists' Union, have also expressed serious concerns. "Today it's cormorants; tomorrow it's probably going to be white pelicans," said Cuthbert, who notes that the USDA Wildlife Services program is conducting preliminary research on pelican control.
The DNR and Leech Lake Band are collaborating on an intensive study that examines cormorant feeding habits. This work, coupled with continued fisheries population monitoring, will help define the number of nesting pairs the lake can maintain and still have a productive sport fishery. The research, as well as the cormorant management program, will likely continue for two or more years.
Erika R.L. Rivers, DNR information officer