Potato Island on Lake Vermilion is a study in black and white. Cormorants and herring gulls nest on the barren, half-acre island in the eastern basin of the lake.
Herring gulls have been nesting on Lake Vermilion in large numbers for years. Double-crested cormorants are native to Northeastern Minnesota and had existed in low numbers on Lake Vermilion until just the past few years. Now, their population is growing significantly. This year 434 nests were counted on Potato Island, up from 338 nests last year, according to the Minnesota Department of Natural Resources.
That's a concern for many anglers on the lake, who believe that cormorants could someday reduce the lake's walleye fishery by preying heavily on yellow perch, a favorite forage fish of walleyes.
"It's been controversial," said Duane Williams, DNR large-lake specialist for Lake Vermilion. "A lot of folks here have been pushing for pre-emptive control, before we get a problem."
On Leech Lake, where cormorant numbers reached about 10,000 in 2004, the Leech Lake Band of Ojibwe has supervised the shooting of about 3,000 cormorants a year since 2005, reducing the fall population to about 2,000 birds. That kind of culling can be done only with the permission of the U.S. Fish and Wildlife Service because cormorants are protected under the Migratory Bird Treaty Act.
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Representatives from the Sportsmen's Club of Lake Vermilion have twice met with representatives from Leech Lake to learn about that lake's experience with cormorants. Some people on Lake Vermilion would like to see the cormorant population reduced there, too, before it begins to cause damage to the fishery.
"You don't wait until the house is burning to buy fire insurance," said Mel Hintz, president of the Sportsmen's Club of Lake Vermilion.
DNR fisheries biologists are concerned that the lake's yellow perch population has been lower than normal, although not beyond historic lows, for the past four years, said Edie Evarts, DNR area fisheries supervisor at Tower. The agency surveys fish populations with gill nets each fall.
"I think our gill-net assessment this fall is going to be kind of key," the DNR's Williams said. "If perch are down again, we'd have to take a pretty hard look at it."
Evarts agreed, saying the DNR is in the "watch, not watch-and-wait mode" with the cormorant issue.
"If we make the link that cormorants are impacting perch or walleye," Evarts said, "we'd be in line to do a request for cormorant control with the Fish and Wildlife Service. But we need that data."
Learning from Leech Lake
Statewide, the cormorant population is stable, said Gaea Crozier, DNR non-game wildlife specialist at Grand Rapids. Surveys done in 2004 and again in 2010 showed no significant change in numbers, but local populations have varied as the birds move around, Crozier said.
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Cormorants often are cited by anglers as a possible detriment to fisheries, which may or may not be the case, biologists say.
"Folks are very quick to infer that one cormorant means the end of a lake," said Doug Schultz, DNR area fisheries supervisor at Walker, on Leech Lake. "That's simply not the case. Currently, we are managing the population on Leech to 2,000 birds, not zero, and the fishery has remained excellent."
On Leech Lake, research has shown that 60 percent of a cormorant's diet, by weight, is made up of yellow perch, Schultz said. Just 5 percent is made up of walleyes. No similar studies have been made at Lake Vermilion.
Success story
Cormorant populations dropped dramatically, as did those of eagles and peregrine falcons, when the use of DDT was widespread as an insecticide in the United States. The chemical weakened their eggshells, reducing reproductive success. After DDT was banned in the early 1970s, all of those species began to make comebacks.
Aquaculture farms in the South provided an additional winter food source for the migratory cormorants, and their population has continued to expand.
"From a conservation perspective, the recovery of the population is a remarkable one," Schultz said. "It's a success story. The focus has since shifted to management, and the U.S. Fish and Wildlife Service oversees where and how that takes place."
Federal protection
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Although cormorants are federally protected under the Migratory Bird Treaty Act, the act allows control of cormorants in specific situations, one of which is damage to a public resource, Williams said.
"There needs to be relatively strong evidence that there's an effect on the resource and that it can be attributed to cormorants," the DNR's Crozier said.
On Leech Lake, cormorants are killed with shotguns by employees of the Wildlife Services division of the U.S. Department of Agriculture, Schultz said. The goal is to keep the adult population at about 500 breeding pairs. Each nest produces an average of two young, Schultz said, yielding a fall population of about 2,000.
Nesting colonies of cormorants also have been established on Mille Lacs Lake, Lake of the Woods, Kabetogama Lake and other large waters around Minnesota.
Difficult to prove
While some anglers have been quick to point out that culling cormorants on Leech Lake has led to much-improved walleye fishing in recent years, DNR officials say that's difficult to say with certainty. In addition to beginning cormorant control in 2005, the DNR also began annual stocking of 7.5 to 22.5 million marked walleye fry in order to better understand rates of natural reproduction on the lake.
Also starting in 2005, a slot limit was established on the lake, requiring anglers to release walleyes from 18 to 26 inches long. All of those practices remain in effect, so it is hard to pinpoint the effectiveness of any single strategy, Schultz said.
But on Lake Vermilion, Evarts remains ready to seek cormorant control if necessary.
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"We've learned a lot from Leech," Evarts said. "We feel we're on top of it. We would take action sooner rather than later."
The DNR plans to analyze results of this fall's netting assessments quickly and share them with the public as soon as possible in a public meeting, Evarts said.
