ST. PAUL - Double-crested cormorants will be controlled at Lake Vermillion this spring in an effort to limit the number of birds that eat yellow perch and potentially small walleye, according to the Minnesota Department of Natural Resources (DNR).
The action follows several years of surveys that show a consistently lower perch population in the lake's east bay.
Perch are the lake's primary forage fish for walleye.
"We believe cormorant predation is the likely cause of fewer perch being caught in survey nets," said Don Pereira, DNR fisheries policy and research manager. "This conclusion is based on the 'weight of evidence' that came from analyzing fish population data."
Double-crested cormorants established 32 nests on Vermillion's Potato Island in 2004.
The colony has steadily increased. In 2012, 424 nests were counted, nearly a 30 percent increase from 2011.
Lower perch counts were first noticed in 2007 and have remained depressed ever since.
Reduced perch numbers have not resulted in significantly lower walleye counts in the 39,000-acre St. Louis County lake.
Edie Evarts, DNR Tower area fisheries supervisor, said the upcoming cormorant control is designed to reduce the possibility of lower walleye numbers in the future.
"Limited control measures are a reasonable approach to insure cormorant impacts to the perch population do not result in a declining walleye population as well," Evarts said. The agency is applying what it has learned about cormorant impacts on fish populations over the past decade, she said.
The proposed control will consist of culling 10 percent of the adult birds present and oiling the eggs of all nesting pairs.
Oiling prevents the eggs from hatching.
Together, this approach controls existing numbers of birds, eliminates new production and reduces fish consumption that would have occurred from feeding and raising young birds.
This initial control strategy will be monitored for effectiveness by measuring perch abundance in annual netting surveys and counting the number of nesting pairs of cormorants each year.
Future control recommendations will be adjusted by the response of perch abundance to the control implemented.
The control is being implemented under a public resource depredation order administered by the U.S. Fish and Wildlife Service.
Cormorants are native to Minnesota.
The statewide population is estimated at about 40,000 birds.
Like bald eagles and other fishing-eating birds, their abundance has increased in recent decades due to the elimination of the pesticide DDT, which had a negative impact on reproduction, and protection under the Migratory Bird Treaty Act.
In most places where colonies exist, popular fisheries have not been affected.