A new study links the rising population of double-crested cormorants in the Les Cheneaux Island to the collapse of the yellow perch fishery here.

Dave Fielder, fisheries research biologist for the DNR based in Alpena, published his findings in the September issue The Journal of Great Lakes Research and noted in a release, "While fish abundance is a result of many different factors, we have learned that the rise of an abundant predator like cormorants can, in fact, substantially reduce some fish populations."

Mr. Fielder's study suggests that the upswing in the cormorant population at Les Cheneaux, from zero nests in 1980 to 5,500 in 2004, may be the single most important factor in the collapse of the yellow perch fishery.

"I examined trends in perch reproduction, weather variables, water levels, and fishing activity," Mr. Fielder noted. "None of those factors were as influential as cormorant predation in contributing to the collapse of the perch population."

His findings led to a cormorant control effort in the Les Cheneaux Islands implemented by Wildlife Services of the U.S. Department of Agriculture. Oil was poured on cormorant eggs to stop their development, for example, and some birds were shot. The number of nests dropped to about 500.

He is continuing to monitor how the perch population at the Les Cheneaux Islands is responding to the management efforts.

Mr. Fielder's report contradicts an earlier study that found cormorants were not eating perch in sufficient numbers to impact their population. A study conducted in the mid-1990s by the Michigan Department of Natural Resources, the U.S. Fish and Wildlife Service, and the University of Michigan concluded that the perch population was not a victim of predatory fisheating cormorants, as fishermen had long suspected. That report did note that the birds were feeding on perch in the spring, but preferred alewives, which have since suffered significant population reductions.

James Diana of the University of Michigan's School of Natural Resources and the Environment, and Mr. Fielder's former professor, stands by his decade-old findings, although he concedes that conditions could have changed between the time of his study and of his student's.

It's possible, he suggested, both studies were right for their time, or that a swell in the number of alewives, which cormorants also like, spared the perch.

"Maybe cormorants are having a bigger effect right now," he said. "The biodiversity of the Great Lakes is so vast that it's almost impossible to determine cause and effect."

Mr. Fielder said he had the advantage of studying cormorant behavior over several years, while Mr. Diana's group focused on the single year of 1995 and, thus, was limited.Mr. Fielder's research used long term netting surveys and creel survey data - physically asking anglers what they caught - to demonstrate that cormorants account for the trends in the perch population.