Cormorant diet is subject of study
Written by Green Bay Post-Gazette   
Tuesday, 11 January 2005 10:28
Double-crested cormorants, those hook-billed fish eaters many anglers love to loathe, have long been blamed for the decline of yellow perch fishing in Green Bay.

"It?s like: They?re big, they?re black, they?re ugly ? let?s kill ?em," said state Department of Natural Resources biologist Paul Peeters of Sturgeon Bay, describing the way some fishermen feel about the protected predators.

But the blame might have been hasty.

In a historic effort to find out if anglers? concerns are warranted, University of Wisconsin-Madison graduate researcher Sarah Meadows ? working under the direction of UW wildlife ecologist Scott Craven ? dug into the bellies of more than 400 Green Bay cormorants shot under a federal permit last year.

Her findings: More than 1,700 yellow perch, a mix of many sizes.

"Is this a significant number?" Meadows said. "That depends on how many perch are out there."

According to the study released today, walleye, white bass, white perch and white sucker together made up about 5 percent of the prey species by numbers. By weight, perch made up about 17 percent of the diet; white sucker, 40 percent; and walleye, 12 percent. A few other fish species showed up in small numbers.

Meadows found no smallmouth bass, trout or salmon. The lack of trout and salmon suggests that the birds fed only in southern Green Bay, where few trout and salmon exist.

Indeed, Peeters said, with a record 2003-year class of perch finning their way around the bay, it might have been simply a case of perch being the most abundant prey species available.

Meadows, halfway through the two-year project, plans to plug her data into a 25-year population model of yellow perch on Green Bay.

"I?d be reluctant to draw any conclusions before the model has factored in cormorants as a source of yellow perch mortality, so we can have an accurate picture of the effect they?re having on the perch populations," she said.

Craven said that if another year of information and the computer modeling suggest that cormorants are an important factor in yellow perch mortality, then some form of active control may be justified.

"If not, cormorants and perch should be able to co-exist in the Green Bay ecosystem," he said.

Cormorants are protected under the Migratory Bird Treaty Act, but last year the U.S. Fish and Wildlife Service issued a Public Resource Depredation Order that permits state wildlife agencies, Native American tribes and United States Department of Agriculture officials in 24 states, including Wisconsin, to control the birds without a federal permit.

After getting approval from the Fish and Wildlife Service, employees of the USDA?s Wildlife Services program shot 436 cormorants on Green Bay between mid-May and mid-September.

Meadows monitored the birds from their spring arrival to fall departure, noting their numbers, locations, and what they ate. The study area runs from the mouth of the Fox River at Green Bay northwest to Peshtigo Point and northeast to Sturgeon Bay.

The 670-square-mile area includes the cormorant-breeding colony on Cat Island at the southern end of Green Bay. Meadows said the colony increased in 2004 despite those losses; it contained about 4,000 breeding adults at the end of summer.

Most of the birds shot had bellies full after returning from the day?s feeding.

One had 80 fish in its stomach, another a 16-inch white sucker that weighed 1.5 pounds ? the equivalent of small human wolfing down a large turkey, Meadows noted.

DNR Great Lakes specialist Bill Horns said the state is using money earmarked for Green Bay yellow perch restoration from the Fox River Natural Resources Damage Assessment settlement funds set aside by former department secretary Darrell Bazzell.

"I know there are concerns about (cormorant) predation on stocked trout and salmon, but this study will not be broadened to take on that question," Horns said. "We just don?t have the funds to support that research."

Perch numbers in the cormorants? stomachs peaked in mid-June, and then dropped off as the fish moved from shallow to deeper water. By mid-July, other species were much more common in the birds? stomachs.

Cormorants have caused serious problems for catfish aqua culturists in southern states, and aggressive cormorant control programs are under way in several parts of the country to protect aquaculture industries and wild fisheries.

In northern Lake Huron, for example, Michigan officials knew from Canadian research that breaking eggs wouldn?t help, as birds will re-nest. But oiling the eggs plugs the tiny holes in the porous shells, killing the embryo. Parent birds continue to incubate in vain.

Michigan?s oiling plan last year was successful, with only 41 eggs hatching out of 3,220 nests.

Since cormorants are long-lived birds, Michigan officials also set a goal of killing about 15 percent of the adult birds out of an estimated 6,400. They came close, taking 909.

Cormorants were listed as an endangered species in Wisconsin until 1982, but their recovery began in the 1970s following a ban on DDT.

Breeding populations of cormorants in Green Bay and Lake Michigan increased about 33 percent a year between 1973 and 1997, Meadows said, but recently appeared to begin stabilizing.

The diet study was prompted by concerns among commercial fishermen and recreational anglers that cormorants are preying on yellow perch and causing or contributing to a 90 percent drop in yellow perch populations between 1988 and 2000.

Prior to the study, DNR avian ecologist Sumner Matteson said the limited data the DNR had suggested perch are a small part of cormorants? diets.

"Until we have this information and more information about other factors affecting the Green Bay fisheries, we are not going to control or manage them," Matteson said.

Said Peeters, "If they?re not truly a problem, it would be a waste of money."

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