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Researchers analyze cormorant diets in lower Green Bay

Over the past decade, Green Bay”s perch fishery has plummeted while double-crested cormorants have thrived. Cormorants eat fish, including perch, and a lot of people are wondering whether hungry cormorants are contributing to the perch decline.

Researchers at the UW-Madison”s Department of Wildife Ecology are helping to answer that question. Wildlife ecologist Scott Craven and graduate researcher Sarah Meadows are midway through a two-year study of cormorant diets in the Green Bay area.

In 2004, 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 at Cat Island.

“To get a valid sample, we needed to collect birds that were distributed over time and space,” Meadows says. Following approval from the U.S. Fish and Wildlife Service, employees of the USDA”s Wildlife Services program shot 436 cormorants between mid-May and mid-September. Meadows noted that the colony size increased in 2004 despite those losses; it contained about 4,000 breeding adults at the end of summer.

Meadows has analyzed the stomach contents of the collected birds. The results: cormorants eat fish – a lot of fish, and nothing but fish.

Meadows found 80 fish in one bird”s stomach. Most of the birds were collected with full bellies after returning from feeding. Some were so full they had fish backing up their throats and oozing out of their mouths. One had swallowed a 16-inch sucker that weighed 1.5 pounds (cormorants weigh five to six pounds; this is the equivalent of a small human wolfing down a large turkey).

Of the 4,712 fish Meadows counted, 1,743 were perch. “Is this a significant number? That depends on how many perch are out there,” she says.

John Netto of the U.S. Fish and Wildlife Service and Wisconsin Department of Natural Resources biologist Justine Hasz used Green Bay yellow perch data collected by the Wisconsin DNR over the past 25 years to develop a yellow perch population model for Green Bay. Meadows will plug her data into this model, and hopes to have an answer to the significance question before the beginning of the 2005 field season. “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 in Green Bay,” she says.

Perch numbers in the cormorants” stomachs peaked in mid-June, then dropped off as the perch moved from shallow to deeper water. By mid-July, other species were much more common in the birds” stomachs. Meadows also noted seasonal peaks in numbers of gizzard shad and round gobies. That”s not all bad in the case of gobies, an invasive exotic that causes problems in U.S. fisheries.

In addition to the perch, Meadows counted 1,348 gizzard shad, 524 spottail shiner, 545 round goby and 232 alewife. Walleye, white bass, white perch and white sucker together comprised about 5 percent of the prey species by numbers. By weight, perch comprised about 17 percent of the diet; white sucker, 40 percent; and walleye, 12 percent. A few other species showed up in small numbers.

Meadows found no smallmouth bass, trout or salmon. The lack of trout and salmon suggests that these cormorants fed only in southern Green Bay, she notes.

While cormorants were listed as endangered species in Wisconsin until 1982, the birds began their recovery in the 1970s following the ban on DDT. “The breeding population of cormorants in the Green Bay-Lake Michigan region increased 33 percent per year between 1973 and 1997, but has recently appeared to begin stabilizing,” Meadows says. “Counts of the birds on the Cat Island colony I am working on have been between 1,500 and 2,000 breeding pairs for the last few years.”

Double-crested cormorants are currrently protected under the Migratory Bird Treaty Act, but the U.S. Fish and Wildife Service recently issued a Public Resource Depredation Order that permits state wildlife agencies, Native American tribes and USDA officials in 24 states (including Wisconsin) to control cormorants without a federal permit. The birds have caused serious problems for catfish aquaculturists in southern states, and aggressive cormorant control programs are underway in several parts of the country to protect aquaculture industries and wild fisheries.

After a series of dismal hatch years, Green Bay”s perch pulled off a spectacular hatch in 2003, so fisheries researchers are especially interested in how many perch from that year class are showing up in cormorants.

“If another year of food-habits 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,” Craven says. “If not, cormorants and perch should be able to co-exist in the Green Bay ecosystem.”

This work is funded primarily by the Wisconsin DNR using Fox River NRDA settlement funds set aside by former department secretary Darrell Bazzell. Cooperators include the Wisconsin DNR (Justine Hasz and Bill Horns, Great Lakes fisheries coordinator); Tom Erdman at the UW-Green Bay; the U.S. Fish and Wildlife Service (Ken Stromborg and John Netto); and the USDA Animal and Plant Health Inspection Service, Wildlife Services Program (Dave Nelson). This research was encouraged by the Lake Michigan Fisheries Forum, a citizen advisory panel that reviewed the Green Bay yellow perch situation through a series of public workshops in 2002.