Wisconsin Perch Anglers Face Tough Competition

USSFN:  1/19/05 - (Madison) - Preliminary research to determine if the resurgence of double-crested cormorants is contributing to the decline of yellow perch in Green Bay found the waterbirds do eat perch, but officials say more research is needed determine if their feeding is significant enough to have an impact on perch populations.

University of Wisconsin researchers are mid-way through a two-year, state-funded study to learn whether the resurgence of double-crested cormorants, a species teetering on the brink of extirpation a generation ago, is contributing to the decline of yellow perch in Green Bay.

Researches examined the stomach contents of 436 birds in 2004 that revealed that of the 4,712 fish counted, 1,743 were yellow perch. That number was higher than for any other fish species counted, but yellow perch ranked second by weight as a food source, at 17 percent, behind white suckers, which comprised 40 percent.

The study will now focus on whether the number of perch the birds eat is significant, says Sarah Meadows, a UW graduate researcher and the study's principal investigator.

“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,” Meadows says. She hopes to have an answer to the significance question after data have been analyzed from the 2005 field season

The Department of Natural Resources contracted with Meadows and Scott Craven, a UW wildlife ecology professor, to examine the food habits of the cormorants nesting in Green Bay, where more than 80 percent of Wisconsin’s breeding population occurs.

The study was prompted by concerns among commercial fishermen and recreational anglers that cormorants prey on yellow perch, which may be causing or contributing to a 90 percent drop in yellow perch populations between 1988 and 2000. Declining yellow perch populations spurred the DNR to reduce sport bag and commercial harvest limits for yellow perch, at the same time populations of the federally protected bird rebounded in Wisconsin and elsewhere.

A quarter-century after cormorants were added to the state endangered species list, populations of the bird are at historic highs, thanks to reduced contaminant levels, ample food in their summer and winter ranges, federal and state protection, and the installation of cormorant nesting platforms in the 1970s. There are more than 1 million cormorants in the United States and Canada, and more than 11,000 breeding pairs in Wisconsin.

The UW researchers’ work in Green Bay focuses on cormorants in an area running from the mouth of the Fox River at Green Bay northwest to Peshtigo Point and northeast to Sturgeon Bay, and includes a cormorant breeding colony at Cat Island.

After getting 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 2,000 breeding pairs at the end of summer.

Meadows examined the contents of their stomachs, noting species, sizes and weight. In addition to the 1,743 perch she found, she counted 1,348 gizzard shad, 524 spottail shiner, 545 round goby and 232 alewife. A combined 200-plus walleye, white bass, white perch and white sucker were also found, together totalling 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 fish species showed up in small numbers.

Meadows noted that perch numbers in the cormorants’ stomachs peaked in mid-June, then dropped off as the perch moved from shallow to deeper water, Meadows says. By mid-July, other species were much more common in the birds’ stomachs. She also noted seasonal peaks in numbers of gizzard shad and round gobies, an invasive species.

Researchers will now try to determine whether cormorants eat enough perch to prevent the robust numbers of young fish hatched in 2003 and 2004 from reaching a fishable size. To learn the answer, Meadows and Craven will plug their data into a computer model that U.S. Fish & Wildlife Service scientist John Netto developed drawing on yellow perch data that DNR fisheries biologists Brian Belonger and Justine Hasz had collected over the last 25 years.

Bill Horns, DNR Great Lakes fisheries specialist, says the research is very timely, and provides a great opportunity to develop a much better, more comprehensive understanding of the cormorants' diet and any effects on yellow perch that could help inform management actions. The study 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.

The federal government has given states the authority to control cormorant populations, but DNR wanted a rigorous study to determine more definitively what role, if any, cormorants were playing in the yellow perch decline. The limited existing data suggested perch were a small part of the birds' diet.

"The timing of this study is really fortunate," Horns says. "We had the money set aside, excellent scientists available with an interest in this work, a strong year class of yellow perch coming through, and a well developed model of the yellow perch population."

Money for yellow perch work was earmarked from settlements reached with paper companies for damage caused to natural resources from historic discharges containing PCBs.

Craven conducted research in the early 1980s into the eating habits of cormorants nesting on the Apostle Islands, and Meadows has studied the food habitats of different kind of cormorant in New Zealand.

DNR's yellow perch data and model has given biologists a good understanding of the population dynamics of yellow perch in general, and the strong yellow perch year classes of 2003 and 2004 provide a large enough population of young perch to be able to measure any significant effects of cormorant preying on this group.

Horns says that if the study demonstrates that cormorants are a problem for perch and the control measures would help, the agency would probably turn to USDA’s Wildlife Services for help with control under a Public Resources Depredation Order issued in 2003 by the Fish and Wildlife Service.

Fisheries biologists and commercial and recreational anglers have also cited other changes on Green Bay in the last decade or so as possible reasons for the yellow perch decline. The proliferation of invasive species, including zebra mussels and white perch, improved water clarity, and lower water levels are among the many changes on Green Bay since yellow perch populations started declining in the late 1980s.

FOR MORE INFORMATION CONTACT: Bill Horns (608) 266-8782, Sumner Matteson, (608) 266-1571