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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

With wings not completely waterproof, cormorants spend time drying out after diving for food. © Rich Phalen
With wings not completely waterproof, cormorants spend time drying out after diving for food.

© Rich Phalen

February 2008

Cormorant conundrum

Island habitats feel the pressure as populations of these once-endangered waterbirds grow.

Jeff Pritzl and Paul Peeters


Feasting on an aquatic smorgasbord
A hazard to habitat
Capping the population explosion

For double-crested cormorants, the good old days in Wisconsin spanned the 1920s through the 1950s. During that time the birds slowly gained a foothold on larger bodies of water in the north, along major rivers and on the Great Lakes. Yet cormorant populations plummeted from the 1950s through the '70s, beset by pesticides, habitat loss and human intervention at nest sites. By the mid-'70s, only 66 nests were counted in three colonies statewide.

The embattled waterbirds got a little help. First, DDT and its byproducts were banned. Then, from 1973-97 some 1,269 artificial nesting platforms were erected on protected islands and on flowages across northern and north central Wisconsin and near deeper waters in east-central, eastern and southwestern Wisconsin. Double-crested cormorants were protected nationwide under the Migratory Bird Treaty Act and placed on the state endangered species list in 1972.

With their adaptable natures, cormorants made a speedy recovery. These birds are not picky: They use habitat as varied as cobble beaches, rocky shores, trees and ledges, and feed on all kinds of fish and other aquatic life. The birds readily accepted the artificial platforms and their numbers grew at better than 16 percent a year.

Now more than 13,700 active nests have been documented in Wisconsin, and it seems happy days are here again for the resilient waterbirds. Except that growth – whether it comes in the form of an expanding subdivision or an expanding bird population – comes at a cost.

Feasting on an aquatic smorgasbord

Only the double-crested cormorant (Phalacrocorax auritus), the most numerous and widespread of six cormorant species in North America, inhabits the Midwest, the Great Lakes and inland waters. The bird has small turquoise eyes and a distinctive, hooked mottled-orange bill that gives it a bemused "grin." This efficient swimmer and diver has webbed feet and water-resistant, but not waterproof, plumage. The wettable feathers are a tradeoff; the cormorant sits low in the water and actually sinks slightly, which cuts its buoyancy, making it easier for the bird to dive and pursue fish, guided by a rudder-like tail.

Cormorants commonly forage five to 25 feet deep, occasionally plunging as deep as 75 feet in pursuit of a meal. They grab fish with their bills and return to the surface to swallow their prey.

After feeding, the birds come ashore or find posts on water where they can stand and hold out their wings horizontally to dry. Only moderately efficient flyers, cormorants prefer to nest, loaf and perch on shores adjoining their feeding areas.

Cormorants are opportunistic feeders and seek schooling fish to improve their odds of catching a meal. They readily eat minnows, alewives, smelt, yellow perch, salamanders, crustaceans and mollusks. Uncovered aquaculture ponds filled with captive fish provide easy pickings for cormorants. Netting and runway designs that exclude birds can dissuade cormorant predation.

Several researchers have studied cormorant feeding habits to better answer questions from commercial fishers and sport anglers who contend that cormorants mainly feed on yellow perch, other panfish and game fish. The research shows that while perch consumption can be substantial, the birds maintain broader eating habits.

A 1985 study of a cormorant colony's feeding habits in the Apostle Islands suggested that cormorants feed much more heavily on shallow forage fish like sticklebacks, sculpin and burbot than on commercially-prized fish like whitefish and lake trout. A three-year study of cormorant feeding habits on Green Bay from 2004-6 examined the stomach contents of 1,429 birds. The researchers determined perch were an important food source for cormorants in mid-June, but perch predation largely dropped off as schooling yellow perch moved from shallow to deeper waters a little later in the summer. By mid-July, other fish, including gizzard shad and the invasive round goby, made up a much more substantial part of the cormorant diet.

No one can accurately assess how cormorant predation fits into perch population dynamics because many factors contribute to fluctuations in yellow perch populations, including fishing pressure, the influx of invasive species in the Great Lakes, mercury levels, runoff and other manmade contaminants. Researching the relationship between cormorants and fish populations is very expensive and has limited value given the fact that fish populations fluctuate based on a lot of variables. Short-term fish studies just are not applicable very far into the future and there are other factors to consider.

A hazard to habitat

Potential losses of panfish and game fish are important, but wildlife managers are more concerned about the hazards that spreading cormorant populations might pose to island vegetation; breeding habitat for herons, egrets and colonial waterbirds; and stop-over habitats for migrating songbirds.

Currently, nearly 90 percent of the state cormorant breeding population inhabits the Lake Michigan shores along Green Bay. Isolated islands off the Door Peninsula have about 10,000 pairs nesting in four distinct colonies. Green Bay's Cat Island, just offshore of where the Fox River empties into the bay, has another 2,400 pairs. On Lake Superior, nesting colonies are on the Apostle Islands (fewer than 500 nests). There are fewer than 20 cormorant pairs along the Mississippi River. On our inland waters, researchers have counted about 1,100 cormorant nests, half of them concentrated in the Millers Bay area off Oshkosh on the west central shore of Lake Winnebago. One management goal is to ensure there is room on islands with existing cormorant populations to accommodate the birds without creating sufficient disturbance so populations would spread to additional islands.

The vegetation likely will not recover on small islands where cormorants and other shorebirds congregate.© Paul Peeters
The vegetation likely will not recover on small islands where cormorants and other shorebirds congregate.

© Paul Peeters

As cormorants perch in trees drying their wings, roosting and scoping out potential breeding territories, it's only natural that they defecate. Given their diet, cormorant feces are highly acidic. The concentrated nutrients can kill off roosting trees, nest sites and the understory. At the Green Bay colonies, the cormorants are staying put, nesting on the ground where trees used to be. Over time, island, cliff and shoreland vegetation is destroyed, and recovery can take a long time.

A team of wildlife managers, fisheries biologists and endangered resources staff has been meeting periodically since 2002 to track the cormorant conundrum. Populations in the Apostle Islands, Mississippi River and on inland waters are small, and managers don't believe they need to intercede in those areas. But the growing numbers of cormorants in Green Bay and along the northern Door County islands are another matter. To protect habitat for other nesting birds on or near Cat Island, the management team hopes to reduce the number of pairs from 2,400 to 1,000 and maintain a smaller population.

The four largest cormorant colonies on northern Door County islands (Hat Island west of Egg Harbor, Jack Island west of Peninsula State Park, Spider Island east of Newport State Park and Pilot Island east of the Northport Ferry docks) contain more than 10,000 pairs; managers recommend these populations be reduced by half to sustain at least 500 nests in each colony, with a total of about 5,000 nests at the four colony sites. Reducing the cormorant population gradually aims to take the populations back to a level when the conflicts (real or perceived) were not so evident. Reducing these populations by about 50 percent will still leave viable, active colonies. We can restore more of a biological balance as we continue to study if management is effective or if human intervention is even needed to keep natural systems in balance.

In April 2006, Governor Doyle signed legislation directing the Department of Natural Resources to develop and direct a program to control cormorant populations where it appears habitat loss and predation may become significant issues. The team's draft recommendations were reviewed at public meetings around the state last October; the comments gathered will help shape a plan to be presented to the Natural Resources Board early in 2008.

Capping the population explosion

Techniques to decrease cormorant populations must be applied gradually and with care. Immediate, drastic controls might merely lead cormorants to disperse more widely to other locations. Previous control techniques include shooting adult birds with guns equipped with silencers, and oiling cormorant eggs in the nest. Oiling eggs once during incubation seals off the pores and prevents development. It's an effective method for reducing hatching success, and the adult cormorants do not immediately restart another nesting cycle. Over time, the population can slowly dwindle until population goals are reached. Thereafter, the populations can be monitored and maintained at lower levels.

Some people don't believe cormorant controls are needed. Critics point out that perch populations have started to rebound at the same time cormorant numbers are growing quickly. They note that cormorants feed on a range of fish and aquatic life.

An aim of the environmental assessment process will be to investigate the concerns raised at public meetings on proposed cormorant management and to develop a range of options for controlling cormorant build-up. The management team will also estimate costs for effective cormorant management. Some experiments with egg oiling were supported by fisheries funds, but the bulk of the research and experimental control work was paid for with settlements from paper mills on the Lower Fox River for the improper disposal of recycled paper pulp residues containing PCBs.

"If cormorant management will be successful, it's going to take time and money," DNR Regional Wildlife Biologist Jeff Pritzl told the Green Bay Press-Gazette. "We want to make sure there will be a return on this investment and that there are not unintended impacts to other bird life."

Tackling the cormorant conundrum presents many challenges for wildlife and fisheries managers, but two in particular stand out. The first is to conduct cormorant control work without harming non-target species like egrets, herons, pelicans, terns and gulls. The second is to monitor and fine-tune cormorant concentrations to keep these waterbirds in balance with available habitat and food supplies while sustaining fishing and other forms of outdoor recreation.

Jeff Pritzl leads the wildlife team for DNR's Northeast Region based in Green Bay. Paul Peeters leads the fisheries team for the Northeast Region.