Evidence mounts against cormorants
Birds are big predators in northern Lake Huron
April 8, 2004
BY ERIC SHARP
DETROIT FREE PRESS OUTDOORS WRITER
Anglers have been complaining for years that a burgeoning cormorant population on the Great Lakes is destroying perch and smallmouth bass stocks. Now they have support from research scientists.
Preliminary analysis of a 20-year study on Lake Ontario shows the goose-sized diving birds are decimating inshore fish the size of perch and young bass and competing for food with salmonids and walleyes. Changing water conditions also have made cormorants more efficient Great Lakes predators than 20 years ago, and they have become a major predator in northern Lake Huron.
“Some people say cormorant numbers go up and down in a natural cycle,” said John Casselman, who appeared at a recent meeting of the Lake Ontario committee of the Great Lakes Fishery Commission in Grand Island, N.Y. “These are very controversial issues.”
Casselman, a fisheries biologist for the Ontario Ministry of Natural Resources, has been following the interaction between cormorants and fish on Lake Ontario for more than 20 years. A few dozen breeding cormorants were first documented on Lake Ontario in 1945, and those numbers have since increased to about 250,000.
The birds’ numbers have exploded in the past decade, and so has their impact on fish stocks. Studies show that cormorants in eastern Lake Ontario ate about one-half of one percent of the available small-bodied fish in 1991 and now take at least 7.5 percent, Casselman said.
“Since 1991, we have 3.8 times as many cormorants feeding on one third as many fish,” he said.
The first documentation of cormorants nesting on Lake Huron was in 1932. There were fewer than 4,000 nests in the 1940s, and the introduction of the pesticide DDT kept their numbers down through the next couple of decades.
But cormorant numbers and their range began expanding when DDT was banned, and by 2000 the estimate was that 40,000 breeding pairs were on Lake Huron. Scientists say there is one immature bird for each nesting adult, so that puts the number of individual cormorants on Lake Huron at 160,000. About 38 percent nest around the main basin, and the rest are evenly divided among the St. Marys River, North Channel islands and Georgian Bay.
Dr. James R. Bence, an assistant professor in the fisheries and wildlife department at Michigan State University, estimated that cormorants in Lake Huron eat more than 30-million pounds of fish a year. That’s about 18 percent of the fish consumed by millions of salmon, burbot, lake trout and walleyes in those waters.
“One issue is what do cormorants do to the overall prey base,” Bence said. “There are people concerned that we are near the edge. I’m not one of them. The other concern is the species they are eating.”
Anglers probably would be far less concerned about cormorant predation if the birds fed only on open waters far from land. But cormorants feed mostly in near-shore waters and prefer fish about five to 10 inches long, which include species like perch and young smallmouth bass. This puts birds in direct competition with — and eyesight of — anglers.
Cormorants target five- to 10-inch fish because “there aren’t enough bigger fish to waste energy chasing them, and there’s not enough energy in the small ones,” Casselman said.
Cormorants can dive to 100 feet and often feed in gangs, pinning schools of fish against a rocky shoreline or in a small bay. Casselman thinks the introduction of zebra mussels made the cormorants even more efficient predators, “because it stands to reason that they can hunt more effectively underwater when they can see the fish 30 feet away rather than three feet away.”
Zebra mussels have made the lakes much clearer.
“Cormorants are much more effective predators than fish,” Casselman said. “A lake trout can’t pick up and fly 30 miles to find out where the prey fish are. And I have no doubt that the clearer water has made it possible for cormorants to see schools of prey fish from the air. I was out in an airplane over eastern Lake Ontario doing a cormorant survey one day, and from 1,000 feet we could look into the water and see the edges of the mussel beds in 80 feet of water.”
The U.S. Fish and Wildlife Service, which manages migratory species like cormorants, has given several states, Indian tribes and the U.S. Department of Agriculture permission to try some experimental cormorant culls to see if their numbers can be reduced.
One plan would involve oiling and killing 90 percent of the eggs at five island rookeries that are home to about 25,000 cormorants in the Les Cheneaux Islands. The islands are in northern Lake Huron off the southern shore of the eastern Upper Peninsula.
David Fielder, a fisheries biologist at the state Department of Natural Resources’ research station in Charlevoix, said cormorants appear to be a major contributor to the decline in the perch population.
“I won’t say that cormorants are completely responsible for the perch crash, but it stands to reason that they are at least a part of the problem,” Fielder said. “A study in the early 1990s showed that cormorants fed heavily on perch, mostly in a narrow window in the spring, then switched over when the alewives moved inshore. But alewives became much less abundant in the late 1990s, and so I suspect that the cormorants would have fed more on perch all year long.
“In 1998 we had a huge perch year-class. Now those fish are nearly gone. Angling pressure doesn’t explain that.”
State and federal officials want to kill about 15 percent of the adult cormorants that nest around the five islands. Peter Butchko, who runs the USDA wildlife control office in Lansing, said Congress approved $125,000 for the Lake Huron cormorant control experiment.
But the experiment has been delayed by animal-rights activists, who have filed a suit in federal court in an effort to prevent the birds from being killed or molested.