The double-crested cormorant (Phalacrocorax auritus ) is a large (30–36 in; 73–90 cm), blackish water bird that eats fish and crustaceans. The double-crested cormorant population around the Great Lakes (but not in other areas) was once in serious decline. It has now has recovered. In fact, some fishermen feel that there are too many double-crested cormorants. These fishermen believe that an abundance of double-crested cormorants is responsible for a decline in fish populations in the Great Lakes. The conservation of this bird entails weighing the interests of the fishing industry against the interests of a now-abundant bird whose range is expanding.
The double-crested cormorant is a large migratory water bird native to North and Central America. The bird is found along the West Coast as far north as southern Alaska, wintering in Southern California and Baja California. On the East Coast it breeds from Maryland to Newfoundland, wintering along the southern coast to Florida, the Bahamas, and Cuba. Inland, double-crested cormorants breed along the upper Mississippi Valley and in the Great Lakes and winter on the Gulf Coast.
The first double-crested cormorant in the great Lakes region were sighted on the western shore of Lake Superior in 1913. By the 1940s, the population around the Great Lakes had increased to about 1,000 nesting pairs. Subsequently, the cormorant population began to decline. By 1973 a survey found only 100 pairs in the region.
In the early 1970s, cormorants were not the only water bird whose population was declining. Around this time, the United States Congress enacted several laws to protect cormorants and other waterfowl. For example, the pesticide DDT was banned in 1972. This chemical had entered lakes and rivers in run-off water and was implicated in reducing the birth rate of fish-eating birds, including the bald eagle . Congress also amended the Migratory Bird Treaty Act, first passed in 1917, to make it illegal to harm or kill cormorants and other migrating water fowl.
The double-crested cormorant populations around the Great Lakes began to increase in the 1980s, and the birds expanded their range eastward to Lake Erie and Lake Ontario. The return of the double-crested cormorant seemed to some conservationists to signal that the Great Lakes, once seriously polluted, were recovering. However, one reason the cormorant population may have increased around the Great Lakes is that overfishing during this time seriously depleted the number of large fish in the lakes. This led to an increase in the number of smaller fish such has smelt (Osmerus mordax ) and alewife (Alosa pseudoharengus ). Cormorants eat these small fish. An increase in their food supply may have contributed to an increase in the cormorant population.
Another ecological problem, the zebra mussel , may also have helped the cormorant. The zebra mussel is an exotic (non-native) invasive mussel that competes with native mussels for food resources. It is a voracious feeder and can clean lakes of green plankton , leaving the water particularly clear. Clear water may have helped the cormorant, which hunts fish by sight, find food more easily.
By the 1990s, some local cormorant populations had grown to unprecedented proportions, and at the same time, sport fish populations had declined. As a result, some Great Lakes fishermen, refusing to consider the role of overfishing in the decline of fish populations, blamed double-crested cormorants for steep drops in populations of fish such as smallmouth bass (Micropterus dolomieui ), rock bass (Ambloplites rupestris ), and brown bullheads (Ameiurus nebulosus ). An adult cormorant weighs about four pounds, and eats about one pound of fish a day. Some areas hosted flocks of thousands of cormorants. A 1991 study of cormorants in Lake Ontario estimated that the birds had consumed about five million pounds of fish. In addition, cormorants tend to eat smaller fish, and if enough fish are eaten before they can reproduce, future generations of fish are threatened.
Although fishermen have blamed the cormorant for declining fish populations, conservationists have insisted on scientific studies to determine if these birds are indeed causing a decline in fish populations. A 1998 study of cormorants on Galloo Island in Lake Ontario found that the smallmouth bass, a popular sport fish, made up just 1.5% of the cormorant's diet. Yet because the cormorants ate small bass that had not yet grown to reproductive maturity, the bird was considered linked to the smallmouth's decline.
Another 2000 study of the Beaver Islands area in Lake Michigan concluded that it likely the large cormorant population was a factor in the declining numbers of smallmouth bass and other fish. Yet the biologist who led the study was unable to conclude that cormorants were entirely responsible for the decline in the fish population. However, wildlife officials took action to manage this cormorant population.
Cormorants also have made themselves unpopular because they nest in large colonies, where thick layers of their droppings can kill off underlying vegetation and leave the area denuded of all but a few trees. Cormorant colonies have endangered some sensitive woodland habitats and contributed to soil erosion by killing shoreline plants.
Because federal law protects double-crested cormorants, people cannot legally harass or kill these birds. In 1999, nine men were convicted of slaughtering 2,000 cormorants on Little Galloo Island in Lake Ontario. These men had illegally tried to reduce the cormorant colony by shooting adult birds. Soon after the incident, however, the New York Department of Environmental Conservation enacted a plan to reduce the Galloo colony from 7,500 to 1,500 birds over five years by spraying vegetable oil on cormorant eggs. The oil-coated eggs do not hatch. This method of thinning the flock was considered less disruptive to other wildlife and more humane than killing adult birds.
The New York program generated controversy, as not all scientists who had studied the birds believed that double-crested cormorants were responsible for the decline of the fish population, and some conservationists feared similar programs would be enacted against other fish-eating birds. Ironically, success in protecting the double-crested cormorant in the 1970s resulted in controversy two decades later. Even though the population of double-crested cormorants substantially increased, the bird is still under federal protection. Any action to manage the cormorant population must be carefully developed, implemented, and evaluated.
[Angela Woodward ]
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Sharp, Eric. "Controversy Surrounds Cormorants." Outdoor Life (August 2000): 119.
"State Studies Show Cormorants' Toll on Fish." New York Times (December 20, 1998): 64.
Wallace, Scott. "Who Killed the Cormorants?" Sports Afield 222, no. 3 (September 1999): 72.