Double-crested Cormorant double-crested-cormorant-decreasing-fish-populations

Double-crested Cormorant

article by:
Angela Woodward - Environmental Encyclopedia

Reviewed by:
On April 8, 2017
Last modified:April 8, 2017


Double-crested Cormorant is is it decreasing fish populations?

The Double-crested Cormorant (Phalacrocorax auritus)

Phalacrocorax Auritus is a significant (30–36 in; 73–90 cm), a blackish water bird that eats fish and crustaceans. The double-crested cormorant population across the Great Lakes (but not in other places) was once in serious decline. It’s now recovered. Some fishermen believe there are too many double-crested cormorants. These fishermen consider an abundance of double-crested cormorants is responsible for a reduction in fish populations in the Great Lakes. The conservation of the bird entails weighing the interests of the fishing business against the benefit of a now-plentiful bird whose range is expanding.

The double-crested cormorant is a big migratory water bird native to North and Central America. The bird is located along the West Coast as far north as southern Alaska, wintering in the south of California and Baja California. On the East Coast, it reproduces from Maryland to Newfoundland, wintering along the south shore 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 very first double-crested cormorants in the Great Lakes region were sighted on the western coast of Lake Superior in 1913. By the 1940s, the people across the Great Lakes had grown to about 1,000 nesting pairs. Later, the cormorant population started to decrease. By 1973 a survey found just 100 pairs in the area.

In the early 1970s, cormorants weren’t the sole waterbird whose population was decreasing. Around this time, the United States Congress enacted several laws to safeguard cormorants and other waterfowl. As an example, the pesticide dichlorodiphenyltrichloroethane (DDT) was banned in 1972. This compound had entered lakes and rivers in run-off water and was implicated in lessening the birthrate of fish-eating birds, for instance, the bald eagle. Congress also amended the Migratory Bird Treaty Act, first passed in 1917, to allow it to be illegal to damage or kill cormorants and other migrating waterfowl.

The double-crested cormorant populations across the Great Lakes started to grow in the 1980s, as well as the fowl enlarged their range eastward to Lake Erie and Lake Ontario. The return of the double-crested cormorant appeared to some conservationists to indicate the Great Lakes, once severely polluted, were regaining. Nevertheless, one reason the cormorant population might have grown around the Great Lakes is that overfishing in this time severely depleted the variety of big fish in the lakes. This caused a rise in the quantity of smaller fish such has smelt (Osmerus mordax) and alewife (Alosa pseudoharengus). Cormorants eat these little fish. Growth in their food supply might have led to growth in the cormorant population.

Another environmental issue, 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. This is a voracious feeder and may clean lakes of green plankton, making the water exceptionally clear. Clearwater might have helped the cormorant, which hunts fish by sight, locate food more readily.

By the 1990s, some local cormorant people had grown to unprecedented proportions, and at the same time sport fish populations had decreased. Because of this, some Great Lakes anglers, refusing to think about the role of overfishing in the decline of fish populations, attributed double-crested cormorants for steep falls in populations of fish including smallmouth bass (Micropterus dolomieui), rock bass (Ambloplites rupestris), and brown bullheads (Ameiurus nebulosus).

An adult cormorant weighs around 4 pounds (1.8 kg) and eats about 1 pound (0.45 kg) of fish a day. Some places hosted flocks of tens of thousands of cormorants. A 1991 study of cormorants in Lake Ontario estimated the fowl had had about 5 million pounds (2.25 million kg) of fish. Additionally, cormorants often eat smaller fish; and if enough fish are eaten before they can copy, future generations of fish are endangered.

Although anglers have attributed the cormorant for decreasing fish populations, conservationists have insisted on scientific studies to find out whether these birds are causing a decline in fish populations. A 1998 study of cormorants on Galloo Island in Lake Ontario found the smallmouth bass, a favorite sports fish, made up only 1.5 percent of the cormorant’s diet. Since the cormorants ate little bass that hadn’t yet grown to reproductive adulthood, the fowl was considered linked to the smallmouth’s decline.


Another 2000 study of the Beaver Islands region in Lake Michigan reasoned it probably the big cormorant population was a factor in the decreasing amounts of smallmouth bass and other fish. The biologist who headed the study was not able to decide that cormorants were wholly in charge of the decline in the fish population. Nevertheless, wildlife officials took actions to manage this cormorant population.

Cormorants additionally have made themselves unpopular since they nest in large colonies, where thick layers of their droppings ca

Double-crested Cormorant Minn-Kota-Shop-1


n kill off native plant life and leave the place denuded of all but a couple of trees. Cormorant colonies have endangered some sensitive woodland habitats and led to soil erosion by killing coastline plants.

Because national law shields double-crested cormorants, individuals cannot lawfully harass or kill these birds. In 1999, nine men were convicted of slaughtering 2,000 cormorants on Little Galloo Island in Lake Ontario. These guys had illegally attempted to decrease the cormorant colony by shooting mature fowl. Shortly following the event, but, the New York Department of Environmental Conservation enacted a plan to lessen the Galloo colony from 7,500 to 1,500 fowl over five years by spraying vegetable oil on cormorant eggs. The oil-coated eggs don’t hatch. This technique of thinning the flock was considered less disruptive to other wildlife and much more humane than killing mature fowl.

The New York plan created controversy, as not all scientists who’d examined the fowl considered that double-crested cormorants were responsible for the decline of the fish population, and some conservationists worried similar plans would be enacted against other fish-eating birds. Paradoxically, achievement in shielding the double-crested cormorant in the 1970s resulted in controversy two decades after. Despite the fact that the population of double-crested cormorants significantly raised, the fowl continues to be under national protection. Any actions to control the cormorant population has to be cautiously developed, executed, and assessed.