The Double-crested Cormorant (Nannopterum auritum) is a large, dark-colored diving water bird found widely across North America. These highly specialized predators propel themselves underwater with webbed feet to catch fish, then stand with wings outstretched to dry their non-waterproof feathers. The species’ dramatic population increase has fueled conflict with human interests, leading to the common but incorrect assumption that the cormorant is an invasive species. In reality, the issue is the overabundance of a native species, a distinction central to understanding the controversy surrounding its management.
Native or Introduced Status
A species is classified as invasive only if it is non-native to an ecosystem and its introduction causes environmental or economic harm. The Double-crested Cormorant does not fit this definition because it is unequivocally native to North America, with a natural range extending across the continent. Records document their presence in the Great Lakes region as far back as the late 1700s.
The core conflict lies in the difference between an invasive species and a native species that has become overabundant. A native species originated and developed within its current habitat without human intervention. Overabundance describes a native population that has grown to a level causing significant negative impacts on the environment or human activities.
The cormorant’s current status exemplifies this distinction: a native bird whose unchecked population growth mimics the effect of an invasive one. This complicates management efforts, as the bird is a protected part of the native fauna. Control efforts must therefore balance ecological health with the animal’s protective status.
Ecological and Economic Impact
The dramatic increase in cormorant numbers has led to significant conflicts with human economic interests, particularly in fisheries and aquaculture. These conflicts are most pronounced at commercial freshwater aquaculture facilities, where cormorants cause considerable financial losses by preying on stocked fish. For instance, a small number of cormorants feeding on a six-hectare pond over a winter season can result in a biomass decline of over 20% by harvest time.
Concerns also extend to wild and publicly stocked fisheries, where cormorant predation has been implicated in the decline of commercially and recreationally important species. Studies in the Great Lakes region have linked cormorant activity to negative impacts on local populations of yellow perch and walleye. The economic loss to recreational fisheries in some regions, such as Oneida Lake in New York, has been estimated to range from $5 million to $66 million annually.
Beyond direct predation, cormorant colonies inflict extensive damage to nesting habitats. Their highly acidic guano changes soil chemistry and acts as a defoliant, leading to the death of nesting trees and ground vegetation. This habitat destruction can displace co-nesting species, such as Great Blue Herons, and threaten rare plant communities.
Rapid Population Recovery and Growth
The current overabundance issue stems from the cormorant’s historical decline and subsequent rapid rebound. Early in the 20th century, numbers were reduced by human persecution, driven by competition with commercial fishing. A second, more severe decline occurred from the 1950s through the early 1970s due to the widespread use of the pesticide DDT.
The breakdown product of DDT, DDE, accumulated in the birds, causing eggshell thinning, widespread reproductive failure, and population crashes. By the early 1970s, the species had been extirpated from several areas, including parts of the Great Lakes.
The recovery began with the ban of DDT in the United States in 1972, which allowed reproductive success to increase. Simultaneously, the species was granted protection under the Migratory Bird Treaty Act (MBTA), ending the era of unchecked persecution. This combination of regulation, legal protection, and increased forage fish populations resulted in a spectacular resurgence. For instance, the Great Lakes population rebounded from only a few hundred nesting pairs in the 1970s to over 100,000 by 2000, creating the current management challenge.
Legal Status and Population Control
As a native migratory bird, the Double-crested Cormorant is protected in the United States under the Migratory Bird Treaty Act (MBTA). This federal law prohibits the “take” (killing or capture) of the birds without prior authorization from the U.S. Fish and Wildlife Service. Therefore, managing or reducing cormorant populations requires specific permits, often issued as depredation orders to address localized conflicts.
Management strategies mitigate damage while ensuring the overall health of the native population. Non-lethal methods are attempted first, including hazing with lasers or boats, and visual deterrents such as Mylar tape or floating decoys. These methods aim to deter the birds from high-value areas like aquaculture ponds or recreational fisheries.
If non-lethal methods prove insufficient, permits may be issued for lethal control, including nest destruction and the humane culling of birds in specific conflict zones. A common method for managing nesting success without killing adult birds is egg oiling, which prevents the embryo from developing.