The history of the Double-crested Cormorant (Phalacrocorax auritus) in the Great Lakes is complex, marked by dramatic population swings. This large, fish-eating waterbird is native to the region, documented in the Great Lakes basin for centuries as part of the ecosystem. However, its current abundance is a recent phenomenon that has created ecological and social conflict, often leading to the perception that the bird is an invasive species.
Defining the Bird’s Native Status and History in the Great Lakes
The Double-crested Cormorant is a native species, evidenced by its historical presence in North America, predating the population explosion of the late 20th century. Cormorants established colonies in the Great Lakes, including the Michigan waters of Lake Michigan and Lake Huron, between the 1920s and the 1950s. This initial colonization was followed by a sharp population decline beginning in the mid-1950s and continuing through the early 1970s.
The population crash was linked to the widespread use of organochlorine pesticides, particularly DDT and related compounds like PCBs. These contaminants accumulated in the aquatic food web, causing reproductive failure in fish-eating birds. The chemicals caused eggshell thinning, making the shells fragile and prone to cracking under the weight of the incubating adult. By 1970, the breeding cormorant population was extirpated from Lake Michigan, with only an estimated 89 nesting pairs remaining across the Great Lakes system.
Factors Driving Rapid Population Recovery
The recovery of the Double-crested Cormorant population began in the 1970s, primarily due to the federal ban on DDT use in 1972 and improved water quality. The removal of persistent contaminants allowed the birds’ reproductive success to rebound, enabling more chicks to hatch. The cormorant also gained protection under the Federal Migratory Bird Treaty Act, which limited human persecution and provided a regulatory safeguard for the recovering species.
The resurgence was also fueled by changes in the food supply, particularly the proliferation of non-native forage fish. Invasive species like Alewife and Rainbow Smelt became abundant prey sources, providing an energy-rich diet for the expanding cormorant colonies. Intensive aquaculture operations in the southern United States also contributed by providing a reliable food source during winter migration, increasing survival rates. This combination of reduced environmental toxins, legal protection, and increased accessible prey allowed the population to grow exponentially, reaching over 100,000 nesting pairs region-wide by the mid-2000s.
Ecological Role and Current Management Strategies
The population rebound shifted the cormorant’s ecological role from a threatened native species to a source of conflict for resource managers and fishing communities in Michigan. Concerns center on the bird’s consumption of sport fish and the physical damage large colonies inflict on island habitats. Cormorants are efficient visual predators that dive between 5 and 25 feet to catch fish, and a single bird can consume roughly one pound of fish per day.
While cormorants primarily consume smaller, non-commercial species like Alewife and Round Goby, studies in Michigan have documented localized impacts on sport fish populations. For example, management efforts in the Les Cheneaux Islands and Brevoort Lake were initiated after cormorant foraging was implicated in the decline of local Yellow Perch and Walleye populations. The concentrated waste from large nesting colonies, known as guano, is highly acidic and toxic to vegetation. This leads to the death of trees and the conversion of forested islands into barren habitats.
The management of cormorants in Michigan falls under the jurisdiction of the U.S. Fish and Wildlife Service (USFWS) and the Michigan Department of Natural Resources (DNR), guided by the Migratory Bird Treaty Act. Current strategies focus on localized population control, including both lethal and non-lethal methods. Techniques such as egg oiling, which prevents the embryo from developing without destroying the nest, limit reproduction in specific colonies. In areas with documented conflicts, such as the Les Cheneaux Islands, lethal control measures (culling) reduce nesting adults. Non-lethal harassment, using methods like pyrotechnics or limited shooting, is also used to scare birds away from sensitive spawning grounds, particularly during spring migration.