Damage to Structures
Cormorants may foul docks and navigation devices with feces while roosting or drying their wings when foraging.
Damage to Fisheries
Cormorants may cause damage to private property by feeding on stocked fish in privately-owned lakes and ponds. On larger lakes and reservoirs, cormorants may take sport-fish and locally impact the fishing industry. Foraging flocks of birds are easy to identify, and often are reported by local fishermen.
Damage to Vegetation
Cormorants may damage vegetation on privately-owned lands, especially nesting islands. The strong odor of droppings near roosts and nesting areas, along with the loss of vegetation, may reduce nearby property values. Tourists attracted to the natural beauty of waterfront areas may view the areas as unattractive once cormorants take up residence. On a local scale, decreasing property values and reduced tourism and recreation, may cause economic losses for area residents and businesses that rely on income from tourism.
Health and Safety Concerns
Humans should avoid direct contact with excrement from wildlife, including droppings from cormorants. Cormorants can present a bird-strike hazard when their populations and nesting or foraging habitats occur near and in plane flight paths.
Newcastle disease is a viral disease that can affect all bird species, and was first recognized in double-crested cormorants in the St. Lawrence River Estuary, Quebec, in 1975. In 1992, double-crested cormorants in 7 different states died from the disease. This widespread epidemic affected cormorants from the interior population, causing juvenile mortality rates ranging from 10 to 90%. By the late 1990s, outbreaks had occurred in cormorant populations across North America.
Possible transmission of Newcastle disease from free-ranging, wild birds to poultry is a concern, although there has only been one reported incident directly linking double-crested cormorants to an outbreak in domestic poultry. There has been no report of extensive mortality in other wild birds that share habitat with infected double-crested cormorants. However Newcastle disease identical to that found in cormorants has been isolated from American white pelicans (Pelecanus erythrorhynchos) and ring-billed gulls (Larus delawarensis).
People also can contract Newcastle disease. Symptoms, including conjunctivitis, mild fever, headache, and malaise are usually mild and last 3 to 4 days. Newcastle disease is transmitted through bird guano, or by humans who have been in contact with infected birds. Therefore, people working with double-crested cormorants should take measures to prevent infecting other birds, wild or domestic. After handling cormorants, disinfect hands, footwear, and equipment, and wash all clothing.