Cormorants, fish-eating birds, are often discussed due to their impact on fish populations and ecosystems. These aquatic birds are recognizable by their dark plumage, long necks, and habit of perching with wings outstretched to dry. While known for their diving capabilities to catch fish, questions arise about their status and environmental impact. The debate over whether cormorants are “invasive” is common, fueled by their growing numbers.
Defining an Invasive Species
An invasive species is defined as a non-native organism whose introduction to an ecosystem causes or is likely to cause economic, environmental, or human health harm. Not all non-native species are invasive; many coexist without significant negative impacts, like agricultural crops. The distinction rests on the proven detrimental effects of the introduced species on the new habitat’s native species, biodiversity, or economic activities.
Cormorants and Their Status
Double-crested cormorants are native to North America. Despite this, their populations have significantly increased, leading to them being perceived as invasive in some areas. This resurgence is largely attributed to the ban of the pesticide DDT, which caused eggshell thinning and population declines, and regulatory protections under migratory bird acts. Their high numbers in specific locations prompt management actions similar to those for invasive species.
Their historical presence in North America spans thousands of years, supported by archaeological evidence. A dramatic population recovery since the 1970s, after declines from human persecution and contaminants, has led to a continent-wide population estimated at over one million birds. This growth is notable in regions like the Great Lakes, where breeding pairs rebounded from hundreds in the early 1970s to over 100,000 by the early 2000s.
Ecological and Economic Impacts
High cormorant populations can lead to ecological and economic impacts. They impact local fish populations, especially in areas with concentrated fish like aquaculture facilities or sport fisheries. While opportunistic feeders, their diet includes commercially valuable fish, causing conflicts with human fishing interests. Studies in Europe show high cormorant numbers can negatively affect species like brown trout, Atlantic salmon, and European eel, leading to significant economic losses for fisheries and aquaculture farms.
Beyond fish consumption, cormorants cause habitat degradation on nesting islands. Their guano is highly acidic, altering soil chemistry and killing trees and vegetation in colonies, forming “cormorant trees.” These habitat changes can also affect other co-nesting bird species. Economically, aquaculture operations, particularly catfish farms, report significant losses due to cormorant predation, sometimes millions of dollars annually from direct fish consumption and deterring costs.
Management Approaches and Perspectives
Managing cormorant populations involves non-lethal and lethal strategies. Non-lethal deterrents include visual scares (Mylar tape, predator eye balloons, lasers), auditory methods (air horns, bioacoustics mimicking distress calls), and physical barriers (netting, grid wires) to prevent access to fish in aquaculture ponds.
When non-lethal methods are insufficient, lethal control measures may be implemented under specific regulations. These include egg oiling (preventing hatching by blocking gas exchange) and nest destruction. Authorized shooting may be permitted as a last resort under strict federal or local regulations, such as those issued by the U.S. Fish and Wildlife Service. Lethal control is typically part of an integrated management plan, often requiring prior non-lethal attempts.
Cormorant management often sparks debate among stakeholders like conservation groups, fisheries managers, and aquaculture operators. Some view cormorants as a natural ecosystem component, highlighting their recovery as a conservation success. Others emphasize the economic and ecological impacts of high populations, advocating for control to mitigate perceived damage to fish stocks and habitats. These differing perspectives underscore the complexities of balancing wildlife conservation with human interests.