Overview of Damage Prevention and Control Methods
- Remove nest trees on islands
- Release fish at night
- Monofilament grid lines
- Harassment with boats and pyrotechnics
- High-powered lasers at night
- Mylar® tape at nest sites
- 12-gauge shotgun, size No. 4 or 6 steel shot
- Modified foot-hold traps, No. 3 padded-jaw
- Clap-net traps
- Walk-in traps
- Spotlight and net by hand at night
Other Control Methods
- Nest destruction
- High-pressure water spray to destroy nests
- Oiling or puncturing to destroy eggs
Damage Prevention and Control Methods
Integrated Pest Management
Timing, Economics, and Methods
Cormorants may be managed whenever their damage justifies the granting of the permits necessary to control them. An adult cormorant eats approximately 19 ounces of food per day, so local impacts of large flocks on fisheries can be substantial. Although cormorants frequently are blamed for reductions in fish harvests, this claim has not always withstood scrutiny by researchers. Sometimes other factors, such as pollution, invasive species, and habitat loss, are the real causes of fisheries declines.
Impacts on Recreational Fisheries
There is a long history of conflict between human fishery interests and cormorants. As North American cormorant populations expanded following a low point in the 1960s and 1970s, concerns about fishery impacts also increased. By the late 1990s, natural resource agencies in 27 states reported losses to free-ranging fish stocks. Agencies in 10 states, ranging from the southwest to the northeast, considered cormorant predation to be of moderate to major concern in fishery management.
In reviews of cormorant diet studies, scientists have concluded that fish species of recreational or commercial significance generally make up only a small percentage of the cormorant diet. Cormorants, however, are opportunistic predators whose diet varies considerably with local prey availability. For example, in one study, trout comprised 1% of great cormorant diet (by weight) in one English river, and 85% of the diet in another. In another study, investigators found that the percent of sport and commercially significant species in the diet of double-crested cormorants feeding at a Wyoming river varied from less than 1% to 93%. At Rice Island in the Columbia River, estuary salmonids, some of which are federally-listed as threatened or endangered, are the most important prey of double-crested cormorants. However, diet studies by themselves say more about the importance of the fish to the cormorant, than about the impact of the cormorant on the fish stocks.
Reviewers considering the effect of cormorant predation on fisheries often have concluded that they have no clearly defined impact. Studies are inconclusive, because obtaining information can be difficult and expensive, particularly in large open-water systems. Fisheries operate under a wide variety of physical, biological, and cultural conditions, so that even when comprehensive information is available, it may be impossible to separate the effects of the various influences. Any conclusions generally are specific to the fishery in question.
In recent years, several large studies of fishery-cormorant interactions have been conducted. Not surprisingly, the conclusions have varied. However, studies on eastern Lake Ontario, using a 20-plus-year fishery database, concluded that cormorant predation was associated with an increase in mortality of young smallmouth bass (Micropterus dolomieui) that contributed to a major decline in bass abundance and in the quality of the bass fishery. Researchers at the Cornell University Biological Field Station have studied the walleye (Sander vitreus) population, recreational fishery, and cormorant diet at Oneida Lake, New York, for decades. They concluded, based on over 40 years of fish population data, that cormorant predation was likely a significant source of sub-adult walleye mortality that negatively impacted the fishery.
Cormorant diets are highly variable depending on availability of local prey. Impacts on fisheries likely are to be even more variable due to the complex set of conditions under which they operate. When considering the potential impact of cormorant predation on any given fishery, be aware that such impact is likely to be specific to that set of conditions. Information developed to date reveals that concerns about cormorant impacts on open-water fisheries are widespread, but aquatic systems are extremely complex, and the impacts of any single predator species are difficult to demonstrate with a high degree of certainty.
Nests or nesting trees can be removed or physically broken up with the hope that adult birds will either leave the area or fail to rebuild and re-nest successfully that season. This method may be useful for discouraging cormorants from nesting in new areas, especially if nests are destroyed early on. It requires more effort in already established colonies.
Site-specific impacts on fisheries often occur when large concentrations of easily accessible fish are present. Fish are particularly vulnerable when large numbers of hatchery-raised lake fish are released at once, or when natural movements, like salmon smolt runs, or fish spawning behavior may concentrate fish in small areas. Fish harvest methods that congregate fish in enclosed areas that cormorants can access also leave fish vulnerable. Releasing fish at night so they have time to disperse before cormorants begin feeding in the morning can reduce predation. In lakes, releasing fish in deep water rather than from the shore may also reduce predation. In streams, fish can be stocked early in the season before cormorants return from their wintering grounds. Harassment conducted in coordination with stocking may also relieve pressure on recently stocked fish.
Netting or grid wires can prevent or deter cormorants from preying on fish in hatchery or aquaculture ponds (below). Nets provide a physical barrier and are effective as long as the edges of the nets extend to the ground surrounding the pond. If nets do not extend to the ground, cormorants may learn to walk into the water and around the netting. Although netting can be effective, the cost may be prohibitive for large ponds. In some instances, the levies between ponds are too narrow to hold net support structures, and netting may interfere with machinery needed for daily operations.
Overhead-wire systems work by making it difficult for cormorants to land on and take off from ponds. Although these systems are effective at preventing large flocks from landing, individual birds often learn to fly between the lines, or land on levies and walk into the pond despite the wires. Grid wires may reduce access to people as well, and present hazards to non-target species like osprey (Pandion haliaetus) or swallows (Hirundinidae sp.), as well as bald eagles (Haliaeetus leucocephalus). Ropes with plastic floats, sometimes called bird balls, are a less expensive and less labor-intensive alternative to wire systems. Floating ropes can be strung parallel to each other about 50 to 55 feet apart. The success of both wire systems and floating ropes depends on the availability of alternative foraging areas nearby. Birds that are able to find other food sources easily are more likely to be deterred.
Wire-grid systems can also protect nesting colonies of other waterbirds. Along with gulls, cormorants can out-compete common terns for favored nesting islands. Grid wires suspended above tern nesting colonies can enhance tern nesting success and productivity by discouraging larger birds from nesting. This method effectively reserves nesting space for common terns until they are able to establish and defend a colony.
Harassment, or scare tactics, applied in an integrated and consistent fashion can discourage cormorants from using specific sites. Birds can be hazed at fish hatcheries and aquaculture facilities, as well as roosting and nesting sites on larger ponds, lakes, and the marine environment. Harassment is most effective when the birds are not nesting, or when birds have not become habituated to a location. Cormorants learn quickly and these methods often do not deter the birds for long. For harassment to be effective, a variety of techniques should be used in combination, and the location and combination of devices should be changed frequently for best results.
Devices that make noise, including pyrotechnics such as shell crackers, screamers, whistling or exploding projectiles, bird bangers, propane cannons, and live ammunition have been tried, with varying success. Live ammunition is often the least expensive and most readily available form of pyrotechnics, however other methods may be more effective, and extra precaution should be taken to avoid injuring or killing cormorants and other protected species.
Hand-held lasers have been used successfully to disperse roosting cormorants and are most effective in low light conditions, such as at night roosts. In addition, lasers are silent and can be used to move cormorants without disturbing other non-target species. The regular presence of humans may frighten cormorants from smaller aquaculture or hatchery facilities, as well as from roosting sites and potential colonies. Encouraging visitors and frequent human activity near valuable stocks at hatcheries and aquaculture facilities also may help to reduce depredation on fish.
Visual harassment techniques like scarecrows, human effigies, and balloons also have been tried with varying degrees of success. However, stringing Mylar® tape (left) between stakes near roosting and loafing sites has proven effective in reducing cormorant use of these areas on Oneida Lake in New York. In addition, chasing cormorants with boats has been used successfully to disperse roosts and flocks from ponds and larger bodies of water.
Use of limited lethal control with harassment techniques will significantly improve the effectiveness of harassment.
Shooting allows for a rapid reduction in cormorant numbers. Shooting can be most effective on breeding colonies, where large numbers of bird congregate each day, although open-water shooting and removal at night roosts also can be used to protect specific sites. Use 12-gauge shotguns with non-toxic shot sized #4 or #6. Shooters should be knowledgeable in waterbird identification to prevent the killing of non-target species. Shooting adults also helps to reduce cormorant conflicts by harassment of the remaining birds. Shooting can be combined with pyrotechnics to enhance the effectiveness of non-lethal control options.
Spring-loaded net traps have been used to capture nesting colonial waterbirds. During trapping attempts, dummy eggs should be placed in the nest, and once a bird is caught and released, the actual eggs should be put back in the nest. Observers usually monitor nests from a nearby blind so that birds can be rapidly handled and removed from the net to prevent injury.
Modified foot-hold traps can be placed at nests to capture adult birds during the breeding season in April and early May. Cormorants have been captured with modified No. 3, padded-jaw, foothold traps by replacing factory coil springs with weaker No. 1.5 trap springs (below). The trap chain should also be replaced with aircraft cable and a shock cord to minimize injury to trapped birds. Dummy eggs should be placed in the nest during trapping, and once a bird is caught and released, the actual eggs should be put back in the nest.
Netting with Spotlights
At night, cormorants can be removed effectively by spotlighting and netting. This method works best on dark nights with low ambient light.
Other Control Methods
A variety of methods can be used to reduce or stabilize cormorant populations, or to deter them from taking up residence in new areas. Any technique that involves egg or nest destruction, or removal of cormorants will likely require federal and state permits. Nest destruction is relatively labor intensive, although it can be practical on smaller colony sites. In order to be effective, control must be repeated throughout the nesting season, and likely on an annual basis. Nest removal may shift cormorants to other locations where they may cause continued conflicts.
High-pressure water sprays have been used to destroy cormorant nests in trees. This method can be used to disperse nesting birds and limit population growth.
Egg oiling can be used to prevent or reduce population growth, and may be useful for eliminating colonies at a specific location, especially if combined with other harassment or population reduction methods. Spraying eggs with food-grade corn oil prevents the exchange of gases through the shell, causing asphyxiation. The benefit of egg oiling over destroying eggs is that cormorants will continue to incubate the eggs and are less likely to attempt to re-nest. Management strategies that include egg oiling are best suited to situations where the presence of cormorants can be tolerated, and rapid population reduction is not the goal. Because cormorants often re-nest, some reproduction may still occur if persistent effort is not applied. In some states, a pesticide applicator license may be required for oiling eggs.