Reducing Aquaculture Damage from Cormorants Non-Lethally

Double-crested cormorants have made a remarkable comeback after decades of persecution by humans and exposure to DDT and other toxins. Though the return of the cormorant over the past few decades is certainly a conservation success story, its recovery has been blamed for economic losses experienced by some, which in turn threatens cormorants and the ecosystems in which they play a vital role. In particular, the U.S. aquaculture industry has grown during the period of cormorant population recovery. Many aquaculture operators have discovered that they are not equipped to protect their vulnerable fish from predation by cormorants and other fish-eating birds.

The Humane Society of the United States is working to promote peaceful coexistence between those who produce fish for human consumption and the wild birds who must consume fish to survive. If you have determined that fish losses because of double-crested cormorants are resulting in economic losses, please consider the following suggestions for non-lethal cormorant damage reduction at aquaculture facilities.

Non-Lethal Harassment

Cormorants can be non-lethally harassed and dispersed at fish farms and at nearby night roosts via a variety of methods and products. Devices and techniques include human activities and vehicles, exploding and whistling pyrotechnics, propane exploders and cannons, electronic noise-making devices or sirens, electronic bird distress calls, and visual repellents such as human effigies (scarecrows), eye-spot balloons, flash tape, or flagging. Preliminary research by the U.S. Department of Agriculture indicates that lasers may also be effective in dispersing cormorants from night roosts. Before using noise-making devices or other harassment techniques, determine whether state or local laws restrict the use of these devices and be sure that you understand how to use these devices safely and with minimal risk of injury to the birds or other nearby wildlife.

Cormorants, like most animals, quickly habituate to these techniques (and ultimately may stop responding) if devices are simply installed and used repeatedly in the same locations and at predictable times. To delay or prevent habituation, harassment devices and techniques should be moved around the farm and rotated in and out of use frequently. As one cormorant control expert suggests, "keep 'em guessing!" Frightening devices that are activated by movement—and therefore contingent on the bird's own behavior—may be less likely to result in habituation.

Many producers assign teams of workers to patrol their farms and harass cormorants feeding at fish ponds. However, a combination of periodic harassment and human effigies (scarecrows) positioned near ponds can be even more effective. Effigies are thought to be more effective when they are made to appear similar to the people who do the harassment (e.g., if they are wearing a distinctive yellow raincoat and hat and holding a stick or flag).

Regional Coordination Increases the Effectiveness of Harassment

Efforts from areas as diverse as the U.S. Mississippi Delta region and Israel indicate that the effectiveness of non-lethal harassment—at aquaculture facilities or at nearby night roosts—is greatly increased if harassment efforts are coordinated regionally. Regional coordination of harassment may also make non-lethal control more cost-effective for individual producers. Unless accompanied by a coordinated, region-wide effort, harassment at a single farm may simply move the birds to a neighboring farm, and cormorants may return within a few days. Sharing the costs and time required for harassment with other producers may make this regional harassment more affordable then going it alone. If cormorant damage is causing an economic loss for you, consider starting an organized cormorant harassment team with other aquaculture producers in your county or region.

Some waterbird control experts have found that a team of workers conducting regional harassment (mostly using pyrotechnics, along with human effigies) can be even more effective in some circumstances if alternative feeding ponds are made available. The reasoning is that cormorants may be more likely to stay away from areas where they have been harassed if they are simultaneously provided with an area in which they are allowed to feed. Like all animals, cormorants must minimize energy expenditure during foraging to survive; thus, they should prefer to forage in areas where they can feed with minimal disturbance.

Please keep in mind, however, that artificial feeding of wildlife is never an ideal solution to human-wildlife conflicts because it can disrupt wildlife movements, increase or decrease reproduction and survival, and may result in increased transmission of wildlife diseases both within and between wildlife species.

Exclusion

Netting, fencing, and other exclusion devices can either keep cormorants out of fish ponds, or can simply make access to ponds more difficult. Exclusion devices can increase the energy expended by foraging cormorants and make them look elsewhere for food that is easier to obtain.

For small ponds, netting suspended over the ponds can provide effective protection. One caveat to the use of netting is the potential for cormorants, waterfowl, and other birds to become entangled, which can cause injury or death and may also damage the netting. Research indicates that, in general, netting made of thick, dark colored material (not thin, monofilament lines) with small mesh sizes (less than 5 centimeters) may be safer and less likely to inadvertently trap cormorants and other birds.

We urge producers using this technique to choose netting material that is less likely to entangle birds; producers should also monitor netting frequently to remove any trapped birds before they are injured. However, note that netting may be impractical or cost prohibitive for large ponds (more than 5 hectares).

Installing floating ropes in ponds can help provide partial protection for catfish and other farmed fish by reducing cormorants' ability to land and take off from ponds. Because cormorants may adjust to the presence of the ropes, this technique may be best used temporarily, for ponds containing fingerlings or other especially vulnerable fish.

Underwater barriers—artificial structures or anything that provides shelter for fish and a more complex underwater habitat—may also impede cormorants from foraging. However, this technique may be more practical for farms using single-batch cropping systems.

Recent research by the USDA and other researchers suggests that twine strung across ponds—at 100-foot intervals, held approximately 3 feet above the surface of the water—can significantly reduce cormorant depredation. This may prove to be a cost-effective solution for at least some areas.

This type of exclusion, with minor variations, was previously found to be effective at excluding the related great cormorant from aquaculture ponds in Hong Kong. In Hong Kong, the use of wires over ponds spaced at 5- or 10-meter intervals reduced cormorant visits by 99.5% and 98.5%, respectively. Numbers of other piscivorous birds visiting wired ponds also decreased significantly. Only minor increases in visits to unwired ponds were observed. Researchers at Asia Ecological Consultants, Ltd., determined that the reduction in cormorant visits is cost-effective; many aquaculture facility operators agreed with this assessment.

Facility Design and Management and Roost Habitat Management

The most effective long-term way to minimize losses to fish-eating birds likely lies in the design and management of the facility. To protect against losses caused by cormorants, design aquaculture facilities and management practices with predatory birds in mind. For example, constructing smaller ponds can make it easier to exclude cormorants and other birds using netting. Fingerlings and other especially vulnerable fish can be protected by moving them to ponds in areas with high human activity; the close proximity to humans may itself be a deterrent, and should also make non-lethal harassment easier and less time-consuming.

Producers who use a multiple-cropping system (with fish of two or more different size classes within a pond), may want to consider switching to a single-batch system to reduce the period of time during which fish will be especially vulnerable to avian predators. This practice may be even more effective at reducing predation when combined with delayed introduction of fingerlings into food-fish ponds. Reducing stocking rates may also make ponds less attractive to cormorants.

Redesigning existing facilities may be prohibitively expensive for many producers and changing management practices may also result in a financial burden. However, where feasible, changes to design and management have the potential to provide long-term protection and to reduce the need to use other methods to prevent or control cormorant damage. If you are planning new facilities or additions to existing facilities, we urge you to consider incorporating some of the above facility design suggestions into your plans. If redesign of existing facilities is not an option, altering your management of fish within existing ponds may be worth serious consideration.

Finally, some researchers have suggested that modification of habitat used by cormorants for roosting may help alleviate depredations at nearby aquaculture facilities. Contact your state wildlife agency to determine if cormorant roost habitat modification is an option for you.

For a list of products useful in non-lethal harassment, please see our list of vendors and products.

If you have questions, comments, or would like to suggest additional non-lethal wildlife damage control techniques, please call the Wildlife Section of The HSUS at 202-452-1100 or send an e-mail to wildlife@hsus.org.

Sources

Agricultural Research Service, Science Update: "Water turkeys foiled by twine." Available online (scroll down) at www.ars.usda.gov/is/AR/archive/jan04/sci0104.html

Anon. 2002. Field trials on implementation of preventive measures to reduce cormorant predation in commercial fishponds. Final report prepared by Asia Ecological Consultants Ltd. submitted to Agriculture, Fisheries and Conservation Department, Hong Kong, Special Administrative Region. For a copy of this report, contact Mr. S.T. Tsim (st_tsim@afcd.gov.hk) or Bette Stallman (bstallman@hsus.org).

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Glahn, J.F., Werner, S.J., Hanson, T., and Engle, C.R. 2002. Cormorant depredation losses and their prevention at catfish farms: Economic considerations. In: L. Clark, J. Hone, J.A. Shivik, R.A. Watkins, K.C. VerCauteren, and J.K. Yoder (eds.), Human Conflicts with Wildlife: Economic Considerations. Proceedings of the Third NWRC Special Symposium. National Wildlife Research Center, Fort Collins, CO.

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Reinhold, D.S. and Sloan, C.A. 1997. Strategies to reduce double-crested cormorant depredation at aquaculture facilities in Mississippi. Symposium on Double-Crested Cormorant Population Status and Management Issues in the Midwest. December 9, 1997, Milwaukee, Wisconsin. Available online at http://www.aphis.usda.gov/ws/nwrc/is/cormorants/pdf/corsym10_nogfx.pdf.

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