Wildlife Services

Protecting People 

Protecting Agriculture 

Protecting Wildlife

National Wildlife Research Center

FY 2017

Reducing Predation, Habitat Damage and 

Disease Potential of Fish-eating Birds in 

Aquaculture Systems

Contact Information:

Dr. Fred Cunningham 

Supervisory Research Wildlife Biologist 

Mississippi Field Station 

P.O. Box 6099 

Mississippi State, MS 39762-6099 

Phone: (662) 325-8612 

FAX: (662) 325-8704 

Email:

fred.l.cunningham@aphis.usda.gov 

Website: www.aphis.usda.gov/

wildlifedamage/nwrc/

Groups Affected:

Aquaculture producers, distributors

and retailers

Catfish Farmers

Consumers

Sportfish guides and outfitters

Wildlife managers

Major Cooperators:

Canadian Wildlife Service

Catfish Farmers of America

Cornell University

Delta Research and Extension

Center, Thad Cochran National

Warmwater Aquaculture Center

Idaho Department of Fish and

Game

Michigan Department of Natural

Resources

Mississippi Agricultural and

Forestry Experiment Station

Mississippi State University,

College of Veterinary Medicine

Mississippi State University,

Department of Wildlife, Fisheries,

and Aquaculture

Ontario Ministry of Natural

Resources

Ontario Parks

Southern Regional Aquaculture

Center

U.S. Environmental Protection

Agency-Great Lakes Restoration

Initiative

Vermont Fish and Game

Department

Wildlife Services Operations

Wisconsin Department of Natural

Resources

National Wildlife Research Center Scientists Address Aquaculture Losses

Wildlife Services' (WS) National Wildlife Research Center (NWRC) is the only Federal research facility

devoted exclusively to resolving conflicts between people and wildlife through the development of

effective, selective and socially responsible methods, tools, and techniques. The NWRC's field station

in Starkville, MS, is located in the heart of the primary aquaculture producing area of the southeastern

United States and was established to develop methods to reduce the economic impacts of fish-eating

birds, such as double-crested cormorants, pelicans, egrets, and herons, on aquaculture stocks. 

In the past several decades, populations of fish-eating birds have increased dramatically and caused

substantial economic impacts on aquaculture production. For the catfish aquaculture industry alone,

costs associated with bird damage and damage prevention are estimated at approximately $25 million

annually. NWRC's research goal is to determine the economic impact of fish-eating birds on

aquaculture production and natural resources and to develop methods to reduce depredation impacts

on southeastern catfish, baitfish, and crawfish industries. Current research aims to gain information

about the abundance, distribution, and foraging behavior of fish-eating birds; economic impacts

associated with their foraging activities; and diseases they transmit at aquaculture facilities. Information

gathered as a consequence of this research provides a basis for developing new strategies,

techniques, and tools for reducing damage.

Applying Science and Expertise to Wildlife Challenges

Isotopes Used to Track Cormorant Diets. — Hidden in the feathers of birds are clues to their diet

and migratory behavior. That's because, over time, isotopes that make up the food animals eat remain

in their feathers and other tissues. To learn more about double-crested cormorant diet and behavior,

NWRC and Cornell University researchers looked for differences in the ratios of carbon, nitrogen and

sulfur atoms in the feathers of cormorants from eastern North America. Researchers discovered that

the feathers from birds feeding on Mississippi farm-raised catfish showed a unique isotope signature,

compared to the feathers of birds feeding on natural or marine resources. Birds that ate farm-raised

catfish showed relatively low levels of sulfur and nitrogen in their feathers. They also showed carbon 13

values that were in between those found for birds feeding in natural marine and freshwater habitats.

This isotope approach confirmed what researchers suspected about cormorant migratory patterns and

winter habitat use. All sampled cormorant colonies use aquaculture habitats during the winter and were

from colonies in Lakes Huron and Erie. However, more male cormorants feed at aquaculture habitats,

while marine habitats were used more by females. Researchers note targeting birds at wintering

grounds in the Southeast, may be more effective at reducing damage.

Environmental Impacts of Cormorants to Forested Islands. — Within the last 10-15 years, doublecrested cormorants have begun nesting on islands in Alabama causing concern among managers and

recreationalists regarding their impacts on the fisheries, vegetation, soil, and water quality. To better

assess these environmental impacts, NWRC and Mississippi State University researchers compared

water quality, soil chemistry and tree health of six forested islands with and without nesting cormorants.

Results showed that water quality and chemistry did not differ between islands with and without

cormorant colonies. However, soil from islands with nesting cormorants was more acidic and had

greater concentrations of phosphorous than soils from islands without nesting cormorants. In addition,

researchers found evidence that cormorants were negatively affecting tree health and vigor. Although

nesting cormorants are impacting forested islands, researchers note this is a natural process and

recommend that management efforts to reduce damage should be considered within a framework that

allows for natural ecological processes, including changes in soil chemistry and vegetation.

Competition between Pelicans and Anglers. — The Idaho Department of Fish and Game (IDFG)

annually stocks about 1.8 million hatchery catchable trout state-wide at a cost of about $2.5 million.

Because of increasing concerns over pelican predation and potential competition with anglers for

recently released hatchery trout, NWRC and IDFG researchers internally PIT-tagged and externally

United States Department of Agriculture 

Animal and Plant Health Inspection Service
The U.S. Department of Agriculture is an equal opportunity employment provider and employer

anchor-tagged stocked rainbow trout at 12 areas in Idaho. After

juvenile birds fledged and left their nesting grounds, researchers

recovered PIT tags from fish that were deposited by the birds.

Pelican predation on hatchery trout averaged 18 percent and

ranged from 0 to 48 percent. Angler catch averaged 21 percent

and ranged from 0 to 82 percent. Results also showed that the

mean angler catch was nearly 4 times higher when pelican

predation was less than 25 percent. Findings suggest that

predation by American white pelicans on catchable-sized

hatchery rainbow trout stocked in southern Idaho waters often

exceeds the total catch of those fish by anglers who compete

directly with avian predators for use of stocked trout. 

Pelican and Cormorant Parasitology and Disease 

Epidemiology. — NWRC researchers conducted several studies

with pelicans, cormorants, and other fish-eating birds to determine

their role in spreading Bolobophorus damnificus trematodes and

the virulent strain of Aeromonas hydrophilia

(VAh) on aquaculture farms. The B. damnificus trematode costs 

the Mississippi catfish industry approximately $27 million in 

damages each year. Study results indicated that adult B. 

damnificus trematodes were found only in American white 

pelicans, the parasite's definitive host. In other studies, 

researchers found that cormorants, wood storks, great egrets, 

and pelicans could shed viable VAh after consuming infected fish 

and can shed bacteria that caused a VAh disease outbreak in 

naive catfish ponds--implicating them as a disease reservoir 

and a vector capable of spreading the pathogen to

uninfected areas. These studies stress the need to reduce 

predatory feeding and scavenging on commercial catfish 

operations.

Cormorant Predation on Multiple-Batch Production of

Channel Catfish. — Double-crested cormorants are the primary

bird predator on commercially produced channel catfish. To learn

more about cormorants' economic impacts on catfish farms,

NWRC researchers simulated different levels of cormorant

predation on channel catfish in a multiple-batch cropping system.

A multiple-batch cropping system contains ponds that include fish

of varying sizes. This method allows faster growing fish to be

selectively harvested while fingerlings are added to replace the

harvested fish. The process continues for years without draining

the pond. Results of varying predation levels on 40, 0.05-hectare

catfish ponds showed that production costs for catfish farming

increase as cormorant predation increases. The maximum

increase in production cost is $0.143 per kilogram. However,

researchers note that losses due to cormorant predation are

partially offset by increases in individual fish growth due to lower

catfish densities.

Selected Publications:

Craig, E.C., B.S. Dorr, K.C. Hanson-Dorr, J.P. Sparks, and P.D.

Curtis. 2015. Isotopic discrimination in the Doublecrested cormorant (Phalacrocorax auritus). PloS One 

10(10):e0140946. doi: 10.1371/journal.pone.0140946. 

Craig, E.C., D.T. King, J.D. Sparks, and P.D. Curtis. 

2016. Aquaculture depredation by Double-Crested Cormorants 

breeding in eastern North America. Journal of Wildlife 

Management 80:57-62; doi: 10.1002/jwmg.989. 

Dorr, B.S. and C.R. Engle. 2015. Influence of simulated 

double-crested cormorant, Phalacrocorax auritus, predation on 

multiple-batch production of channel catfish, Ictalurus 

punctatus. Journal of the World Aquaculture Society 46(3): 

319-327. doi:10.1111/jwas.12187.

Jubirt, M.M., L.A. Hanson, K.C. Hanson-Dorr, L. Ford, S.

Lemmons, P. Fioranelli, and F.L. Cunningham. 2015. Potential for

Great Egrets (Ardea alba) to transmit a virulent strain of

Aeromonas hydrophila among channel catfish (Ictalurus 

punctatus) culture ponds. Journal of Wildlife Diseases 

51(3):634-639. doi: 10.7589/2014-06-156.

Lafferty, D.J.R., K.C. Hanson-Dorr, A.M. Prisock, and B.S. Dorr.

2016. Biotic and abiotic impacts of Double-crested cormorant

breeding colonies on forested islands in the southeastern United

States. Forest Ecology and Management 369:10-19. doi:

10.1016/j.foreco.2016.03.026. 

Meyer, K.A., C.L. Sullivan, P. Kennedy, D.J. Schill, D.M. Teuscher,

A.F. Brimmer, and D.T. King. 2016. Predation by American white

pelicans and double-crested cormorants on catchable-sized

hatchery Rainbow trout in select Idaho lentic waters. North

American Journal of Fisheries Management 36(2):294-308. doi:

10.1080/02755947.2015.1120835.

Major Research Accomplishments:

WS and Cornell University researchers discovered that the

feathers from double-crested cormorants feeding on Mississippi

farm-raised catfish showed a unique isotope signature, compared

to the feathers of birds feeding on natural or marine resources.

WS and Mississippi State University researchers compared

water quality, soil chemistry and tree health of six forested islands

with and without nesting cormorants. Results showed that water

quality and chemistry did not differ between islands with and

without cormorant colonies. However, soil from islands with

nesting cormorants was more acidic and had greater

concentrations of phosphorous than soils from islands without

nesting cormorants.

WS research suggests that predation by American white

pelicans on catchable-sized hatchery rainbow trout stocked in

southern Idaho waters often exceeds the total catch of those fish

by anglers who compete directly with avian predators for use of

stocked trout.

WS disease studies indicated that adult B. damnificus

trematodes were found in American white pelicans. Furthermore,

cormorants, wood storks, great egrets, and pelicans could shed

the virulent strain of Aeromonas hydrophilia after consuming

infected fish--implicating them as a disease reservoir and a

vector capable of spreading the pathogen to uninfected areas.

WS research found that production costs for catfish farming

increase as cormorant predation increases. The maximum

increase in production cost is $0.143 per kilogram. However,

researchers note that losses due to cormorant predation are

partially offset by increases in individual fish growth due to lower

catfish densities.