National Wildlife Research Center (NWRC)

PROJECT GOAL: Develop an understanding of the economic impacts of damage inflicted on aquaculture production systems by cormorants, pelicans, wading birds, and waterfowl and develop tools and techniques for reducing that damage.  

Project Accomplishments 2007

NWRC Scientists Study the Reproductive Status of Double-Crested Cormorants (DCCOs) in the Eastern United States—The recent abundance and foraging habits of DCCOs have thrust this species into conflict with the aquaculture industry and fisheries interests over both perceived and documented impacts to natural and commercial resources.  Consequently, resource managers are aggressively employing control measures on DCCOs’ breeding, migratory, and wintering areas to alleviate damage.  Unfortunately, the breeding biology of cormorants is poorly understood, and without this basic knowledge, scientists cannot accurately predict the effects of management on cormorant populations nor attribute observed changes in reproductive success and population size to management actions.  To better understand the breeding biology of cormorants, NWRC scientists began a study in 2007 to determine their reproductive status in the Northeastern, Midwestern, and Southern United States.  This study will allow researchers to determine the proportion of nonbreeding individuals associated with breeding assemblages of cormorants and use this knowledge to improve population estimates of cormorants across the breeding range.

The Role of American White Pelicans in the Life Cycle and Spread of the Bolbophorus Catfish Trematode—The bolbophorus catfish trematode (Bolbophorus damnificus) is associated with high mortality rates in catfish fingerlings and the formation of cysts in the fillets of mature catfish.  NWRC scientists collaborated with parasitologists from the Mississippi State University College of Veterinary Medicine to determine the role American white pelicans play in the life cycle and spread of B. damnificus in catfish.  NWRC scientists confirmed the life cycle of this trematode and compared it to the known life cycle of a similar species.  The research provided further evidence that B. damnificus is a distinct species and the causative agent for trematode-associated deaths in catfish.  The research also showed that the American white pelican is the definitive avian host.

Additional studies were conducted to identify other potential intermediate hosts for the infectious trematode.  Results confirmed that the Planorbella trivolvis (a freshwater snail) population native to the Mississippi Delta is a first intermediate host for B. damnificus.  The studies also demonstrated that the P. trivolvis population found in North Dakota may also serve as intermediate hosts.  Populations of these snails are present in both the breeding and wintering ranges of the American white pelican and are susceptible to B. damnificus infection, which may serve to perpetuate the B. damnificus life cycle in the North-Central United States.

Cormorant Movement Patterns Identified—NWRC scientists attached satellite telemetry transmitters to DCCOs captured at roosts near aquaculture facilities in Alabama, Arkansas, Louisiana, and Mississippi to develop a better understanding of the winter and summer movements of cormorants using aquaculture farms in the Southeastern United States.  The winter home ranges for all cormorants averaged 37,384 km², with 3,467 km² in the core use area.  In summer, cormorants occupy a smaller territory—29,300 km² on average—and 2,889 km² in the core use area.  There were no differences in the ranges of adult v. immature birds or between capture regions (e.g., the Mississippi River flood plain of Arkansas, Louisiana, and Mississippi or eastern Mississippi and western Alabama).

This study verified that cormorants using aquaculture facilities in the Southeast are from the same population as those that are thought to impact natural resources in the Great Lakes region. Data from this study may be used to provide information necessary for regional and/or flyway-based DCCO population management strategies.

Scope of Cormorant Damage to Mississippi Catfish Production—The U.S. catfish industry is valued at more than $650 million, with nearly 65% of catfish production originating from Mississippi.  Biologists at NWRC’s Mississippi field station completed a study that evaluates the distribution and numbers of DCCOs on catfish aquaculture facilities, tying together almost a decade of research on cormorant food habits, bioenergetics, and abundance data.

A significant finding is that most depredation occurs on food-fish production ponds, where economic losses are high.  Study results demonstrate that cormorants used these ponds extensively during January through April, with the greatest economic damage occurring in February and March.  During the years of this study (winter 2000–01 and 2003–04), an estimated 1,775 and 1,347 metric tons of catfish were consumed by cormorants in the Delta Region of Mississippi.  This depredation translated into a loss to the industry of $10.3 million to $13.2 million annually.

Future studies will aim to develop efficient methods for monitoring cormorant abundance and distribution for purposes of damage estimation and cormorant management.

Development of New Aging Techniques for Birds—With collaborators at West Virginia University, NWRC scientists have found pentosidine concentrations in the skin of DCCOs to be a linear predictor of age.  This information may lead to a rapid, more cost-effective technique for identifying the age of cormorants and many other species of birds.  Pentosidine concentration analysis will provide scientists a better understanding of the demographics of cormorants that are impacting catfish production in the Southeast.  In addition, NWRC scientists are developing aging techniques for other bird species of concern, including monk parakeets and black vultures.

Cormorant Distribution and Abundance on Catfish Ponds in the Delta Region of Mississippi—Biologists evaluated the efficiency of aerial and ground surveys conducted twice monthly for detecting changes in cormorant abundance on aquaculture facilities.  Results showed that the sampling effort would need to be increased by approximately 40% to meet the desired level of detection (a 15% change in population levels).  Habitat modeling efforts evaluated the factors that affect cormorant use of catfish facilities.  Results suggest that cormorants are most likely to be present at food-fish ponds v. fingerling and broodfish ponds.  Additionally, bird use of catfish pond clusters was related to season of the year, distance of ponds from cormorant roosts, distance from roads, and the number of cormorants in nearby roosts.

Response of DCCOs to Management Activities on the Breeding Grounds in the Les Cheneaux Islands of Lake Huron, Michigan—Scientists from NWRC’s Mississippi field station collaborated with the WS Operational Program in Michigan and the Michigan Department of Natural Resources to use DCCO management as a means of improving the local yellow-perch fishery.  Several studies addressed different aspects of the damage attributed to cormorants or the management approach for dealing with the damage.

Management efforts resulted in more than a 90% reduction in the number of young cormorants produced annually and more than a 60%t reduction in cormorant numbers overall.  Research results indicate that cormorants from the colonies in question are feeding extensively in the specific areas where perch populations have declined, and that perch numbers following the first three years of management have improved.

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Starkville, MS, Field Station