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Project Accomplishments

Defining Economic Impacts and Developing Strategies for Reducing Avian Predation in Aquaculture Systems

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 2010

In the past 30 years, populations of fish-eating birds have increased dramatically, causing substantial economic impacts to aquaculture production. Aquaculture industry costs associated with bird damage and damage prevention are estimated to exceed $25 million annually. The goal of National Wildlife Research Center (NWRCs research is to determine the impact of fish-eating birds on aquaculture production and natural resources and to develop methods to reduce depredation of southeastern catfish, baitfish, and crawfish industries. Current research is aimed at gaining information about the abundance, distribution, and foraging behavior of fish-eating birds, the economic impacts associated with their foraging activities, and the diseases they transmit at aquaculture facilities. This information will help to develop new techniques for reducing damage.

photo of cormorantEvaluation of Cormorant Harassment To Limit Depredation on Sport Fish— The interior population of double-crested cormorants (Phalacrocorax auritus) has shown a substantial resurgence over the past 35 years, from approximately 32,000 breeding pairs in the mid-1970s to more than 226,000 breeding pairs (including the Great Lakes States and Canadian provinces) by the mid-1990s. Diverse management techniques are used to mitigate conflicts between humans and cormorants, including harassment (e.g., pyrotechnics) supplemented by lethal removal.

In this study, NWRC scientists evaluated impacts of cormorant harassment programs on the walleye (Sander vitreus) fishery in Brevoort Lake, MI, and the yellow perch (Perca flavescens) and walleye fisheries at Drummond Island, MI. Cormorant foraging declined significantly at both locations after scientists initiated harassment programs in the spring. Overall harassment deterred 90% of cormorant foraging attempts, with less than 6 percent of the cormorants taken lethally on average at each site. Yellow perch were a predominant prey item in number and biomass at both locations. Walleye made up a small proportion of the diet at both locations. However, both walleye and yellow perch abundance increased significantly at Drummond Island following cormorant harassment. The estimated cormorant consumption of age-1 walleye in the absence of management at Brevoort Lake during 2005 accounted for 55% of the 2006 age-1 walleye population. Walleye abundance increased to record levels in 2008 following three years of cormorant management at Brevoort Lake. These results support the hypothesis that cormorant predation on spawning aggregations of sportfish was a significant mortality factor, and that cormorant management could reduce sportfish mortality and increased abundance at both locations. Continuation of cormorant harassment programs and fishery assessments will determine whether improvement of targeted sport fisheries is sustainable.

Sex and Age Bias in American White Pelicans Captured on Wintering Grounds—American White Pelican ( Pelecanus erythrorhynchos) use of aquaculture facilities in the southeastern United States has increased during the last 20 years. While capturing loafing pelicans with foot-hold traps and rocket nets for studies evaluating impacts of foraging pelicans on the aquaculture industry, NWRC scientists noticed an apparent age and sex bias among the captured and collected pelicans, which suggests segregation of age and sex classes among pelicans.

In a study to examine this issue further, scientists analyzed data from 284 pelicans captured or collected near aquaculture ponds in Alabama, Arkansas, Louisiana, and Mississippi from 1998 to 2009. Immature male pelicans (73% of those captured) appeared more vulnerable to capture than female and adult male pelicans. This vulnerability may be due to immature birds photo of juvenile pelicansbeing more naive than older birds and less wary of humans or disturbances at a loafing site. The scientists also found a preponderance of males (38% adult male, 47% immature male, 15% female) among pelicans removed from catfish farms and loafing sites. Female American white pelicans appear to be less available for capture or collection at loafing sites in the southeastern United States because female and male pelicans segregate at loafing sites, or males and females spend the winter in different geographic areas. Based on these findings, the scientists recommended additional research to determine if sex ratios of pelicans are male-biased at birth and/or fledging and to confirm whether pelicans of known age and sex are spatially segregated on their wintering grounds.

Feeding Behavior and Diet of Black-Crowned Night Herons—Little is known about the effects of black-crowned night herons (Nycticorax nycticorax) on catfish production in the southeastern United States. Because these fish-eating birds inhabit important catfish production areas and opportunistically exploit abundant food resources, NWRC researchers monitored the movements of free-ranging night herons during 2004 to 2006 and documented their nocturnal use of catfish ponds in Mississippi to assess their potential impacts on catfish aquaculture facilities. The researchers also collected 75 night herons for stomach content analysis.

photo of black-crowned night heronDuring the summer and early fall each year, the researchers observed approximately 85 night herons per biweekly survey. In 2005 and 2006, on randomly selected ponds, few birds were present, indicating selective or clumped use of the study area. Night herons left the study area beginning in November and were gone by January of each year. The birds returned in late spring or summer, with peak abundance occurring in September of each year. Seventy-two percent of the stomachs analyzed contained catfish fingerlings, with 0 to 26 fingerlings per stomach. The fingerlings' mean length and weight was 9.8 centimeters and 11.0 grams, respectively. More research is needed to document the birds' economic impact and to assess the issue of compensatory mortality.

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Starkville, MS, Field Station



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