Do cormorants injure fish without eating them? An underwater video study
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The populations of European great cormorants (Phalacrocorax carbo) and North American double-crested cormorants (Phalacrocorax auritus) have increased sharply over the last decades and these piscivorous birds are suspected to deplete valuable fish stocks and compete with human fisheries. Beyond direct consumption of fish, cormorants are accused of injuring vast numbers of prey without eating them. Using underwater video systems, one of them mounted onto the back of tame Chinese cormorants, we evaluated the proportion of successful pursuits of cormorants on live fish. Trials were conducted with 6 great cormorants and 9 double-crested cormorants and involved a total of 676 prey pursuits. We show that, although they are regarded as highly efficient predators, cormorants aborted about half of their pursuits. However, detailed analysis of prey-capture behaviour in double-crested cormorants revealed that only 0.4% of the prey pursued was injured without being ingested. Further studies using miniature video systems deployed on free-ranging cormorants are required to complement our knowledge of their hunting tactics.
KeywordsGreat Cormorant Juvenile Rainbow Trout Dive Bout Predatory Performance Phalacrocorax Auritus
This study was funded by the K C Wong Foundation, the DRI of the CNRS, the CNRS headquarters in Beijing, the British Council Paris and the European Union (IMPRESS Project; QRRS 2000-30864). For the Chinese part of the study grateful thanks are due to Marie-Pierre van Hoecke, Florence Hesters, Thi-Ngeune Lo, Bruno Ormile-dautel, Céline Le Bohec, Jean-Yves Georges and Yvon le Maho for their support at different stages of our project. Many thanks also to Zhang shiyuan and his family for hosting us in Anxin, and to Gerrit Peters (Earth & Ocean Technologies, Kiel, Germany) for tuning the cormo-cam. For the Canadian part of the study we would like to thank Dave Jones (UBC Zoology) for access to the Animal Care and diving facilities at South Campus. Arthur Vanderhorst and Sam Gopaul provided excellent support in caring for the birds. Bruce Gillespie and Don Brandies (UBC Zoology Mechanical Workshop) were of great help for technical questions. Ken Scheer from the Fraser Valley Trout Hatchery is thanked for generously providing us with live rainbow trout. Tom Hoeg was indispensable in setting up the Underwater Camera System within the dive tank and generously donated knowledge, expertise and time. Thanks also to Robin Liley (UBC Zoology) for tolerating the takeover of his fish holding tanks and access to video equipment. We are indebted to Magali Grantham for her enthusiastic help with animal care and experimentation and to Lovena Toolsy who helped with the video analysis. The British Columbia Ministry of Environment granted holding permits for the double-crested cormorants. All experimental procedures were approved by the UBC Animal Care Committee (Animal Care Certificate # A02-0122) and were in compliance with the principles promulgated by the Canadian Council on Animal Care. Two anonymous referees provided very useful comments.
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