Changes in Breeding Population Size of Brandt's and Double-crested Cormorants in California, 1975-2003

• Main Authors: Phillip J. Capitolo, Harry R. Carter, Richard J. Young, Gerard J. McChesney, William R. McIvers, Richard T. Golightly, Franklin Gress
• Format: Report
• Language: English
• Published: U.S. Fish and Wildlife Service Contract # 10154-2-G106; Humboldt State University; Apex Houston Trustee Council; U.S. Geological Survey; California Department of Fish and Game; U.S. Navy 2012
• Subjects: Population size; California; Breeding colonies; Phalacrocorax penicillatus; Double-crested cormorant; Phalacrocorax auritus; Brandt's cormorant; Uria aalge; uria
• Online Access: http://hdl.handle.net/2148/930

In 2003, Humboldt State University (Department of Wildlife) and the U.S. Fish and Wildlife Service (San Francisco Bay National Wildlife Refuge) cooperated to conduct a survey of all coastal colonies of Brandt’s and Double-crested Cormorants (Phalacrocorax penicillatus, P. auritus) in California. Our goals were: a) to continue annual aerial photographic surveys of these colonies, which have been conducted since 1985-91; and b) to rapidly assess possible changes in populations of these species through comparisons to previous state-wide summaries in 1975-80 and 1989-91. In northern and central California, numbers of nests at each colony in 2003 were counted from aerial photographs, except at two large colonies of Double-crested Cormorants on bridges in San Francisco Bay, where nests were counted from boat surveys. Due to insufficient funds to count all 2003 photographs, 2001 aerial photographic survey data were substituted for southern California colonies. For Anacapa Island colonies, 2001 boat and ground survey data collected by the California Institute for Environmental Studies were used. Numbers of Common Murres (Uria aalge) and Caspian Terns (Sterna caspia) also were counted at specific colonies in California in 2003. In 2001-03, 26,718 Brandt’s Cormorant nests were counted at 97 active colonies and 6,477 Double-crested Cormorant nests were counted at 42 active colonies in coastal California. The nest total for Brandt’s Cormorants was 29.1% lower than in 1989-91 surveys and less than 1% lower than in 1975-80 surveys. The nest total for Double-crested Cormorants was 48.2% higher than in 1989-91 surveys and 592.0% higher than in 1975-80 surveys. Numbers of Brandt’s Cormorants in northern California declined from 1975-80 to 2003. Numbers of Brandt’s Cormorants in central and southern California increased from 1975-80 to 1989-91 before declining between 1989-91 and 2003 and 2001, respectively. In central California, numbers of Brandt’s Cormorants declined at the South Farallon Islands from 1975-80 to 2003, but ...