Technical Report

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

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 increased from 1975-80 to 1989-91 at all nearshore colonies combined. Some Brandt’s Cormorants apparently moved from this large offshore colony to nearshore colonies in central California by 1989-91. While numbers of Double-crested Cormorants increased from 1975-80 to 2003 in northern and central California, numbers in southern California increased from 1975-80 to 1989-91 but declined form 1989-91 to 2001. Survey techniques and seasonal timing for counting nests at colonies were similar in 1975-80, 1989-91, and 2001-03, and surveys did not occur immediately following cyclic population dips of Brandt’s Cormorants related to severe El Nino events in 1983, 1992, and 1998. Numbers of Cormorant nests were generally comparable between these three surveys. However, survey error may amount to ± 10%, which must be taken into account when interpreting comparisons. Mild to moderate El Nino conditions existed in the California Current prior to and during 2003 surveys. Mild effects on breeding Brandt’s Cormorants were noted in central California (but not in northern California) and no effects were observed on Double-crested Cormorants. Upwelling conditions in southern California in 2001 were stronger than normal but had no major observed effect on either species. In 2003 at Castle Rock National Wildlife Refuge, 62,504 Common Murres were counted. Using a k correction factor of 1.67, a breeding population of 104,400 birds was estimated. At Arcata Bay Sand Island (a colony lost in 1969 with recolonization after 1989), 262 Caspian Terns were cunted. At Knight Island in San Pablo Bay (a colony established in the 1970s), 321 terns were counted.

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