Using aerial photography, we examined Brandt’s Cormorant Phalacrocorax penicillatus breeding population trends during the 1979–2006 period in the Gulf of the Farallones (GF), California, at the center of the species’ breeding range and where its largest single breeding assemblage has occurred (South Farallon Islands [SFI]). In 1979, about 22 000 Brandt’s Cormorants bred in the GF, mostly at SFI and mainland colonies north of San Francisco Bay. By 1985, breeding populations were greatly reduced at all active GF colonies following impacts from the strong 1982–1983 El Niño event. During the 1985–1995 period, no trend was detected for SFI, whereas mainland colonies combined increased by 19% per annum. Alcatraz Island inside San Francisco Bay and Año Nuevo Island at the southern end of the GF were colonized, and the new colonies grew rapidly. In 1998, the GF breeding population was again reduced during a strong El Niño. Subsequent growth during the 1998–2006 period was dramatic for all colonies (range 13–29% per annum; 18% combined), associated with strong La Niña conditions during 1999–2000 and high reproductive success during 1999–2006 (except 2003). By 2006, the GF total breeding population was the largest ever recorded (34 876 birds), and the SFI colony had reached a size (~23 500 breeding birds) similar to the previous peak estimate in 1974. Population increase at mainland colonies since the late 1980s likely reflected: 1) immigration of SFI birds; as a result of 2) reduced availability of juvenile rockfishes Sebastes spp. offshore and increased use of Northern Anchovies Engraulis mordax nearshore as prey; and 3) protection of potential breeding habitat.
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