Effects of the 1997-1998 El Niño and 1998-1999 La Niña events on breeding waterbirds and sea lions in the Upper Gulf of California, México

Abstract

During 1998 and 1999 I monitored waterbirds nesting on Isla San Jorge, at La Purinera and on Isla Montague, in the northern Gulf of California, as well as sea lions on Isla San Jorge. During El Niño the brown boobies on San Jorge ceased breeding and left the island, coinciding with an influx of several thousand blue-footed boobies. El Niño conditions coincided with an increase in the numbers of nesting least, elegant, and royal terns and black skimmers on Montague. Double-crested cormorants on San Jorge, least terns on La Purinera, and snowy egrets, black-crowned night-herons, laughing gulls, and gull-billed terns on Montague did not seem affected by this event. Sea lions at San Jorge about doubled their number, but the number of pups did not increase. La Niña conditions coincided with a return to population levels before El Niño in all these species. During 1999 Heermann's gulls and Brandt's cormorants nested on San Jorge. Heermann's gulls had stopped breeding there long ago, and Brandt's cormorants had never been know to nest there.