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Seabirds off Southern California—Surveys Reveal Patterns in Abundance and Distribution

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Average density of seabirds counted during January surveys, 1999-2003
Above: Average density of seabirds counted during January surveys, 1999-2003, in the central part of the study area (study was conducted from Cambria, CA, to the United States-Mexico border). Largest dots indicate 589 to 1,172 seabirds/km2; smallest dots indicate 0 to 41 seabirds/km2. In general, seabirds were concentrated near the northern Channel Islands and along mainland and island coasts. [larger version]

Aerial view of a nesting colony of Brandt’s cormorants
Above: Aerial view of a nesting colony of Brandt’s cormorants in the Channel Islands. Learn more about this large seabird at the Patuxent Bird Identification InfoCenter. Photograph by Gerry McChesney. [larger version]

From 1999 to 2003, a team of researchers from the U.S. Geological Survey (USGS) and Humboldt State University (HSU), in cooperation with the Minerals Management Service (MMS), conducted a project to study the at-sea distribution and abundance of seabirds off the southern California coast, from Cambria, CA, to the United States-Mexico border. This region includes critical habitat for numerous seabird species, and more than 20 species of seabirds breed here, primarily on the Channel Islands, including 4 threatened or endangered species. Southern California waters also are used extensively by people and are economically important, contributing about $9 billion annually to local economies through offshore oil production, oil transportation by tankers, commercial shipping, commercial fishing, military activities, and public recreation. Such high demands on southern California waters can cause conflicts between ecological and economic interests, and these potential conflicts prompted our study.

Using aerial surveys, researchers identified 54 species of seabirds, belonging to 12 families, and counted more than 135,000 seabirds during surveys conducted in January, May, and September of each year. In total, researchers flew more than 102 days and surveyed 55,000 km of transect lines. These data indicate that about 1 million seabirds use the area off southern California during January, with slightly reduced abundances in May and September. Seabirds were mainly concentrated near the northern Channel Islands and along mainland and island coasts. Seabird species that were most abundant at sea during the January surveys include California gulls, western grebes, and Cassin's auklets, whereas sooty shearwaters, phalaropes, and western gulls were most abundant during the May and September surveys.

Current seabird densities were also compared with historical data collected in the same area more than two decades previously. Current seabird densities were determined to be 14 percent (January), 57 percent (May), and 42 percent (September) below historical estimates. Species with notable declines included common murres, sooty shearwaters, and Bonaparte's gulls. But not all species declined—in fact, brown pelicans, Xantus's murrelets, Cassin's auklets, ashy storm-petrels, western gulls, and Brandt's cormorants have all increased in abundance. Changes in the marine environment, both biologic and anthropogenic, probably have affected the abundance and distribution of seabirds off southern California. USGS researchers will continue seabird research off southern California and are currently conducting radio-telemetry studies on ashy storm-petrels.

Science Team: John Mason (HSU at time of study, now with USGS and University of California, Davis), Gerry McChesney (HSU at time of study, now with U.S. Fish and Wildlife Service [FWS]), Bill McIver (HSU at time of study, now with FWS), Harry Carter (HSU), John Takekawa (USGS), Rick Golightly (HSU), Josh Ackerman (USGS), Dennis Orthmeyer (USGS at time of study, now with California Waterfowl Association), Bill Perry (USGS), Julie Yee (USGS), Mark Pierson (MMS), and Mike McCrary (MMS).

Aerial view of Prince Island Observer Bill McIver records data.
Above Left: Aerial view of Prince Island, off the northeast coast of much larger San Miguel Island in southern California’s Channel Islands. Photograph by Josh Adams, USGS.

Above Right: Observer Bill McIver records data. Photograph by Gerry McChesney.

Related Web Sites
U.S. Geological Survey (USGS)
Patuxent Bird Identification InfoCenter
U.S. Geological Survey (USGS)

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in this issue: Fieldwork cover story:
Sediment Impacts on Reef Corals

Coastal Ground Water Discharge

Benthic Habitats Near Oil Platforms

Research Abundance and Distribution of Southern California Seabirds

Suspended Sediment, Turbidity, and Fish Feeding Behavior

USGS Monterey Bay Science Prototype

Outreach Woods Hole Science Center Participates in Open House

10 Years of Ask-A-Geologist

Awards Scientists Rescue Boy from Rip Current

USGS Biologists Receive DOI Honor Awards

Staff & Center News Oceanographer Joins Western Coastal and Marine Geology Team

Netherlands Students Assist USGS in Florida

USGS Represented in Parade

Publications Report on Hazards Offshore Ventura County

USGS Contributes to New Book About Point Loma

November Publications List

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