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1 October 2009 At-Sea Distribution and Abundance of Seabirds Off Southern California: A 20-Year Comparison
David G. Ainley
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Thirty years ago, the U.S. Department of the Interior, Minerals Management Service (MMS), organized the Outer Continental Shelf Environmental Assessment Program to gather the information necessary to meet requirements under the National Environmental Policy Act toward development of minerals extraction on the U.S. continental shelf. A portion of the assessment involved marine birds, especially their abundance and distribution in waters off California, which were deemed by industry to have significant hydrocarbon potential (in production since the 1960s). A few years of intense aerial censusing on a grid extending from the U.S.-Mexico border to the California-Oregon border resulted in an impressive data set, available electronically and summarized in Studies in Avian Biology, no. 11 (Briggs et al. 1987). As of 1983, therefore, the status and distribution of marine birds in California and waters within 50 miles (80 km) of the coast were well quantified. These data were used to approve lease sales and, subsequently, to assess effects of oil spills in California waters. Since completion of that project, several studies have detailed significant changes in the populations of a number of avian species in these waters (e.g., Ainley et al. 1995a, Veit et al. 1997, Ainley and Divoky 2001, Oedekoven et al. 2001, Hyrenbach and Veit 2003). Those findings apparently led to the study under review here—a reassessment, again using aerial censusing, of seabird populations in the portion of California's continental-shelf waters still available, or currently being used, for minerals extraction.

Mason et al.'s study area included waters from the coast to the outer edge of the continental shelf (3,000 m isobath, 80–100 km from the mainland beach) from Morro Bay to San Diego. Flights were flown on 102 days during three months: January, May, and September (May 1999–January 2002). Using the data obtained and reanalysis of Briggs et al.'s (1987) data, the present volume compares spatial distributions and populations of seabirds in 1975–1978, 1980–1983, and 1999–2002. The methods used were carefully considered, and the data are of high quality.

This work is in the same vein as many of the early volumes of Pacific Coast Avifauna and Studies in Avian Biology: sound, informative reporting on the status of various populations on the West Coast. The present volume is 101 pages long, but 70 pages are taken up by tables and figures. Therefore, this “monograph” would fit better into the category of a “long paper.” A short introduction and short discussion, two pages each, bracket a series of 32 species accounts, with a map showing January, May, and September survey results accompanying each. Given that the study was designed to provide data for environmental assessment reports, and to determine the extent to which seabird population size had changed in the study area, no hypotheses were involved in study design and no findings were directly related to answering ecological questions. However, pertinent literature was reviewed in each account with regard to the status of each species, including a substantial amount of unpublished information on colony sizes in the study region. Acquisition of the survey data represents a valuable achievement, and no doubt the data will be extensively used in the near future.

Consistent with the above-referenced studies detailing significant avifaunal change, this one confirmed that overall seabird density has declined in the southern portion of the California Current (CC) during recent decades. Accordingly, particularly large decreases were evident in three of the more abundant CC species, Sooty Shearwater (Puffinus griseus), Bonaparte's Gull (Chroicocephalus Philadelphia), and Common Murre (Uria aalge); but increases were found in 10 species that contribute much less to the overall California seabird fauna (i.e., “common” instead of “abundant”), including Ashy Storm-Petrel (Oceanodroma homochroa), Brown Pelican (Pelecanus occidentales), Brandt's Cormorant (Phalacrocorax penicillatus), Western Gull (Larus occidentalis), Xantus's Murrelet (Synthliboramphus hypoleucus), and Cassin's Auklet (Ptychoramphus aleuticus). These indications of increase may be problematic for wildlife managers given the proposed removal (warranted, in my opinion) of the Brown Pelican from the federal list of endangered species and proposals now being considered to add Xantus's Murrelet and Ashy Storm-Petrel to the list. In the Discussion, the authors attempt to reconcile their findings with other data that indicate these species are in jeopardy. In regard to the spatial results of this study, they confirm, demonstrating consistency over time, Briggs et al.'s (1987) results showing that waters between the northern Channel Islands and Point Conception form an avian “hotspot”—owing, I might add, to a conspiracy of oceanographic factors that enhance ocean production and zooplankton retention. Such information on the spatial occurrence of CC seabirds will be informative in the process currently gathering momentum, nationally and internationally, to designate at-sea “important bird areas.”

The monograph ends with a very brief statement about what factors may account for the reduced avian density in the CC over recent decades. Fingers are pointed, of course, to global (climate) change, with little elaboration. In that regard, the most surprising gap in the interpretation of the study's results is apparent ignorance among the authors of the oceanographic monitoring program that completely overlaps this study, both spatially and temporally: the California Cooperative Oceanic Fisheries Investigations (Cal-COFI), underway annually since the 1950s. A seabird component of Cal-COFI has existed since 1987 (Veit et al. 1997, Hyrenbach and Veit 2003, continued by PRBO Conservation Science). Cal-COFI is, to date, one of the most comprehensive biological oceanographic efforts anywhere on the planet and has contributed hugely to our understanding of changes in ocean foodwebs of the southern CC specifically, and of climatic and other impacts on marine foodwebs generally. Like the present MMS aerial study, Cal-COFI vessels and remote vehicles occupy a closely spaced grid, though oriented perpendicular to CC flow rather than along latitude lines as in the MMS study. Should MMS have an interest in understanding the factors behind changes in marine bird populations off Southern California (i.e., the extent to which they may be linked to remotely driven vs. immediate environmental factors), I must disagree with the authors' recommendation that this aerial effort be repeated periodically and recommend that the continuous Cal-COFI effort be supported instead. Monitoring efforts of this latter sort, especially multidisciplinary ones based on more than just remote sensing or snapshots, are increasingly difficult to fund; thus, available resources should be directed toward maintaining their continuity when appropriate (the case here). The present volume spends about one third of its Discussion assessing the degree to which MMS snapshots have been representative of patterns in general or to what degree each has been skewed by El Nino and La Nifta events—and, I might add, El Niño-like non-El Niños-known to perturb seabird patterns off California dramatically (Ainley et al. 1995b, and references above). The long-term, annual effort of Cal-COFI would overcome this problem. Fear not, I have no financial or vested interest in Cal-COFI or in the Long-Term Ecological Research grant that the National Science Foundation recently bestowed to facilitate, in part, its continuation. It's just great science that could be used to develop a better understanding of the spatial and temporal variation in bird populations, at sea in a world of accelerating change. In any case, for the latest on the status of marine birds in coastal and offshore waters of Southern California, Studies in Avian Biology, no. 33 should be your go-to reference.



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David G. Ainley "At-Sea Distribution and Abundance of Seabirds Off Southern California: A 20-Year Comparison," The Auk 126(4), 937-939, (1 October 2009).
Published: 1 October 2009

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