Volume 49, No. 2

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Distribution patterns and population size of the Ashy Storm Petrel Oceanodroma homochroa


1 RG Ford Consulting, 2735 NE Weidler Street, Portland, Oregon 97232, USA
2 HT Harvey & Associates Ecological Consultants, 983 University Avenue, Los Gatos, California 95032, USA *(
3 Shearwater Journeys, Inc., PO Box 190, Hollister, California 95024, USA
4 National Marine Fisheries Service, Southwest Fisheries Science Center; current address: Marine Mammal Institute, Oregon State University, Newport, Oregon 97365, USA


FORD, R.G., TERRILL, S., CASEY, J., SHEARWATER, D., SCHNEIDER, S.R., BALLANCE, L.T., TERRILL, L., TOLLEFSON, M. & AINLEY, D.G. 2021. Distribution patterns and population size of the Ashy Storm Petrel Oceanodroma homochroa. Marine Ornithology 49: 193 - 204

Received 02 April 2020, accepted 27 Feb 2021

Date Published: 2021/10/15
Date Online: 2021/06/22
Key words: At-sea aerial surveys, California Current, hotspots, seabird rafts, at-sea vessel surveys, world population size


The Ashy Storm Petrel (ASSP) population has been surveyed intensively, both at sea within the California Current and on the adjacent breeding grounds. Nevertheless, colony-based estimates of breeding populations have significantly underestimated the species' abundance, especially in the northern portion of its range compared to the southern portion. The species has been described as abundant in central-northern waters, whereas it is considered rare or uncommon in the southern portion. We analyzed aerial and ship tracks from 1980 to 2017 (~497 000 km covering 91 267 km2), to estimate total ASSP population size and at-sea distribution. Modeling the results of these formal surveys (7211 ASSPs seen) led to an estimated total population of about 13 445 ASSPs (95% confidence interval = 10 128-27 820), with 62% of the population frequenting the northern portion of the range, i.e., north of Point Conception (especially Monterey Bay north through the Gulf of the Farallones to Cordell Bank), compared with 38% in the south (especially waters around the northern Channel Islands). Based on over a thousand well-organized bird-watching surveys, almost the entire population can occur in a single flock in the northern portion during the California Current's autumn Oceanic Period. These flocks occur in trophic hotspots: areas where submarine canyons cut into the shelf. Such large “molting flocks” have been shifting north from Monterey Bay to Cordell Bank since about 2007, but reasons for the northward shift remain a mystery. The apparent mismatch in at-sea vs. colony-based population sizes likely involves the non-breeding portion, which is especially difficult to estimate because it includes immature individuals as well as “floating” individuals: adults capable of breeding but denied access to breeding sites, which are limited by intraspecific competition.


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