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Recent Population Size, Trends, and Limiting Factors for the Double-Crested Cormorant in Western North America

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Title Recent Population Size, Trends, and Limiting Factors for the Double-Crested Cormorant in Western North America
Names Adkins, Jessica Y. (creator)
Roby, Daniel D. (creator)
Lyons, Donald E. (creator)
Courtot, Karen N. (creator)
Collis, Ken (creator)
Carter, Harry R. (creator)
Shuford, W. David (creator)
Capitolo, Phillip J. (creator)
Date Issued 2014-09 (iso8601)
Note To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The article was published by John Wiley & Sons Ltd. and is in the public domain. The published article can be found at: http://onlinelibrary.wiley.com/journal/10.1002/%28ISSN%291937-2817.
Abstract The status of the double-crested cormorant (Phalacrocorax auritus) in western North
America was last evaluated during 1987–2003. In the interim, concern has grown over the potential
impact of predation by double-crested cormorants on juvenile salmonids (Oncorhynchus spp.), particularly
in the Columbia Basin and along the Pacific coast where some salmonids are listed for protection under
the United States Endangered Species Act. Recent re-evaluations of double-crested cormorant
management at the local, flyway, and federal level warrant further examination of the current population
size and trends in western North America. We collected colony size data for the western population
(British Columbia, Washington, Oregon, Idaho, California, Nevada, Utah, Arizona, and the portions of
Montana, Wyoming, Colorado and New Mexico west of the Continental Divide) by conducting aircraft-,
boat-, or ground-based surveys and by cooperating with government agencies, universities, and non-profit
organizations. In 2009, we estimated approximately 31,200 breeding pairs in the western population. We
estimated that cormorant numbers in the Pacific Region (British Columbia, Washington, Oregon, and
California) increased 72% from 1987–1992 to circa 2009. Based on the best available data for this period,
the average annual growth rate (λ) of the number of breeding birds in the Pacific Region was 1.03, versus
1.07 for the population east of the Continental Divide during recent decades. Most of the increase in the
Pacific Region can be attributed to an increase in the size of the nesting colony on East Sand Island in the
Columbia River estuary, which accounts for about 39% of all breeding pairs in the western population and
is the largest known breeding colony for the species (12,087 breeding pairs estimated in 2009). In contrast,
numbers of breeding pairs estimated in coastal British Columbia and Washington have declined by
approximately 66% during this same period. Disturbance at breeding colonies by bald eagles (Haliaeetus
leucocephalus) and humans are likely limiting factors on the growth of the western population at present.
Because of differences in biology and management, the western population of double-crested cormorants
warrants consideration as a separate management unit from the population east of the Continental Divide.
Genre Article
Topic Cormorant
Identifier Adkins, J. Y., Roby, D. D., Lyons, D. E., Courtot, K. N., Collis, K., Carter, H. R., Shuford, W. D., & Capitolo, P. J. (2014). Recent population size, trends, and limiting factors for the double-crested cormorant in western North America. Journal of Wildlife Management, 78(7), 1131–1142. doi:10.1002/jwmg.737

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