The first assessment of population size for double-crested cormorants (Phalacrocorax auritus) along the Pacific coast of North America summarized data collected prior to 1993 in British Columbia, Washington, Oregon, California, and northwestern Mexico, as well as examining historical trends and conservation issues (Carter et al. 1995). More recent status assessments have been completed for sub-populations in California (Capitolo et al. 2004, Shuford 2010) and in British Columbia (Moul and Gebauer 2002). The breeding distribution of this population has changed dramatically over the past 50 years, with increases at some known colonies and formation of many new colonies; the largest increase has been in the Columbia River estuary, where the number of breeding pairs grew from 6,620 in 1992 to 14,032 in 2007. Fisheries managers have raised concerns over the impact of predation by cormorants from this large breeding assemblage on survival of out-migrating juvenile salmonids (Oncorhynchus spp.) from throughout the Columbia Basin, especially those evolutionarily significant units (ESUs) of salmonids that are listed under the U.S. Endangered Species Act.
The geographic area included in this updated status assessment of the Western Population of double-crested cormorants extends from the Pacific Coast east to the Continental Divide, north into southern British Columbia (following the breeding range of the species), and south to the international border with Mexico. The current size of the entire western breeding population is estimated to be about 29,240 breeding pairs. The estimate for the current breeding population in British Columbia, Washington, Oregon, and California, which support the great majority of the Western Population, is approximately 26,390 breeding pairs. This represents an increase of nearly 10,000 breeding pairs (ca. 60% increase) since 1987-1992 (Carter et al. 1995, Moul and Gebauer 2002). Based on the best available data for the three Pacific coastal states and British Columbia during the periods ca. 1992 (Carter et al. 1995, Moul and Gebauer 2002) and ca. 2009, our best estimate of the average annual population growth rate for this population is 1.03, indicating that the population has grown at an average annual rate of about 3% per year over the last two decades. This overall trend apparently reflects continued recovery of the Western Population due to various statutory and ecological factors during the latter half of the 20th Century, including inclusion of the species in the Migratory Bird Treaty Act, the banning of DDT, and the shift by the species toward increased use of artificial nesting habitats. Most of this population increase, however, can be attributed to increases in the numbers of breeding pairs in the Columbia River estuary and at a few inland sites, which currently account for approximately 41% and 29% of breeding pairs in the Western Population, respectively. Concurrently, numbers of breeding pairs in coastal British Columbia, northern Washington, and southern California have declined since 1987-1992.
The size of the Western Population of double-crested cormorants is still more than an order of magnitude less than the population that inhabits central and eastern North America. Increasing bald eagle (Haliateeus leucocephalus) populations, episodic human disturbance, and long-term impacts of certain organochlorine and hydrocarbon pollutants may be important factors causing declines in some portions of the range of the Western Population. Parallel studies on the genetic structure of the Western Population and the post-breeding dispersal of double-crested cormorants nesting in the Columbia River estuary suggest a high degree of population connectivity from southeastern California to southern British Columbia. Alaska, extreme southern California, and areas east of the Continental Divide have more limited connectivity to the Western Population of double-crested cormorants.
- J.Y. Adkins, D.D. Roby and 27 co-authors
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