Recent population size, trends, and limiting factors for the double-crested cormorant in western North America

Abstract

The status of the double-crested cormorant (Phalacrocorax auritus) in western North America was last evaluated during [1987][1988][1989][1990][1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001][2002][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 (l) 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. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Predation by American White Pelicans and Double‐Crested Cormorants on Catchable‐Sized Hatchery Rainbow Trout in Select Idaho Lentic Waters

“…Similar to American white pelicans, double-crested cormorants Phalacrocorax auritus (hereafter cormorants) have also increased in abundance in recent decades throughout North America (Wires and Cuthbert 2006;Adkins et al 2014), including in Idaho. The increased abundance of this fish predator has led to numerous conflicts with important economic fish industries in North America, especially Great Lakes sport fisheries (Burnett et al 2002;Lantry et al 2002;Fielder 2008) and the aquaculture industry in the southeastern United States (Glahn et al 2000;Dorr et al 2012).…”

Conservation Objectives for Wintering and Breeding Waterbirds in California's Central Valley

“…For example, nesting numbers for three species of terns in 2010-2012 were well below those recorded in the Central Valley on earlier surveys during a wetter period in 1998-1999 (Shuford et al 2016c). By contrast, numbers of pairs of the Double-crested Cormorant (Phalacrocorax auritus) nesting in the Central Valley increased from 1998-1999 to 2011-2012 consistent with upward trends in the interior of California overall (Shuford 2014b) and throughout western North America as a whole (Adkins et al 2014). Unfortunately, data from prior comprehensive surveys of the Central Valley are not available for seven species (one species of grebe, six species of wading birds) surveyed in that region from 2010 to 2012.…”

Demographics of Piscivorous Colonial Waterbirds and Management Implications for ESA-listed Salmonids on the Columbia Plateau

“…2009) estimated the entire western population to be approximately 31,200 breeding pairs (Adkins et al 2014), of which roughly 44% and 4% nested in the Columbia River estuary and on the Columbia Plateau in 2010, respectively. As with Caspian terns, most nesting cormorants in the Columbia River Basin were located at East Sand Island in the Columbia River estuary in 2010, with four smaller colonies located on the Columbia Plateau.…”