Cormorant Colony on the Rise in Columbia Estuary
"Buffered to a certain extent by more stable food resources" near East Sand Island in the Columbia River estuary, the nesting double-crested cormorant colony there has blossomed, while the number of breeding pairs elsewhere along the Pacific coast in Washington and British Columbia have dropped by some 66 percent, according to the soon-to-be published research article, "Status Assessment of Double-Crested Cormorants in Western North America."
The research compares colony populations calculated for the 1987-2003 period with estimates during 2009 that were derived from aircraft-, boat-, or ground-based surveys and by cooperating with government agencies, universities, and non-profit organizations.
The article will be included in the August issue (online and print) of the Journal of Wildlife Management, but is currently available on the publisher's website with a subscription.
The article can also be found at the Northwest Bird Research web site.
Bird Research Northwest is an ongoing research program investigating the ecology of piscivorous colonial waterbirds (primarily, Caspian terns, double-crested cormorants, American white pelicans, and several gull species) and their impacts on the survival of juvenile salmonids in the Columbia Basin and elsewhere along the Pacific Coast. Much of the funding for the research comes from the Bonneville Power Administration and the U.S. Army Corps of Engineers.
Lead author for the article is Jessica Y. Adkins, who the time of the writing was with the Department of Fisheries and Wildlife at Oregon State University.
Contributing authors include Daniel D. Roby of the U.S. Geological Survey, Oregon Cooperative Fish and Wildlife Research Unit at OSU; Donald E Lyons and Karen N. Courtot of OSU's Department of Fish and Wildlife; Ken Collis of Real Time Research; Harry R. Carter of Carter Biological Consulting in Victoria, British Columbia and Humboldt State University's Department of Wildlife in Arcata Calif.; W. David Shuford of Point Blue Conservation Science in Petaluma, Calif., and Phillip J. Capitolo of the University of California, Institute of Marine Sciences.
The researchers collected colony size data for the western population of double-crested cormorants (British Columbia, Washington, Oregon, Idaho, California, Nevada, Utah, Arizona, and the portions of Montana, Wyoming, Colorado and New Mexico west of the Continental Divide.
"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 percent from 1987-1992 to circa 2009," the article says.
Based on the best available data for this period, the average annual growth rate 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 percent 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 percent during this same period," the report says.
The East Sand cormorant population has grown and thrived "because of ample habitat for ground-nesting, abundant food supply, few mammalian predators, low rates of human disturbance, and possibly the greater security from predation by bald eagles (Haliaeetus leucocephalus) afforded by this large and dense colony," the article says.
The abundant food supply includes marine forage species but is richly supplemented with juvenile salmon and steelhead, including many wild stocks that are listed under the Endangered Species Act.
That has caused concern for federal agencies that are charged with assuring that those listed fish species' survival is not jeopardized by federal actions such as the operation of the Columbia River hydro system, and the maintenance of the lower Columbia River navigation channel. East Sand is a natural island that has been grown through the deposit of dredge spoils from channel maintenance activities directed by the Corps.
"No site elsewhere in the region has the combination of factors necessary to sustain a super colony of this size," the research article says. The forage base supports more than 50,000 piscivorous birds that breed or roost at East Sand Island every summer, including Caspian terns, double-crested cormorants, Brandt's cormorants, brown pelicans (Pelecanus occidentalis), and gulls.
The Corps, which also operates many of the dams on the Columbia and Snake Rivers, earlier this month offered for public comment a draft environmental impact statement that evaluates various options for reducing cormorant predation on salmon at East Sand. The preferred alternative would involve the culling or shooting of birds. Other options involve the hazing of cormorants and/or manipulation of habitat to encourage them to move elsewhere.
"Double-crested cormorants from the Columba River estuary have demonstrated a strong connectivity with coastal breeding sites to the north but less so with southern coastal and inland or interior sites. The number of coastal colonies to the north of the East Sand Island has declined by approximately 50 percent since the early 1990s, and numbers nesting at the remaining northern coastal sites have also declined, resulting in a 66 percent decline in numbers of breeding pairs within this subpopulation.
"If management is employed to dissuade double-crested cormorants from nesting at East Sand Island in an effort to reduce their impact on ESA-listed juvenile salmonids, some individuals may likely disperse northward to prospect for breeding sites.
"We are unclear how successful dispersal to these areas would be, however, given the declines in this region," the article says. Because of the unique characteristics of the double-crested cormorant colony at East Sand Island and the tenuous states of colonies elsewhere, the future of this colony will likely influence the entire western population."
According to Bird Research Northwest's 2012 annual research report, in 1989 fewer than 100 pairs of double-crested cormorants nested on East Sand Island. Steady growth has been witnessed ever since.
The researchers estimate that 12,300 breeding pairs attempted to nest at the East Sand Island colony in 2012, compared to 13,045 breeding pairs in 2011. The size of the East Sand Island double-crested cormorant colony grew rapidly from 1997 to 2007, nearly tripling in size.
In 2008, however, the colony experienced an unexpected decline (20 percent) before rebounding to nearly the previous peak in colony size by 2010, the annual report says.
"The growth of the East Sand Island colony appears to be exceptional among colonies of double-crested cormorants along the coast of the Pacific Northwest, most of which are stable or declining," the Bird Research Northwest 2012 annual report says.
Northwest Bird Research estimates total smolt consumption by double-crested cormorants nesting on East Sand Island in 2012 was 18.9 million smolts, which is not significantly different from the number of smolts consumed by cormorants from this colony in 2011.
Annual smolt consumption by double-crested cormorants nesting on East Sand Island has been trending upward since 2003, until 2012 when estimated consumption leveled off.
Estimates of total smolt consumption by East Sand Island cormorants were significantly higher than that of Caspian terns nesting on East Sand Island in 2012.
Also see, CBB, July 13, 2014, "Draft EIS Proposes Culling Thousands Of Cormorants To Reduce Salmonid Predation"
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