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Annual Avian Predation Report: Estuary Terns, Cormorants Consume 15-20 Percent Wild, Hatchery Smolts
Posted on Friday, December 02, 2011 (PST)

Combined losses of juvenile salmon and steelhead to predation by Caspian terns and double-crested cormorants in the Columbia River estuary were about 27 million smolts, according to preliminary data presented Tuesday to researchers during a gathering in Walla Walla, Wash.


If that number holds up, that consumption by avian predators nesting on East Sand Island would represent a toll of from 15-20 percent on the overall number of hatchery and wild fish that survived their journey down the Snake, Willamette and Columbia rivers and tributaries to near the Columbia’s mouth.


Of that total, an estimated 22.6 million smolts were taken by the double-crested cormorants, which is up from 19 million in 2010, according to Dan Roby, who presented the latest available data during the annual Anadromous Fish Evaluation Program Annual Review.


A final vetting of the data should be completed by March, along with a 2011 draft annual report, “Research, Monitoring, and Evaluation of Avian Predation on Salmonid Smolts in the Lower and Mid-Columbia River Draft 2010 Annual Report.”


Roby of the U.S. Geologic Survey’s Oregon Cooperative Fish and Wildlife Research Unit and Oregon State University and Ken Collis of Real Time Research, Inc. are co-principal investigators for the research, which has been ongoing since 1990s to evaluate avian impacts on salmon and steelhead stocks that include fish listed under the Endangered Species Act. Funding for the work comes from money appropriated to the U.S. Army Corps of Engineers and from the Bonneville Power Administration.


Since 1952, the Corps has sponsored biological studies concerning anadromous fish passage on the lower Snake and Columbia rivers and other related issues. Each year, a forum is held where AFEP-funded researchers present current year study results to the region. Scientists and researchers from federal and state agencies, private contractors, non-profit organizations, tribal fishery groups, and universities attend the annual review to hear those research results.


The main purpose of AFEP is to provide scientific information that will assist the Corps in making decisions regarding the safe passage of anadromous fish past the eight dams and reservoirs on the lower Snake and Columbia rivers and otherwise improve fish survival. Topics covered this week include estuary, lamprey, predation, bull trout, turbine survival, passage survival for adult salmonid and steelhead, and delayed mortality/transportation studies as well as system survival and the programmatic sediment management plan. Specific agenda topics can be found at


East Sand Island, built with dredge spoils from Corps navigation channel maintenance, was the target of what was perhaps the first lower Columbia River bird management effort. The number of avian predators, primarily Caspian terns, visiting the estuary had steadily grown over the 1980s and 1990s. Most settled in Rice Island, another dredge spoils creation.


With the number of salmon stocks also growing, fish managers became concerned about the impact on fish numbers caused by predation from what had become the world’s largest colony of Caspian tern.


In 1999 an effort was started to dissuade nesting at Rice Island and attract birds to East Sand, which is nearer the ocean where the birds can better take advantage of marine forage fish. The plan worked. The entire tern colony settled on East Sand, where marine fish became the main prey species and the consumption of salmonids was reduced.


A Rice Island colony that included as many as 16,000 nesting terns ate as many as 12.4 million juvenile salmonids (1998). Since 2000 tern consumption at East Sand has averaged less than 5.5 million smolts per year. The estimate for 2011 is about 5 million.


Back at the turn of the century only a few thousand double-crested cormorants nested at East Sand. But the number stair-stepped upward from about 5,000 in 1997 to nearly 14,000 in 2006, before leveling off. The population estimates in 2010 and 2011 were about 13,600.


Estimates of cormorant smolt consumption averaged about 9 million between 2003 and 2006 before spiking in 2010.


This year juvenile salmonids represented about 20 percent of the double-crested cormorants’ diet, up from 17 percent last year, according to the research abstract prepared for the AFEP meeting.


The vast majority of the cormorants’ smolt consumption both in 2010 and 2011 consisted of subyearling fall chinook salmon, Roby said.


In 2011, the researchers initiated a pilot study to test strategies for limiting the size of the East Sand cormorant colony. An 8-foot-high privacy fence was built to visually separate about 15 percent of the nesting site from the rest of area, and hazing by humans was employed literally around the clock to dissuade the big birds from nesting in that area. The effort was successful, and that strategy may be tested again as a potential means of reducing the size of the double-crested cormorant colony.


Roby said that the Corps expects to complete a draft environmental impact next year that focuses on possible means for reducing the cormorants’ impact on salmon. The DEIS is being developed in coordination with the U.S. Fish and Wildlife Service, NOAA Fisheries Service, states, tribes and other parties.


Meanwhile, the Caspian tern colony numbers about 7,000 nesting pairs, which was down from 8,300 in 2010.


The tern colony was besieged by natural forces in 2011. La Nina conditions and high river flows negatively influenced nesting.


But “the proximal factor responsible for colony failure and the decline in colony size was intense disturbance by bald eagles and associated gull predation on tern eggs and chicks,” the abstract says.


Not a single fledgling bird was produced by the colony, which is the first complete breeding failure recorded in the history of the research.


The eagle raids that began in early to mid-May targeted the cormorant too, disrupting nesting activity. But the attacks lessened there within a couple weeks. The cormorants did regroup to some degree, but productivity was likely the lowest it has been since 2000, Roby said.


Despite the eagle dive bombing, most of the terns stayed at East Sand during the spring-early summer nesting periods. But some made their way to newly created habitat.


This year the suitable habitat for Caspian terns at East Sand was reduced to two acres, which is about 40 percent of the original total. The habitat reduction is part of a management plan launched in 2008 as an attempt to get some of the birds to move elsewhere, and as a result reduce predation on salmon.


The Corps has over the past three years built a total of eight new islands as alterative nesting sites. Five are in interior Oregon and three are in the upper Klamath region in northeastern California.


“Four of these eight new islands supported nesting Caspian terns in 2011, including the new 2-acre rock-core island at Tule Lake National Wildlife Refuge, where 34 pairs nested,” the abstract says. “Adverse weather conditions, apparent low forage fish availability, and avian nest predation limited Caspian tern nestling and fledgling production at these alternative islands in 2011; however, a substantial number of terns from the Columbia River estuary are visiting these sites as 92 terns originally banded in the Columbia River estuary were seen at the Upper Klamath islands this past season.”


The researchers affix colored bands on the legs of a portion of the birds so their movements can be tracked. Data received from the sighting of banded birds also feeds into a study of survival rates of Caspian terns along the Pacific Coast region.


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