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2011 Avian Predation Report: Cormorants Consume 20 Million Salmonids, Terns Take 4.8 Million
Posted on Friday, July 20, 2012 (PST)

The Caspian tern colony on the lower Columbia River estuary’s East Sand Island, the largest of its kind in the world, was smaller in 2011 than the previous year and was beset by a variety of disturbances, but the birds present still managed to eat a sizeable number of juvenile salmon and steelhead headed for the Pacific Ocean.


And while the terns’ bite on salmon was slightly smaller than years’ past, the impact of double crested cormarants – another avian predator – continued to grow. Cormorants last year consumed more than 20.5 million salmonids, according to researchers’ estimates. The cormorants’ toll on is the largest on record for research dating back to the 1990s.


The tern colony consisted of about 7,000 breeding pairs in 2011, a substantial decline from 2010’s 8,300 breeding pairs. And the Caspian tern colony’s nesting efforts on East Sand Island did not produce a single fledgling in 2011, the first time that a complete breeding failure has been recorded at this colony since research was focused on the birds more than 10 years ago.


But the terns present did make a killing on salmon, many of which are targeted for protection under the Endangered Species Act.


“Juvenile salmonids continued to be a large part of the diet of Caspian terns nesting on East Sand Island, comprising 36 percent of the diet (percent of prey items) in 2011, somewhat higher than the average during 2000-2010 (30 percent),” according to a research report completed last month.


“Despite complete colony failure, Caspian terns nesting at the East Sand Island colony consumed about 4.8 million juvenile salmonids (95 percent c.i. = 4.0 – 5.6 million) in 2011, lower than the 11-year average but not significantly different than the smolt consumption estimates from the previous two years,” the report says.


The preliminary report, “Research, Monitoring, and Evaluation of Avian Predation on Salmonid Smolts in the Lower and Mid-Columbia River,” has been prepared for the Bonneville Power Administration and the U.S. Army Corps of Engineers for the purpose of assessing project accomplishments. The federal agencies are amidst a program aimed at reducing tern nesting at East Sand by two-thirds with the goal of reducing predation there on salmon and steelhead stocks. A total of 13 wild fish stocks that are funneled past the island in their migration to the Pacific are listed under the Endangered Species Act.


The long-running research is headed by co-principal investigators Daniel D. Roby of the U.S. Geological Survey’s Oregon Cooperative Fish and Wildlife Research Unit and Department of Fisheries and Wildlife at Oregon State University and Ken Collis of Real Time Research, Inc., in Bend, Ore. The report includes contributions from Donald E. Lyons, Jessica Y. Adkins, Yasuko Suzuki, Peter Loschl, Timothy Lawes, Kirsten Bixler, Adam Peck-Richardson, Elise Dykstra, Jeff Harms, Will Mashburn, James Tennyson and Nicola Ventolini of OSU, Allen Evans, Bradley Cramer, Mike Hawbecker and Nathan Hostetter Real Time Research, Richard D. Ledgerwood of NOAA Fisheries’ Northwest Fisheries Science Center and Scott Sebring of the Pacific States Marine Fisheries Commission.


The report can be found at:


The 2011 research was intended to assess the impact of avian predation on survival of juvenile salmonids in the Columbia River estuary and monitor the efficacy of on-going Caspian tern management actions designed to reduce their impact on salmonid smolt survival in the estuary. It also aimed to test management strategies to limit nesting habitat availability for double-crested cormorants at East Sand Island in the Columbia River estuary and evaluate the impacts on smolt survival of piscivorous colonial waterbirds such as Caspian terns, double-crested cormorants, American white pelicans, California gulls and ring-billed that nest in the Columbia Plateau region.


The 2011 research season (birds typically flock north in springtime to nest and leave at summer’s end with young birds in tow) was unusual with high water conditions and predation by other species. The result was a total failure in tern reproduction. The dearth followed a 2010 season in which tern fledgling production was almost nil.


“The proximal factors responsible for colony failure and the decline in colony size were intense disturbance by bald eagles (Haliaeetus leucocephalus) and associated gull predation on tern eggs and chicks,” the draft report says. “Climate conditions associated with a very strong La Niña and the resultant exceptionally high river flows also apparently contributed to the lack of nesting success through their effects on marine forage fish availability.


“Further reductions in smolt consumption by Caspian terns nesting on East Sand Island will require further reductions in the size of the tern colony; future management plans are designed to reduce the size of the East Sand Island tern colony to about one-third its pre-management size (ca. 9,500 breeding pairs),” the draft report says.


Management of Caspian tern nesting habitat at the East Sand Island colony continued in 2011, with the Corps further reducing the area of suitable tern nesting habitat on East Sand Island to 2.0 acres, 40 percent of the original area of managed tern nesting habitat on East Sand Island. This habitat restriction caused Caspian terns to nest at higher densities (0.85 nests/m2) than previously recorded in the Columbia River estuary.


Since early 2008, the Corps’ Portland District has built a total of eight new islands as alternative Caspian tern nesting sites. The plan is to draw terns away from the estuary’s salmon convergence point. Five of these new tern islands are in interior Oregon and three are in the Upper Klamath Basin of northeastern California.


Six of the eight new tern islands were surrounded by water throughout the 2011 nesting season, and thus suitable as tern nesting habitat. Four of these six suitable islands supported nesting Caspian terns, including the new 2-acre rock-core island at Tule Lake Sump 1B in northern California’s Tule Lake National Wildlife Refuge, where 34 pairs nested.


“Adverse weather conditions (severe La Niña) and avian nest predators (gulls and great horned owls) limited Caspian tern nesting success and fledgling production at these alternative islands in 2011; however, a substantial number of terns banded at East Sand Island in the Columbia River estuary visited these sites, including 92 terns banded at East Sand Island that were seen at the Upper Klamath Basin tern islands during the 2011 breeding season,” the draft report says.


East Sand Island in the Columbia River estuary is also home to the largest double-crested cormorant colony in western North America, which consisted of about 13,000 breeding pairs in 2011, very similar to 2010.


“Juvenile salmonids represented about 19 percent of the diet (percent biomass) of double-crested cormorants nesting on East Sand Island in 2011, the highest percentage observed since 1999 (24.6 percent of the diet).


“Double-crested cormorants nesting at the East Sand Island colony consumed approximately 20.5 million juvenile salmonids (95 percent c.i. = 15.2 – 25.9 million) in 2011, the highest annual point estimate of smolt consumption for the East Sand Island cormorant colony so far recorded.


“During the past two years, smolt consumption by double-crested cormorants nesting on East Sand Island has been significantly greater than smolt consumption by Caspian terns nesting on East Sand Island. Federal, state and tribal fisheries officials are at work on a management plan to control cormorant predation on salmon in the lower Columbia estuary.”


Likewise, “Further up-river in the Columbia Plateau region, Caspian terns and double-crested cormorants are also the two bird species responsible for most of the smolt losses to avian predators. Management options to reduce the impacts of these two avian predators on smolt survival along the mid-Columbia River and lower Snake River are currently being considered by resource managers,” the report says.


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