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Inland Waterbird Colonies Show Unexpectedly High Predation Rate On Specific, Listed Salmonid Stocks
Posted on Friday, April 22, 2011 (PST)

A new “synthesis” of research data points to three bird colonies, out of nine total, in the mid-Columbia/lower Snake River region that might be the best targets for management actions to reduce predation on migrating wild juvenile salmon and steelhead that are listed under the Endangered Species Act.

 

During the 2004-2009 study period Caspian terns nesting at Goose Island in the Potholes in south-central Washington took at minimum an estimated 10 percent of the juvenile Upper Columbia steelhead that passed by.

 

“That has been a shocker to us,” said Oregon State University’s Dan Roby, who is principal investigator for an ongoing research program aimed at evaluating eating habits of birds from colonies in the Columbia Plateau. The birds have to fly straight east from the Potholes 21 miles or so to get to the Columbia.

 

“It looks like the primary reason for going there is to work on salmonids, and in particular steelhead,” Roby said.

 

“Based on the results of this study, the greatest potential for increasing survival of smolts from ESA-listed salmonid stocks by managing inland avian predators would be realized by focusing management efforts on Caspian terns nesting at colonies on Crescent Island, Goose Island, and the Blalock Islands,” according to the executive summary for the collection of research reports. “Reductions in the size of these tern colonies would enhance survival of upper Columbia River and Snake River steelhead stocks in particular.”

 

The Blalock Islands are in the mid- Columbia River and Crescent Island is at the confluence of the Columbia and Snake rivers.

 

The package of four research reports, “Impacts of Avian Predation on Salmonid Smolts from the Columbia and Snake Rivers: 2004-2009 Synthesis Report,” can be found at:

http://www.birdresearchnw.org/default.aspx

 

The synthesis report was prepared for the U.S. Army Corps of Engineers, Walla Walla District, for the purpose of assessing avian research project accomplishments. The data compiled in the reports will inform processes to decide what, if any, management actions might be taken to reduce avian predators’ impacts on juvenile salmon and steelhead stocks, some of which are protected under the Endangered Species Act. The Corps has funded much of the inland avian predation research.

 

NOAA Fisheries’ 2008/2010 Federal Columbia River Power System biological opinion calls for the federal action agencies, which include the Corps, to develop an avian management plan (for double-crested cormorants, Caspian terns, and other avian species as determined by research, monitoring and evaluation for Corps-owned lands and associated shallow water habitat in inland Columbia-Snake river basin areas such as the Columbia Plateau.

 

The Corps operates many of the hydro projects that make up the FCRPS. BiOps judge whether federal activities, such as the operation of the dams, jeopardize the survival of listed species. The research focuses on Caspian terns, double-crested cormorants, American white pelicans, California gulls and ring-billed gulls -- all piscivorous colonial waterbirds with a history of nesting in the Columbia Plateau region.

 

The Corps is particularly interested in the inland colonies because they have a much higher per capita (bird) predation rate on salmon and steelhead. The inland birds’ diet includes a higher percentage of salmon than tern and cormorant colonies in the lower Columbia estuary. The estuary colonies are much larger, however, so they eat far more salmon and steelhead overall.

 

“Due to the relatively high observed predation rates in the estuary, and because all anadromous salmonids must migrate through the estuary, our results indicate that the management of terns and cormorants nesting on East Sand Island near the mouth of the Columbia River has the greatest potential to enhance survival of juvenile salmonids from all Columbia Basin stocks combined,” according to the report’s executive summary.

 

But managing the number of birds at the three inland colonies singled out in the report “have at least the potential” to reduce mortality on particular evolutionarily significant units, i.e. listed species, Roby said.

 

“Impacts to survival of specific stocks of salmonids from avian predation were also associated with some inland colonies of Caspian terns and double-crested cormorants. Predation rates by Crescent Island terns on Snake River summer steelhead (7.7 percent) and by Goose Island terns on upper Columbia summer steelhead (10.0 percent) were substantial during the study period.

 

“Predation rates on Snake River summer steelhead (2.0 percent) and Snake River sockeye (1.7 percent) by cormorants from the inland colony at Foundation Island, although lower than tern predation rates, were high compared to those of other inland bird colonies,” the report says.

 

“While inland colonies of terns and cormorants are much smaller than their counterparts in the estuary, these inland colonies can be more reliant on salmonids as a food source,” the executive summary says. The East Sand colonies can take advantage of numerous marine species found in the lower estuary, as well as salmon and steelhead.

 

“This greater reliance on salmonids, coupled with lower diversity of available salmonid stocks compared to the estuary, is responsible for the unexpectedly high impact of some inland tern and cormorant colonies on specific stocks of salmonids, particularly steelhead,” the report says.

 

“Current management efforts to increase smolt survival through reductions in tern and cormorant predation in the estuary could result in higher predation rates on certain ESA-listed salmonid stocks if terns and cormorants dissuaded from nesting in the estuary recruit to inland colony sites.”

 

“Reductions in the size of these tern colonies would enhance survival of upper Columbia River and Snake River steelhead stocks in particular. More limited enhancement of smolt survival for Snake River steelhead and Snake River sockeye could be achieved by managing the double-crested cormorant colony at Foundation Island,” the report says.

 

“Based on smolt predation rates as inferred from PIT tags recovered on-colony, management of other inland piscivorous waterbird colonies in the Columbia Plateau region would provide relatively small and perhaps undetectable increases in stock-specific smolt survival.” Much of the diet data is based on the numbers of PIT tags, which are placed in fish for other research purposes, retrieved from the colony sites.

 

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