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The Birds: Report Analyzes Benefits Of Reducing Estuary Cormorants’ Predation On Salmon, Steelhead
Posted on Friday, March 16, 2012 (PST)

The complete elimination of the West Coast’s largest double-crested cormorant colony, located each spring and summer just inside the mouth of the Columbia River at East Sand Island, could, as a result of reduced predation on juvenile fish, boost populations of upriver steelhead by as much as 2.5 percent, according to a recent research report.


But even in that best-case fish news scenario, there would still be a lot of work to do, according to the analysis and report prepared by Oregon State University, private and Pacific States Marine Fishery Commission scientists.


Reducing cormorant predation on protected salmon and steelhead in the lower Columbia estuary could well match other efforts, such as habitat and hydro system passage improvements, to improve the survival of wild salmon and steelhead that are listed under the Endangered Species Act.


“…actions to reduce predation on juvenile salmonids by double-crested cormorants nesting at East Sand Island will not by themselves recover ESA-listed anadromous salmonid populations originating upstream of Bonneville Dam,” according to the December 2011 report. “Reductions in cormorant predation in the estuary could, however, result in increases in salmonid population growth rates comparable to some other salmonid recovery efforts in the Columbia River basin, particularly for steelhead populations.”


Potential increases in salmon and steelhead population growth rates resulting from complete elimination of predation by East Sand Island double-crested cormorants, assuming no other mortality factors would compensate for this reduction in predation, ranged from 0.6 to 1.2 percent for chinook salmon “evolutionarily significant units” originating above Bonneville Dam, 1.6 percent for the Snake River sockeye salmon ESU, and from 1.9 to 2.5 percent for steelhead designated population segments, according to data compiled from “passive integrated transponder” tags found on the island.


Such tags are affixed to juvenile fish for a variety of research purposes. ESUs, which are ESA species designations, are aggregations of genetic and geographically related salmon populations. DPSs are like designations for wild steelhead, which are rainbow trout that, like salmon, are born in freshwater, mature in saltwater and return to rivers as adults to spawn.


“If a moderate level of compensatory smolt mortality (e.g., 50 percent) occurred in response to a complete elimination of cormorant predation, … values would drop below 1 percent for chinook and sockeye salmon ESUs, but remain 1 percent or greater for steelhead DPSs,” according to the estimates.


Compensatory mortality means that a certain number of the fish saved from the jaws of East Sand-based cormorants will fall prey to, perhaps, other birds elsewhere or other causes of death.


The December 2011 report, titled “Benefits to Columbia River Anadromous Salmonids from Potential Reductions in Predation by Double- Crested Cormorants Nesting at the East Sand Island Colony,” was prepared for the U.S. Army Corps of Engineers.


It was compiled by Donald E. Lyons and Daniel D. Roby of the U.S. Geological Survey-Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife at Oregon State University, Allen F. Evans, Nathan J. Hostetter and Ken Collis at Real Time Research, Inc., and Scott H. Sebring of the Pacific States Marine Fisheries Commission. Lyons is lead author.


The report can be found at:


In its 2008 biological opinion and 2010 supplemental BiOp on the proposed operation of the Federal Columbia River Power System and its potential impacts to ESA-listed salmonid populations, NOAA Fisheries directed the federal “action” agencies – the Corps, Bonneville Power Administration and Bureau of Reclamation -- to analyze impacts of estuary cormorants on survival of Columbia River juvenile salmonids and develop a management plan to reduce cormorant predation, if warranted. The Corps and Bureau operate Columbia and Snake river basin hydro projects that make up the FCRPS, and BPA markets the power generated in the system.


That process is under way. The Corps early last month released a draft environmental assessment for public comment regarding a proposal to fence off a portion of the spawning area used each spring and early summer. The planned experiment would involve hazing the big birds in an attempt to discourage them from nesting on one side of the fence. Monitoring would evaluate the activities of displaced cormorants to determine if they moved to the more tranquil side of the fence, elsewhere on the island, or relocated off the island.


The research results could help the researchers devise long-term solutions for reducing the number of cormorants that nest at the site, and therefore reduce the species’ consumption of juvenile salmonids in the Columbia River estuary.


East Sand, a low profile island five miles upriver from the Pacific Ocean -- built up with the deposition of dredging spoils -- has over the past three decades seen the growth of the annual double-crested cormorant nesting colony from about 90 pairs in 1989 to about 13,000 pairs in 2011.


The birds in 2011 consumed an estimated 22.6 million out-migrating juvenile salmonids.


That consumption includes wild fish from 13 stocks of salmon and steelhead that are listed under the Endangered Species Act.


A long-term management plan is being written by a multi-agency working group to help guide the Corps’ Portland District in developing effective alternatives to reduce salmonid consumption by the cormorant colony at East Sand Island and is expected to be available for public review by May 2012. An end result of that process would be an environmental impact statement under the National Environmental Policy Act.


Caspian terns have also found East Sand Island to be fertile ground. Management to reduce predation on salmonid smolts by the world’s largest colony of Caspian terns on East Sand Island has been ongoing since 1999.


Resource managers are also considering management to reduce predation on smolts by waterbird colonies in the Columbia Plateau region of eastern Washington state, based on recent assessments of predation impacts and potential benefits to ESA-listed salmonid populations.


The large size of the East Sand Island cormorant colony, along with the high daily food requirements of individual cormorants, results in millions of smolts consumed annually by cormorants nesting at this colony, according to the new paper.


The analysis was focused to great degree on potential benefits for eight wild ESA-listed anadromous salmonid stocks that originate above Bonneville Dam (river mile 146) and pass through at least one dam in the FCRPS.


Reductions in predation on juvenile salmonids by cormorants from the East Sand Island colony could be achieved by management that reduces colony size (e.g., habitat management, disturbance, or lethal control); however, if dispersal of cormorants away from East Sand Island is considered a management option, the benefits estimated here would only be accrued if cormorants do not prey on Columbia River salmonids at whatever colony location they disperse to.


“In general, a two-thirds reduction in predation by double-crested cormorants nesting at the East Sand Island colony would produce similar levels of benefit for salmonids originating above Bonneville Dam to benefits projected for the ongoing management to reduce by two-thirds the predation by Caspian terns nesting at the East Sand Island colony,” the new paper says.


“Management to reduce cormorant predation would not be as efficient, however, as Caspian tern management in terms of benefits per managed bird due to the lower per capita impacts of cormorants on survival of salmonids originating from above Bonneville Dam.” That’s because terns have been shown to include a much higher percentage of salmon and steelhead in their diet than cormorants, which zero in more heavily on marine prey species.


More research is needed to firm up the estimated benefits that might be derived from reducing the number of cormorants that nest at East Sand.


“The robustness of these analyses would be strengthened by additional information on (1) the degree to which other smolt mortality factors may compensate for reductions in mortality from cormorant predation, (2) the off-colony deposition rate of PIT tags from PIT-tagged salmonids consumed by cormorants, and (3) the impacts of cormorant predation on survival of ESA-listed salmonids from populations originating primarily below Bonneville Dam.”


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