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Report Addresses Impacts of Growing Tern Population

by Barry Espenson
Columbia Basin Bulletin - October 25, 2002

A draft 2002 research summary shows a continuing trend -- the short-term plan for managing Caspian terns in the Columbia River estuary is succeeding in its goal of reducing the birds' consumption of migrating juvenile salmon and steelhead.

The plan does have its limits, however. The birds continue to chew up millions of salmonids, many of them listed under the Endangered Species Act. And as the number of terns grows -- as it did this year -- more fish eaten.

"If the colony grows the consumption of smolts is going to go up," said Daniel Roby, principal investigator for the Caspian tern research effort. The annual reports are prepared for the Bonneville Power Administration, which funds much of the research, and the Interagency Caspian Tern Working Group that devised the management plan. Some refinements to the modeling and analysis are expected before a final report is produced.

Researchers say that the migratory terns historically used the estuary in relatively small numbers, preferring in many cases nesting sites along the north Pacific coast. The birds, which fly north in April to nest and rear their young, began in the mid-1980s to find sanctuary on estuary islands created with spoils from the dredging of the Columbia River's shipping channel.

By 1997 the estuary Caspian tern colony had grown to about 17,000 birds and was concentrated almost exclusively at Rice Island, 21 miles from the Columbia's mouth. The colony was then and is now the largest known breeding colony of Caspian terns in the world.

Researchers estimated that the Rice Island terns ate about 12.4 million salmonids in 1998, about 13 percent of the estimated 97 million out-migrating salmonids. Analysis of more than 36,000 smolt PIT tags recovered at the colony revealed that, at minimum, 13.3 percent of all tagged steelhead smolts that reached the estuary were consumed by terns in 1998. The National Marine Fisheries Service has estimated that about 18 percent of the steelhead out-migration that year was ESA listed.

"It doesn't appear the birds make a distinction between listed and unlisted steelhead" in selecting their prey, Roby said. Past research has indicated that some hatchery stocks, such as chinook, are more vulnerable to the avian predators than are their naturally spawned cousins.

The state, federal and tribal entities involved began in 1999 to implement a plan to shift the colony from Rice Island to East Sand Island, 16 miles closer to the ocean. It was believed that, with readier access to marine forage fish, the terns would eat fewer salmonids. Vegetation was removed from East Sand to make it more enticing to the terns and the bare sand habitat at Rice Island flagged and seeded to discourage nesting.

About 1,400 pairs of terns moved in 1999 and 8,500 pairs (94 percent of the colony) called East Sand home in 2000. The entire colony has nested at East Sand during the past two seasons -- about 9,000 pairs in 2001 and 9,900 nesting pairs in 2002.

The theory has proved correct. The East Sand Island terns included between 31 and 47 percent salmonids in their diet from 1999 to 2002. Rice Island terns had had 77-percent and 90-percent salmon in their diet in 1999 and 2000.

For the past two years, researchers estimate that the terns ate approximately 5.9 million and 6.5 million smolts, which are 52 percent and 48 percent reductions in smolt consumption as compared to Rice Island consumption in 1998. The estimated percentage of salmonids in the terns' diets was 32.5 in 2001 and 31.1 in 2002.

Roby said increase in the number of smolts eaten this year is related to the increased number of birds on the island. The researchers say the population growth spurt was likely because of the addition of birds from Commencement Bay near Tacoma. The birds had been shooed away from their accustomed nesting habitat -- broken-open sandbags that hold down the lid on an Aasarco Superfund waste depository.

The additional East Sand nesting terns could also have been first-year breeders produced in the estuary in 1999. The nesting success of the terns has been consistently and dramatically higher at East Sand (0.57-1.39 young raised per pair on average during 1999-2002) than it was at Rice Island (0.06-0.445 in 1997-1998 and 0.15-0.55 in 1999-2000).

The colony's population had been relatively stable from 1998 through 2001 at about 9,000 nesting pairs.

Roby said that is unlikely that the current plan will produce additional, large decreases in smolt consumption. Mother Nature could help on occasion by providing an amazing bounty of forage fish as it did last year and to some extent this year. About 18 percent of the terns' diet this past spring and summer was herring or sardine, 14 percent was anchovy, 12 percent were surfperch, 7 percent were smelt and 18 percent were "other" marine species.

Of the 31 percent salmonids in the terns' diet, 53 percent were believed to be coho, 35 percent chinook salmon and 11 percent steelhead.

To achieve further reductions in consumption of juvenile salmonids by Caspian terns in the estuary it will probably be necessary to relocate a portion of the East Sand island colony to alternative sites outside the estuary," according to the draft report. Known inland sites show little promise of attracting or being able to accommodate large numbers of birds. Some, as is the case of Crescent Island in the mid-Columbia, position terns for heavy predation on salmon.

"The best prospects for restoration or augmentation of Caspian tern colonies seem to exist on the coast the Pacific Northwest," according to the report's executive summary.

Those prospects have been, at least to this point, politically bleak. Former nesting sites such as Willapa Bay and Grays Harbor in Washington have been eyed but local and state officials warded off attempts at restoration.

"The welfare of other listed or beleaguered salmonid stocks has been a primary concern in areas considered for restoration of Caspian tern colonies, yet for most former colony sites there is little or no evidence that juvenile salmonids were a significant component of tern diets," according to the draft 2002 season summary.

"Restoration of permanent colony sites for Caspian terns along the coat of the Pacific Northwest appears unlikely without empirical evidence that local salmonid stocks will not be at risk."

A pilot study last year employed a barge as a nesting site and successfully lured displaced Commencement Bay terns. Researchers feel that that success was a sign of a habitat shortage, and that the barge concept could be used in the long run to test the eating habitats of terns at potential habitat restoration sites.

"The rapid and overwhelming response of Caspian terns to the habitat provided on the barge in Commencement Bay is strong evidence for the acute shortage of suitable nesting habitat along the coast of the Pacific Northwest," the summary said. Finding new sites would likely benefit both Columbia Basin salmon and terns.

"The (tern) population is particularly vulnerable to local catastrophes, such as storms, disease outbreaks, oil spills, predation events, or human disturbance," the report said. The concentration of birds at East Sand represents two-thirds of the entire Pacific Coast population.

The researchers say that some of the onus could be put on the fish themselves. Certain stocks are more vulnerable than others so future investigations could focus on identifying the causes of that vulnerability. They suggest the use of genetic markers to help identify species and specific stocks found in stomach samples from avian predators.

Potential future research could also involve the use of net pens near tern colonies to test hypotheses regarding the differences in vulnerability between salmonid stocks. The theory would involve providing the terns a 'cafeteria' menu of fish from which to chose, Roby said.

The researchers also ponder assessments of the condition, contaminant burdens, and health status of smolts consumed by birds to compare with the condition of smolts caught in river. That would allow them to test hypotheses regarding the compensatory/additive nature of avian predation. Some say that the bird predation has little effect on salmon recovery because they naturally select the weaklings -- fish unlikely to survive their ocean sojourn to return to the river and spawn.

"With this information, fisheries managers would be able to modify fish management practices to reduce the impact of Caspian terns and other avian predators on the survival of juvenile salmonids from the Columbia River basin, as warranted," the draft report says.

The report also offers the idea that fish, salmonid or otherwise, in net pens "offer the potential for providing terns with an alternative food source, thereby reducing their reliance on in-river migrants."

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Barry Espenson
Report Addresses Impacts of Growing Tern Population
Columbia Basin Bulletin, October 25, 2002

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