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2003 Tern Report
Colony Eating Fewer Columbia Salmonids

by Barry Espenson
Columbia Basin Bulletin - November 14, 2003

The world's largest known colony of Caspian terns this year turned, for the most part, their attention from juvenile salmon and steelhead to other fish species, with the birds eating fewer salmonids in the lower Columbia River and estuary than in any of the past seven years.

The colony, which now nests five miles from the river mouth on East Sand Island from April to midsummer, gobbled up an estimated 4.2 million juvenile salmonids this year as the fish migrated toward the Pacific Ocean, according to a draft 2003 summary report on the summer field season.

That compares to a peak of an estimated 12.4 million consumed by the terns in 1998, the second year of the study that has been financed by the Bonneville Power Administration.

The research is being carried out by researchers from Oregon State University, the U.S. Geological Survey, the Columbia River Inter-Tribal Fish Commission, and Real Time Research.

The birds in 2003 also included the lowest percentage of salmon in their diet -- 24 percent -- that has been witnessed during the course of the study.

During the 1997-2000 period when all or part of the terns nested 16 miles upriver at Rice Island, the birds' diet was from 73 percent to 91 percent salmonid. The birds nesting nearer the ocean included only from 31 to 47 percent salmon or steelhead in their diet from 1999 through 2002.

The decline in salmonid consumption is the result of a management strategy that lured a portion of the terns from Rice to East Sand Island in 1999. In 2000, about 94 percent of the terns nesting in the estuary -- about 8,500 pairs -- made their home at East Sand and for the past three years the entire colony has nested there.

The colony relocation was made by discouraging nesting at Rice and encouraging nesting at East Sand through habitat manipulation and other means. The goal was to bring the birds closer to the ocean, and the marine fishes that they might target.

It was hoped that the move from Rice Island, which was created artificially as a depository for materials dredged by the Corps of Engineers from the river's shipping channel, would reduce impacts on salmon and steelhead, including some stocks that are listed under the Endangered Species Act. The sandy nesting site perched at the mouth of the salmon run allowed the tern population to burgeon from a handful when first spotted in the mid-1980s to thousands.

"The success of this project is to some degree a stroke of luck," said Ken Collis of Real Research, co-principal investigator for the research project. The same phenomenon that gains much of the credit for improved adult returns to the river -- favorable ocean conditions -- likely improved the lot of marine fish species.

The birds benefited indirectly from the "strong coastal up-welling and associated high primary and secondary productivity along the coast of the Pacific Northwest, particularly in 2001," the draft report says. "The favorable ocean conditions have been linked to the regime shift associated with the Pacific Decadal Oscillation and may ensure relatively high availability of marine forage fishes near the mouth of the Columbia River for several years to come."

The salmonids remained the No. 1 item on the terns' diet, at 24 percent. But the abundance of anchovies during 2003 forged a tie -- also making up 24 percent of the terns' diet. Next came herring/sardines at 19 percent, smelt at 18 percent, surfperch at 7 percent and 8 percent for "other."

"The real test of this is when oceans turn poor again. What will the birds do then," Collis said. If marine forage dwindles, the terns might again look upriver towards the millions of juvenile salmon being funneled downriver.

Since the terns transport the fish to their mates and young in their bills, researchers can collect the diet data visually -- "bill load observations." To differentiate between salmon and steelhead species, however, researchers have to actually collect and examine those bill loads.

Of the salmonids brought to the island this year, 43 percent were coho salmon, 26 percent were spring chinook, 17 percent were subyearling chinook and 14 percent were steelhead. That ratio, though relatively constant at East Sand, is radically different then when the birds were all nesting upriver at Rice where salmon were the primary prey. At Rice Island during 1997-99 the subyearlings or fall chinook juveniles were a favorite prey, including 1999 when they represented more than half of the salmonids consumed.

Collis reasoned that the smaller subyearlings linger further up in the saltwater-freshwater mixing zone as their bodies ready for their entry into the ocean. That made them easy prey near Rice Island. On the other hand, by the time they pass East Sand they moving unhesitantly toward the ocean so the terns window of opportunity is narrowed.

The plentiful food supply and the fact that the terns themselves have been relatively predator-free at East Sand has resulted in much improved nesting success. The 1990-2003 average at East Sand has been 1.06. The 2003 rate was 1.08. That compares to rates of 0.06 and 0.45 young per pair in 1997 and 1998 at Rice Island (and 0.55 and 0.15 in 1999 and 2000 when fewer numbers of terns nested there).

That nesting success would normally portend and ever-increasing colony size when the young birds reach 3 or 4 years of age and join the breeding population. Yet this year the population took a dramatic decline -- 16 percent from 2002 -- when the bumper crop from 2000 might have been expected to join the population.

"We didn't really see that to a large degree," Collis said of new breeder "recruitment." Colony size ebbs and flows depending on the size of that recruitment of those young birds into the breeding population and with the transfer of birds in or out from other colonies.

Aerial photographs taken during the peak of the 2003 nesting season showed that the East Sand tern population had dropped to 8,325 breeding pairs percent from last year's count of 9,933. The 2002 count was the largest, having risen from 8,982 pairs in 2001. The population had been relatively stable at about 9,000 pairs since 1998.

"This was a surprise to us. We're not sure what happened," Collis said. It does not appear that the birds have dispersed to colonies elsewhere.

"We've talked to all of our sources and haven't found the birds," Collis said of contacts with researchers monitoring other, smaller colonies down the coast and inland. In fact, the only other significant colony in the Columbia Basin also witnessed a population decline.

The declining count does raise a red flag for the federal agency charged with safeguarding migratory birds -- the U.S. Fish and Wildlife Service. But the lack of a comprehensive means of monitoring the birds' comings and goings along the West Coast means that the missing birds might have just nested in other locales.

"We're not completely worried by this one year of data," said Nanette Seto, a USFWS wildlife biologist and the Pacific regional office's lead in the ongoing preparation of an environmental impact statement on Caspian tern management in the Columbia River estuary. Also participating in that process are NOAA Fisheries and the Corps.

The tern colony on Crescent Island, near the confluence of the Columbia and Snake rivers, consisted of 510 breeding pairs this year, down by 12 percent from last year. The 2001 count was 650 pairs.

While the tern population in the estuary has been stable, or shrunk, in recent years the opposite trend has been noted for double-crested cormorants. A census taken this summer shows that their number grew by 23 percent from 2002 to 2003 with the June 11 census showing 10,646 breeding pairs. Their population trend has been steadily upward to the point that they are likely eating as much or more salmon than the terns.

"We know the size of the East Sand cormorant colony has eclipsed the tern colony," Collis said. While terns have held the spotlight of the research project, there has been enough data collected to produce some assessments of the cormorants' status in the estuary. Collis said that the researchers may produce a supplement to their summary report this winter.

The research effort itself is facing a somewhat uncertain fate. BPA earlier this month decided that the research would be funded at $300,000 during fiscal 2004, well below the nearly $700,000 requested for the complete set of research activities. BPA said, as did the Northwest Power and Conservation Council in recommending no funding for the project, that the research had accomplished its primary task of identifying salmon impacts. Both entities suggested that the USFWS, Corps and NOAA should shoulder the burden for further funding of implementation of management actions.

Those agencies, with budget limitations of their own, are considering requests from the researchers for funding.

"There's a couple things we want to do first," Seto said. The agencies will study the research proposal and identify which of its tasks might provide benefit to the EIS process. Then a price tag would be attached to each task and it would be decided if the work is affordable.

"We certainly have been able to use the data" produced from the research to provide background for the EIS, Seto said. It is also useful in developing a description of current conditions in the estuary.

The summary said "close monitoring is needed to assess the long-term effects of the relocation of the Caspian tern colony on survival of juvenile salmonids, as well as the productivity and demography of Caspian terns in the Columbia River estuary."

The researchers have long advocated the location of other nesting sites that might be rehabilitated to lure terns from the East Sand colony, which would reduce the risk to both birds and Columbia River salmon. Concentrating 67 percent of the West Coast population at one site leaves the population more susceptible to risks such as disease, storms or other calamities.

As part of the EIS work, the research group led by principal investigator Dan Roby of the Oregon State University and Collis is looking at Caspian tern colony status and diet composition in other locations, such as San Francisco Bay and south-central Oregon.

"Restoration of permanent colony sites for Caspian terns along the Pacific coast and elsewhere appears unlikely without empirical evidence that local salmonid stocks and other fish species of special concern will not be at risk," according to the estuary report.

Related Sites:
Columbia River Bird Research:

Related Pages:
Sharp Decline in Terns; Rise of Comorants by Barry Espenson, Columbia Basin Bulletin, 10/10/3

Barry Espenson
2003 Tern Report: Colony Eating Fewer Columbia Salmonids
Columbia Basin Bulletin, November 14, 2003

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