Columbia Estuary Tern Colony's Population Growsby Barry Espenson
Columbia Basin Bulletin - September 6, 2002
What is believed to be the world's largest colony of Caspian terns got bigger from 2001 to 2002, blossoming by nearly 1,000 nesting pairs on their East Sand Island sanctuary in the Columbia River estuary.
The closely monitored congregation of migratory birds, however, experienced its worst nesting success since first being drawn to the island three years ago.
Neither of the trends is surprising, according to researchers who recently released preliminary data regarding the tern colony size and nesting success. A summary report of the birds' 2002 life and times in the estuary is due within a few weeks, according to Ken Collis, co-principal investigator for the tern research project.
That 2002 summary report will include complete data regarding the diet of the birds during their late spring/summer stay in the lowermost Columbia River Basin. The birds fly north in spring to nest, breed and hatch and rear their young.
The birds have, since 1997, been under the watchful eye of a wide range of biologists -- some concerned about the terns' appetite for juvenile salmon and others more concerned about the welfare of the birds' themselves.
The terns first began trickling into the estuary in the mid-1980s, their nesting habitat along Northwest coast in Washington and elsewhere lost for a variety of reasons. The colony grew rapidly at Rice Island, a sandy upwelling created with the deposit of dredge spoils from the U.S. Corps of Engineers' maintenance of the river's navigation channel.
In 1997, Caspian terns nesting on Rice Island consumed an estimated 5 million to 10 million salmonid smolts, or 5-10 percent of the estimated number of smolts to reach the estuary, according to background on the Columbia Bird Research web site,
In 1998, the colony consumed between 8 and 14 million salmonid smolts, or 8-15 percent of the migrants. Those juvenile salmon and steelhead migrants include 12 stocks listed under the Endangered Species Act.
Because of the magnitude of the tern predation on juvenile salmonids, a working group was formed in 1997 and 1998 to explore management options for reducing the predation. The state, federal, tribal and private individuals involved opted for a plan to shift the population from Rice Island to the site 13 miles nearer the ocean -- East Sand Island -- so that the terns would have easier access to marine forage fish.
First implemented in 1999, the effort met modest success -- luring 1,400 pairs or 15 percent of the total colony to East Sand. The effort to discourage Rice Island nesting gained momentum in 2000 with 94 percent of the colony settling in on East Sand's four-acre prepared site, lured by decoys and audio devices.
In 2001 and 2002, 100 percent of the colony settled at East Sand. The plan has worked, both at moving the birds and reducing their consumption of salmon. Juvenile salmonids comprised only 44 percent of the diet of terns nesting at East Sand Island in 2000, compared to 91 percent of the diet of terns nesting at Rice Island. In 2001, with all of the birds at East Sand, salmonids made of 34 percent of the tern diet. A preliminary scan of weekly research reports indicates that salmonids made up about 32 percent of the terns' diets this year, Collis said.
"We're still working on those numbers," he said of the bioenergetics modeling and analysis that remains to be done. "We still haven't estimated the number of smolts consumed this year."
The preliminary data released last week estimates the number of Caspian terns breeding in the Columbia River estuary in 2002. The estimates were produced through the use aerial photographs of the colony taken near the end of the incubation period in late May. Ground counts of incubating and non-incubating terns on 12 different plots within the colony area were used to validate the aerial counts.
As was the case last year, all nesting by Caspian terns in the Columbia River estuary occurred on East Sand Island in 2002. Researchers estimate that 9,933 breeding pairs attempted to nest at East Sand Island in 2002. The best estimate of breeding pairs was 8,982 in 2001.
The number of Caspian terns nesting in the estuary increased significantly from 1997 to 1998, but remained relatively stable from 1998 to 2001 before increasing significantly from 2001 to 2002, according to the preliminary data.
"At this point, we can offer several possible explanations for the differences discussed above," according to the preliminary report. "The increase in colony size in 2002, as compared to the previous year, was likely due to a combination of recruitment of terns that formerly nested in Commencement Bay (and perhaps elsewhere) and first year-breeders that were produced in the Columbia River estuary in 1999."
A nesting ground that held about 1,000 tern pairs in Commencement Bay at Tacoma, Wash., during 2000 was eliminated. The terns had made their spring-summer home atop a Superfund site owned by Aasarco. The contaminated metals repository is covered by large tarp that has been held down by sandbags. Many of the bags have weathered, and broken open, providing ideal sandy nests.
Birds tagged at Commencement Bay were identified this year in the Columbia estuary but researchers have not yet estimated how many might have settled in with the East Sand colony, Collis said. The addition of quite a number of first-year breeders at East Sand is also possible because there was good nesting success in 1999 on the island.
Nesting success (number of young raised per breeding pair) at East Sand was estimated using aerial photos taken of the colony just prior to the fledging period on July 1. The counts from aerial photos ground-truthed via counts of adults and fledglings on 12 different plots within the colony area.
The researchers estimate that 10,715 fledglings were produced at the East Sand Island colony in 2002. This corresponds to nesting success of 1.08 young raised per breeding pair. That's significantly lower than the nesting success at the East Sand Island tern colony in 2001 -- a best estimate of 1.39 fledglings/breeding pair. The nesting success at East Sand was 1.2 fledglings per breeding pair and .57 in 2000, the first year that most of the terns settled there.
The preliminary information says that the decrease in nesting success in 2002, as compared to the previous year, may have been due to lower availability of marine forage fishes. That theory is backed by the suspicion that the terns "foraging trip duration" was longer this year, as compared to the previous year. That data analysis is not yet completed.
The severe rain- and wind-storm in late-June that caused severe flooding on the colony was likely a factor explaining the lower productivity in 2002, as compared to 2001. Up to 1,000 chicks may have died as a direct result of this storm, and many more may have suffered delayed mortality (i.e, from exposure and adult-chick aggression associated with chicks wandering away from flooded areas and into the nesting territories of neighboring terns), according to the researchers.
Finally, it is possible that gull predation on tern eggs and chicks was higher this year as compared to 2001 as a result of the decline in food availability for nesting gulls, according to the researchers.
Despite the reduction in productivity in 2002, as compared to the previous year, productivity in 2002 was higher than was ever recorded on Rice Island -- the former site of the Columbia estuary colony -- and well within the range of other Caspian tern colonies in North America, according to the preliminary information.
The investigations into the causes for the annual differences in colony size and nesting success are ongoing. The research conclusions will be discussed in further detail in the 2002 Season Summary to be posted on the web page in October 2002.
Implementation of the tern management plan was nearly stalled this year after conservation groups concerned about the welfare of the terns won a federal court injunction.
The National Audubon Society, Defenders of Wildlife, Seattle Audubon Society and American Bird Conservancy filed a lawsuit early in 2000 challenging the scientific basis for the tern relocation plan and asking that involved federal agencies' complete an adequate environmental impact study. The groups said the tern management did not take into account the long-term welfare of the birds.
An August 2001 injunction imposed U.S. District Court Judge Barbara Rothstein prohibited any harassment of the birds in the estuary or manipulation of habitat and threatened implementation of the management plan this year.
A settlement agreement was reached this spring just prior to the return birds that requires federal defendants in the case to complete a full environmental impact statement that outlines potential short- and long-term effects on the terns. Meanwhile, they were allowed to again implement the relocation plan.
The defendants in the case include the Corps and the U.S. Fish and Wildlife Service. The Corps is in charge of the dredging. The USFWS has responsibility to protect the migratory birds that fly north to nest and rear their young during the late spring and summer.
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