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Sharp Decline in Estuary Terns;
Steep Rise of Comorants

by Barry Espenson
Columbia Basin Bulletin - October 10, 2003

Researchers this summer witnessed a sharp decline in the number of Caspian terns nesting on the Columbia River estuary's East Sand Island, though their diminished predatory presence was likely counterbalanced by an even more dramatic increase in the number of doubled-crested cormorants that share that locale.

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, a decrease of 16 percent from last year's count of 9,933. The 2002 count was the largest, having risen from 8,982 pairs in 2001.

"We don't have a clue what happened" to cause the decline in tern numbers, said Dan Roby of the Oregon State University. He is co-principal investigator for the research team that has, since 1997, charted the comings and goings, and the diet, of avian predators in the estuary and elsewhere in the Columbia Basin.

The monitoring began in 1997 to assess the impacts of piscivorous waterbirds (i.e., gulls, terns, and cormorants) on the survival of juvenile salmonids in the lower Columbia River. Numerous basin salmon and steelhead stocks are listed under the Endangered Species Act.

Most of the attention has been focused on the terns, avian predators whose numbers in the lower estuary grew from just a handful when first sighted in the estuary in 1984 to thousands. Terns have concentrated in the estuary on, for the most part, islands created by dredge spoils. They have concentrated because historic nesting sites have been lost elsewhere in the Pacific Coast/Western region. The islands offered stable nesting habitat close to abundant supplies of fish -- both marine species and juvenile salmonids migrating to the ocean.

The East Sand tern colony is considered to be the largest in the world from April to late summer before they disperse to points south.

Now, those terns have the honor of sharing that island with the West Coast's largest aggregation of double-crested cormorants. Their number grew by 23 percent from 2002 to 2003 with the June 11 census showing 10,646 breeding pairs, Roby said.

"We don't know where all of the cormorants came from either. We certainly wouldn't have predicted either one of these things," Roby said of the population swings.

Researchers from Oregon State University, U.S. Geological Survey, Columbia River Inter-Tribal Fish Commission, and Real Time Research -- with funding from the Bonneville Power Administration -- have monitored the cormorant population, but not as closely as it has the tern colony.

Both birds prey on fish, but it is much harder to calculate the share of the cormorants' diet that is made up of salmon vs. marine and other species. The terns deliver their prey whole to their young and mates so researchers can visually determine what type of fish it is. The cormorant, on the other hand, swallows the prey and delivers pre-digested meals to the nest. Genetic analysis is the primary means available to determine the fish species in the cormorants' diet.

"It's hard to tell the impact of that predation," Roby said of the growth of the cormorant colony. The cormorants on the island this past summer "probably ate as many salmon as the tern colony."

The Columbia River estuary tern population grew significantly in 1997 and 1998 when its chosen nesting ground was at Rice Island, several miles further inland. A Rice Island tern colony grew to about 17,000 pairs my 1998.

The researchers estimated that in 1998 Caspian terns nesting on Rice Island consumed about 12.4 million juvenile salmonids or approximately 13 percent of the estimated 97 million smolts that reached the estuary during the 1998 migration year.

A pilot study was initiated in 1999 to determine whether the Rice Island tern colony could be relocated 16 miles closer to the ocean on East Sand Island. About 1,400 pairs of Caspian terns nested at the new colony site on East Sand Island in 1999. In 2000, about 8,500 pairs of Caspian terns nested on East Sand Island, or 94 percent of all terns nesting in the estuary. In 2001 and 2002, all Caspian terns nesting in the Columbia River estuary used East Sand Island, with approximately 9,000 and 9,900 pairs nesting at the site.

The diet of East Sand Island terns averaged between 31 percent and 47 percent salmonids during the years 1999-2002, compared to the diet of Rice Island terns, which consisted of 77 percent and 90 percent salmonids in 1999 and 2000, respectively. Total smolt consumption by terns nesting on East Sand Island in 2001 and 2002 was approximately 5.9 million and 6.5 million. That's a drop of 52 percent and 48 percent in estimates of smolt consumption compared to 1998.

The percentage of salmonids in the terns' diet this year at East Sand was 25 percent of the identifiable fish. Roby attributes that decline to the increased availability of marine fish species. Not only is East Sand closer to the ocean than Rice Island, but favorable ocean conditions have allowed populations of those marine species to climb.

Researchers are still puzzling over the large shifts in tern and cormorant populations. The increase in the number of cormorants may be explained by a shift in birds from other West Coast locales. While cormorant populations have grown to troublesome levels in parts of the Midwest and East, the West Coast populations are fairly stable and in some cases even declining, Roby said.

The East Sand colony has grown steadily, from fewer than 100 pairs in 1989 to 4,000 breeding pairs in 1997, to 2003's record total.

"It's an amazingly straight line," Roby said of the cormorant population growth at the island. They are likely drawn by the ready availability of food, and relative security that the large colony provides against eagles and other animals that prey on cormorants and their nests.

The case of the missing terns has been harder to solve, Roby said. At first it was thought they just went elsewhere to nest. But checks of known nesting sites as far south as the Baja peninsula in Mexico revealed no growth spurts.

"If that's what happened, no one seems to know about it," Roby said. The next largest tern colony in the basin, at Crescent Island on the mid-Columbia, showed a decline as well -- about 12 percent.

A draft summary report for the 2003 field season should be completed within the next few weeks, Roby said. That report will detail the researchers' analysis related to tern colony size and nesting success, diet composition and estimates of salmonid consumption, dispersal and survival and monitoring and evaluation of management activities.

Related Sites:
Columbia Bird Research:

Barry Espenson
Sharp Decline in Estuary Terns; Steep Rise of Comorants
Columbia Basin Bulletin, October 10, 2003

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