The agency last week unveiled its latest effort to reduce the graceful seabird's diet of juvenile salmon and steelhead. The corps' draft plan to reduce tern habitat on East Sand Island, located near Chinook, is open for public comment.
The proposal is to cut nesting habitat by about a third, from 1.58 acres currently to 1.08 acres, in hopes of decreasing the number of birds. The corps says it's responding to unexpectedly high nesting density on the island, Steve Helm, a corps wildlife biologist involved with the tern relocation program, said Friday.
Put in layman's terms, the birds were huddled together a lot more closely than biologists ever expected, stalling an effort to disperse the colony to other locations.
Predation by Caspian terns and cormorants is considered one of the major reasons why 13 of 20 salmon and steelhead runs in the Columbia Basin are listed as threatened or endangered under the federal Endangered Species Act.
The birds have taken advantage of sandy islands created by corps dredging near the river's mouth to chow down on ocean-bound salmonids. Biologists estimate that Caspian terns gobbled down 5.5 million smolts annually between 2000 and 2009.
Concentric rings of silt fencing shows how biologists have gradually been reducing habitat for Caspian terns, which nest in patches of open sand. The two white boxes are blinds biologists use to study the birds on East Sand Island near Chinook. The white blotches on the sand are nesting terns.
The corps has been trying to disperse the colony for nearly two decades. Among its first actions was to relocate the birds from Rice Island to East Sand Island by planting vegetation on the bare expanses of open sand where the birds nested. East Sand Island is farther down the river, so the theory was that terns could feed off a wider variety of fish species, reducing their impact on salmonids.
In 2006, the corps adopted a long-range plan to reduce the number of nesting pairs at East Sand island - estimated then to number 10,000 - to 2,500 to 3,100 by 2015. The agency has created seven to eight acres of nesting habitat at inland locations in Oregon and Northern California while gradually reducing the available habitat at East Sand Island to 1.6 acres.
Progress has been slower than expected, Helm said. Last year, for example, researchers estimated 7,600 pairs set up nests there. Biologists had projected the colony should have fallen at least to 4,100 pairs by now, leading to the new project to further reduce the amount of nesting habitat, Helm said.
"We're trying to reduce the colony size by reducing the available habitat," Helm said.
If they don't have room to nest, the terns will look elsewhere, Helm said. In essence, the corps is hoping the terns, which migrate immense distances in the Pacific Basin, start using the new habitat locations created to the south.
The new draft environmental assessment for the proposed activity will be available for public review and comment on the corps' Portland District website. Comments must be received by Feb. 21.