CORVALLIS - Marine birds called Caspian terns are not only eating fewer young coho salmon, fall chinook salmon and steelhead but are raising more young of their own successfully since humans encouraged the birds to switch nesting sites in the Columbia River, scientists from Oregon State University and several public and private organizations reported today (Aug. 9) in a scientific journal.
The article, in the latest issue of the international Journal of Wildlife Management, reports on the dietary and productivity effects of relocating about 9,000 pairs of the birds, the world's largest breeding colony of Caspian terns.
The relocation effort grew out of concerns about juvenile salmonids (salmon-like fish) eaten by Caspian terns when the birds nested on Rice Island, about 21 miles from the mouth of the Columbia River. The fish pass by the island during their migration to the Pacific Ocean.
Studies in 1997 and 1998 suggested that Caspian terns in the Columbia River estuary, particularly those nesting on Rice Island, were consuming millions of juvenile salmonids, including significant numbers of fish listed under the federal Endangered Species Act. The relocation attempt, initiated in 1999, centered on East Sand Island, about 15 miles closer to the mouth of the Columbia.
Workers moved vegetation and debris to create more desirable bare sand nesting habitat, installed tern decoys and playing recordings to attract terns, and discouraged seagulls from eating tern eggs and chicks, said Dan Roby, a federal wildlife ecologist based in OSU's Department of Fisheries and Wildlife who helped plan the effort and coordinated the study of its effects.
In addition, they disrupted tern nesting on Rice Island with tactics such as planting wheat and installing fencing. The impact on young salmon was dramatic, say the scientists.
"In 1999 and 2000 the diet of terns nesting on Rice Island consisted of 77 percent and 90 percent juvenile salmonids, respectively," reports the journal article, "while in 1999, 2000 and 2001 the diet of terns nesting on East Sand Island consisted of 46 percent, 47 percent, and 33 percent juvenile salmonids, respectively."
The researchers speculate that a key reason for the terns' new diet was the greater variety of food available nearer the mouth of the Columbia River.
The birds are consuming more marine forage fishes such as herring, sardines, anchovies, smelt, surf perch and Pacific sand lance, according to the report. It also says the nesting success of Caspian terns has been "consistently and substantially higher on East Sand Island than on Rice Island."
Rice Island was created artificially from dredge wastes. East Sand Island is a natural island.
Organizations participating in the Caspian Tern Working Group that did the relocation work included the National Marine Fisheries Service, the U.S. Fish and Wildlife Service, the U.S. Army Corps of Engineers, the Oregon and Washington Departments of Fish and Wildlife, the Idaho Department of Fish and Game and the Columbia River Inter-Tribal Fish Commission.
The authors of "Effects of Colony Relocation on Diet and Productivity of Caspian Terns," the article in the Journal of Wildlife Management, are: Dan Roby, Don Lyons, David Craig, Anne Mary Myers and Robin Suryan of the Oregon Cooperative Fish & Wildlife Research Unit at OSU; Ken Collis of Real Time Research, Bend; and Jessica Adkins of the Columbia River Inter-Tribal Fish Commission, Portland.