Young salmon surviving the journey down the Snake and Columbia rivers to the ocean are being eaten by the millions just a few miles short of their goal, Oregon State University researchers say.
The smolts are the favorite food of a growing population of Caspian terns on Rice Island, a two-mile stretch of dredged sand eight miles upstream from Astoria, Ore.
The nesting colony of birds was established 11 years ago and has grown from 1,000 pairs to about 8,000, making it the largest colony of Caspian terns in North America and perhaps the world, wildlife biologist Daniel Roby says.
In 1996 and 1997, the researchers released radio-tagged salmon below Bonneville Dam and found that terns on Rice Island ate as many as 15 to 20 percent of the smolts that reached the Columbia River estuary.
Further studies last summer found the colony’s diet is about 85 percent salmon smolts. That could mean 6 million to 20 million salmon smolts were eaten by the terns in one season.
The researchers said other factors could be at work, such as weather variations. Studies have not pinpointed how many salmon have been eaten by other types of birds.
The terns are protected by federal law, but researchers say the birds could be encouraged to move elsewhere by building another sand island from dredge material. In addition, planting vegetation such as silk plants would discourage the terns because they prefer bare sand.