CORVALLIS - Juvenile salmon smolts that survive their journey from Columbia River headwater streams to the Pacific Ocean are being eaten by the millions just shy of their goal, Oregon State University researchers have discovered.
The smolts are the favorite food of a growing population of Caspian terns that have taken up residence on Rice Island, a two-mile strip of dredged sand eight miles upstream from Astoria in the Columbia River estuary.
Since it became established 11 years ago, the nesting colony of Caspian terns has grown from 1,000 pairs to about 8,000. This makes the colony the largest for Caspian terns in North America, and maybe in the world, said OSU wildlife biologist Daniel Roby.
OSU researchers Carl Schreck and Larry Davis discovered the terns' salmon-feeding habit in 1996 as part of their three-year research project into factors causing stress for salmon migrating to the Pacific, with a focus on how the fish respond to barging.
In 1996 and 1997, Davis and Schreck 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.
Roby studied the tern colony's diet last summer and discovered that it is about 85 percent salmon smolts. That could mean six million to 20 million salmon smolts were lost to the terns in one season, Roby said. However, both Roby and Davis are quick to point out that other factors could be at work, too.
Variation in weather conditions and smolt behavior could have led to higher-then-normal losses of smolts to the birds. And research has not pinpointed what percentage of the birds have been eaten by large colonies of gulls or crested cormorants. These birds also live in the estuary and may have eaten some smolts.
What to do about the terns is another matter, Roby said. If the terns continue to feed mostly on young salmon, wildlife biologists might try to encourage them to move by building another sand island from dredge material with the terns in mind.
In addition, planting vegetation such as silk plants on the island would discourage terns, which prefer bare sand.
But first, more research is necessary to prove it was the terns that ate the salmon smolts during the spring Columbia River salmon run. Cormorants in the estuary or gulls nesting further upriver could pose as much of a threat to the smolts.
"Some people don't want to hear about predation being a problem," Roby said.
Further, because terns are among the fish-eating birds protected by federal law, the answer isn't simply to destroy them.
Pending funding of further research, Roby would like to determine how many salmon smolts are being eaten by the birds so that can be factored into continuing salmon recovery projects.
There are some signs that nature is solving the human-made tern problem: Very few nesting pairs of terns produced young during the last breeding season, possibly because of very high levels in the river, ocean conditions and predators such as gulls or eagles raiding the tern nests.