While various attempts to reduce avian predation on young Columbia River salmon and steelhead have been successful, some of the predatory birds have simply moved to new—and potentially more damaging—locations in the estuary, a lead researcher told the Northwest Power and Conservation Council Sept. 17.
After years of killing double-crested cormorants and oiling their eggs at East Sand Island in the Columbia’s estuary, the birds are now nesting by the thousands on Astoria-Megler Bridge, where impacts to out-migrating smolts are likely higher, according to Dan Roby, professor of wildlife ecology at Oregon State University.
Along with Caspian terns, the East Sand Island colonies were consuming as many as 25 million smolts annually, which roughly amounts to 15 percent of the young salmon and steelhead that had survived dams and other predators all the way to the estuary.
Roby, who was involved in a 23-year effort to study and reduce the impacts of bird predation on ESA-listed salmon and steelhead in the Columbia River, said that over 5,000 adult cormorants were killed by wildlife agencies, and eggs were oiled to prevent hatching in about 7,000 nests.
The numbers were lower than their targets, but these efforts ended when the birds largely abandoned their colony and created a new one, he said. Now, he said, more than 3,500 breeding pairs are nesting at the Astoria-Megler Bridge, and the newly established colony is likely to grow to at least 5,000 pairs.
Similarly, Caspian terns dissuaded from nesting at Goose and Crescent islands on the Columbia Plateau have moved to alternative sites like the Blalock Islands, which is part of a wildlife refuge where the U.S. Army Corps of Engineers has been unable to take similar actions, Roby said.
“It was a bit of playing whack-a-mole, and it didn’t work out,” he said.
Other than counting the number of cormorants nesting on the Astoria-Megler Bridge, no one is currently monitoring the impact of this newly established colony of cormorants, Allen Evans, a fisheries scientist with Real Time Research, told the Council. Roby worked with Real Time Research and the U.S. Geological Survey to research the impacts of birds on salmon and steelhead in the estuary.
“We know the farther up river, the higher their impacts are on a per-bird basis,” Evans said, adding, “It’s quite likely the impacts are much greater, but there is no effort to monitor that.”
He said that for some ESA-listed salmonids, bird consumption causes a higher mortality rate than all of the other forms combined, including dam passage, pikeminnow predation and disease.
Research into avian predation in the Columbia Basin began in 1997, and resulted in numerous surprises, Roby said during his presentation to the Council. Researchers discovered that bird consumption is a major source of smolt mortality for multiple ESA-listed salmonid populations, especially steelhead and fall Chinook.
In addition to the huge impact of both Caspian terns and cormorants at East Sand Island, researchers also learned that a Caspian tern colonies at Goose Island and Crescent Island are together responsible for consuming between 5 and 30 percent of some listed steelhead populations, he said.
While smolt consumption rates vary widely depending on the year, the bird species and the location, many fish-eating birds are widespread throughout the basin and their nesting seasons overlap with salmon and steelhead that are migrating to the ocean.
Three management plans were developed to address avian predation.
A management plan for cormorants nesting on East Sand Island called for reducing the size of the colony from 14,900 to 5,600 breeding pairs by killing up to 11,000 adults and oiling the eggs in as many as 26,000 nests to prevent hatching. Agencies also reduced some of their nesting habitat by converting it into wetlands. Cormorants have largely dispersed or abandoned the colony over the last three years, but many of them are now nesting on the bridge at Astoria.
Plans for Caspian terns on East Sand Island called for reducing the colony from about 10,000 to 3,125 breeding pairs by reducing the size of the nesting area and creating alternative nesting sites outside the Columbia Basin. Breeding pairs were reduced to 3,800 pairs, and some terns have relocated, but the sites are under-utilized, Roby said. These actions have so far resulted in a 50 percent reduction in tern impacts on steelhead smolt survival, he said.
On the Columbia Plateau, plans for Caspian terns on Goose and Crescent islands were to reduce nesting areas and create alternative nesting sites outside the Columbia Basin. Terns have not nested at either island over the last four years, but they continue to try. Many have relocated to nearby Blalock Island, where no management actions are taking place. The efforts have resulted in a 44 percent decline in the breeding population.
Roby said researchers have discovered that cormorants and terns have a strong fidelity to the region, and continue to want to nest in the Columbia Basin. He said avian predation continues to be one of the most significant causes of mortality for some smolts, and that adaptive management will be needed to reach the goals of their management plans.