Birds are Devastating Columbia River Steelhead Smolt
Scientists ‘Shocked’ By Impact of Birds On Upper Columbia Steelhead
Scientists who study avian predation in the Columbia Basin have long known that birds can be a significant cause of death for young salmonids–especially upper Columbia River steelhead. But even Allen Evans and Dan Roby were surprised by the numbers after tallying cumulative impacts of 14 bird colonies on this especially vulnerable run of steelhead, listed as threatened under the Endangered Species Act.
With 11 years of results, their study found that between 31 and 53 percent of these juvenile steelhead get snatched up before they reach the ocean by four kinds of birds that have formed colonies along the Columbia–Caspian terns, double-crested cormorants, California gulls and ring-billed gulls.
“I was shocked,” Allen Evans, a lead author of the study, “Cumulative Effects of Avian Predation on Upper Columbia River Steelhead,” told NW Fishletter. Evans is a fisheries scientist at Real Time Research. The paper was peer-reviewed and published in September’s issue of Transactions of the American Fisheries Society.
The study found colonial bird consumption accounts for between 42 and 70 percent of all upper Columbia River steelhead mortality. “Results indicate that avian predation, although not the original cause of steelhead declines in the basin, is now a factor limiting the survival of upper Columbia River steelhead,” the authors state.
Of the four species of birds, Caspian terns have the largest impact, accounting for between 11 and 38 percent of the steelhead deaths.
Evans said steelhead smolt mortality by birds is so great and so variable that it should be accounted for in any work or study looking at juvenile survival rates. “If you don’t account for it, you could work on another study and come to an erroneous conclusion because you didn’t account for the fact that bird predation was cut in half, or doubled. If you want to understand mortality, you need to measure mortality,” he said.
For the study, scientists followed PIT-tagged juvenile steelhead released at Rock Island Dam from 2008 through 2018 through several PIT-tag detection systems at dams and past a PIT-tag array in the Columbia River estuary. They then recovered the tags each year from 14 previously identified bird colonies–ranging from 25 to more than 10,000 breeding pairs–after the birds had left for the year. Scientists made at least two complete sweeps of each colony each year to find the PIT tags deposited there. They used previously published methods for estimating colony-specific PIT-tag deposition and detection probabilities.
Over the 11 years, more than 78,400 smolts were captured, PIT-tagged and released–between 5,800 and 7,800 each year. The smolts were randomly selected and tagged in proportion to the number collected for between nine and 11 weeks each year during nesting season.
Steelhead smolt survival from Rock Island Dam to Bonneville Dam ranged from 27 to 55 percent annually, “indicating that a large portion, and in many years the majority, of steelhead smolts died prior to reaching Bonneville Dam,” the study says. Their survival in the estuary before reaching the ocean was undetermined as there are no detection sites in the lower estuary downstream from the major bird colonies.
Comparing total smolt mortality to mortality caused by birds “indicated that avian predation was often the greatest source of steelhead mortality during outmigration through both Reach 1 and Reach 2,” or from Rock Island Dam to McNary Dam, and from McNary to Bonneville. From Rock Island to McNary, avian predation “was the dominant mortality factor in many but not all study years,” and accounted for between 28 and 87 percent of all smolt mortality through that stretch of river, the study said.
Curtis Dotson, a co-author of the study who works for Grant County PUD, said the PUD became involved because it recognized a few local bird colonies were eating a lot of steelhead smolts, and the PUD was not meeting the juvenile steelhead survival rates called for in its Federal Energy Regulatory Commission license.
By including bird colonies on Goose Island and Crescent Island, the PUD could also offer some funds for the study through the Priest Rapids Coordinating Committee, which includes members from state, federal and tribal governments working to implement anadromous fish activities for Priest Rapids Dam.
“Now we have the data that says this tern colony is a problem for the Priest Rapids Project,” he noted, which frees up funds and agency support to resolve the problem.
Dotson said starting in 2014 and 2015, agencies planted vegetation on Crescent Island, which has discouraged enough Caspian terns to convince the colony to move elsewhere. On the rockier Goose Island, they installed flags and sent crews to harass the birds.
“It has been very successful,” Dotson said. “The bird population was decreasing at the same time that my survival rates were going up.” Those rates are tallied based on the past three years, and are now being met, he said. That’s good for both steelhead and Grant County PUD ratepayers who no longer have to fund the efforts to improve those survival rates.
Evans noted several studies have looked at mortality caused by individual colonies–like the Goose Island colony of Caspian terns–but this study is the first to determine the cumulative impact of avian predation. He said the same mark-and-recapture methods can be used to measure the impacts of other predators.
The issue of bird predation isn’t unique to the upper Columbia. Evans said another study with just one year of data shows that Snake River steelhead smolts appear to have similar mortality rates due to birds as upper Columbia River steelhead. “I think upper Columbia steelhead are still the worst-case, but Snake River steelhead aren’t too far behind,” he said.
Roby–also a co-author of the study, and who has led work in the Columbia River estuary to reduce the impact of Caspian terns and double-breasted cormorants on juvenile Columbia River salmon and steelhead–stressed these bird colonies are known to prey on steelhead more than any other salmonid, and the cumulative predation rates would not likely be as significant for salmon smolts. “There’s good reason to believe that the picture is not quite so gloomy for most other listed [Endangered Species Act] salmonids in the Columbia Basin,” he said.
Roby explained that steelhead smolts are particularly vulnerable because they tend to swim closer to the surface of the water compared to Chinook or sockeye, making them an easier target for birds. Steelhead smolts also tend to be larger, so they’re more attractive to hungry birds looking for a nice meal, he said.
Dams on the Snake and Columbia rivers may be contributing to the problem. Several studies have “hypothesized” the dams may cause smolts to be more vulnerable because of delays in travel times, or because smolts that are stunned, injured or killed in turbines become easier prey for birds, the study notes.
“We know the dams, by creating reservoirs, slow the passage of water, and that increases the exposure time that these smolts have to birds,” Evans said. “You can’t look at results in a vacuum,” he added, noting that avian mortality rates may increase or decrease due to any number of factors, including the presence of hydroelectric dams.
The study suggests that, given the magnitude of predation by birds, “reducing avian predation should be a high priority for those concerned with the recovery of ESA-listed steelhead.” However, terns, cormorants and gulls are all native species and are protected by the U.S. Migratory Bird Treaty Act.
Recognizing that birds often prey on juvenile salmon and steelhead at the outfall pipes below their hydroelectric projects, the U.S. Army Corps of Engineers has tried numerous methods for deterring them, including sprinkler systems and physical harassment, which may include setting off fireworks, firing shotgun blanks and going out to the area in boats.
While harassment works, it is only effective in the short term, the Corps notes. So when a sprinkler system used to keep birds away from the outfall pipe below McNary Dam was swept away in high water, the agency began researching new methods, and this year purchased a laser to try to deter them.
With a range of about 950 feet, the laser emits a bright green light, creating a large dot. “Birds see the dot produced by that color of laser as a solid object moving towards them and fly out of the way because they think they are being chased or about to be hit by something,” Corps mechanical engineer Caleb Willard said in a Corps news release.
The laser was tested at the outfall pipe last spring, and while it was somewhat successful, it was operating at the edge of its range. The Corps purchased another laser and will test a dual laser system in April. If it works, more may be installed at other Corps dams. The agency is also looking into a so-called long range acoustic device that sends out avian distress calls and calls from predatory birds, the news release says. -K.C. Mehaffey