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.
Read the rest of the story in Northwest Fishletter here: