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PORTLAND, Ore. (Reuters) - Hundreds of spring Chinook salmon have been found dead in Oregon rivers over the past week, in a sign that abnormally high water temperatures are taking a toll on the threatened species, wildlife officials said on Friday.
Low snowpack linked to a historic drought has prevented icy-cold runoff from entering rivers as normal this year, according to federal hydrologists.
Temperatures in the Willamette River, a tributary of the Columbia River, have risen from 70 degrees Fahrenheit (21 Celsius) to 75 degrees Fahrenheit (24 Celsius) over the past week, about 12 degrees F (6.5 Celsius) higher than it was the year prior, Oregon Department of Fish and Wildlife spokesman Rick Swart said.
"Anything above 70 degrees, the fish are really stressed," Swart said.
Overall, Swart said it would take several more years of warm rivers to create a significant long-term setback for Chinook salmon populations, which have been returning to the Willamette River at levels not seen for decades.
As of June 14, Oregon Department of Fish and Wildlife officials had counted more than 51,000 Chinook passing through a fish-counting station on the river, far above the 50-year average of 41,000.
A majority of the fish found dead so far also were hatchery raised, rather than the wild fish designated as "threatened" under the U.S. Endangered Species Act, Swart said. He added that a biologist's spot-check showed at least 11 wild Chinook had died on the Clackamas in recent days.
While spring Chinook typically die in the fall after spawning and it is not unusual for some to die every spring and summer, pre-spawning deaths this year are both more numerous and earlier in the season than is typical, Swart said.
To cope with the conditions, some salmon have pushed into tributaries of the Willamette, where temperatures, while higher than normal, are below the 70 degree threshold (21 Celsius), he said.
Some 50 fish that attempted to make the journey from the Willamette to the Clackamas were found dead this week in that tributary.
Warm waters could ultimately also challenge fish hatcheries, but at present these government-overseen breeders do not expect an impact this year, he said.
Reporting by Courtney Sherwood; Editing by Eric M. Johnson and Sandra Maler