About 400,000 baby fish died Sunday at a hatchery near Roseburg.
The entire run of pre-smolt spring Chinook that the Rock Creek Hatchery planned to release next year died when a clogged intake valve cut off their access to fresh water, said Greg Huchko with the Oregon Department of Fish and Wildlife.
Water flow was only interrupted for about an hour, but river temperatures are so high that the fish could not survive.
State wildlife officials said the fish are not endangered. Most would normally die before spawning, but about 5,000 usually return after spending two years in the ocean.
Fish managers said the catch in the North Umpqua River will likely be very limited in 2018.
The hatchery said it’s planning to test and improve alarms that failed to notice the water flow problem.