A chinook salmon.

A chinook salmon.

U.S. Fish & Wildlife Service

Record heat that has warmed rivers in the Northwest has caused another fish die-off. The Oregon Department of Fish and Wildlife reported 109 wild spring chinook salmon died last week in Central Oregon.

Water temperatures in the the Middle Fork John Day River reached the mid-70s. Biologists say those high temperatures combined with low stream flows are what likely caused the die off.

Biologists say they expect more salmon die-offs this summer, until spawning begins in September. Overall the John Day Basin has seen strong salmon returns this year.

This is the third such die off in the Middle Fork John Day River — others happened in 2007 and 2013. There is not much riparian vegetation alongside the shallow river, which causes its temperatures to rise quickly.