Amid last summer’s drought and heat wave, some 98 percent of Okanogan basin sockeye salmon died before they reached spawning grounds, a new report says.

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Amid last summer’s drought and heat wave, some 98 percent of Okanogan basin sockeye salmon died before they reached upstream spawning grounds, a report presented to the Northwest Power and Conservation Council Wednesday says.

The analysis by a federal fishery biologist is a grim reminder of how vulnerable sockeye salmon are to elevated freshwater temperatures, which last June and July climbed far above historical averages in the Columbia River and peaked at 80 degrees in its Okanogan River tributary.

These Okanogan fish formed the majority of the 475,000 sockeye that returned from ocean feeding grounds to swim up the Columbia and over Bonneville Dam east of Portland.

Some of the sockeye were caught by fishermen. But most of them perished in warm waters that weakened their immune systems and left them vulnerable to disease.

The heat also clobbered endangered Snake River sockeye.

Some 4,000 of those fish made it past Bonneville, but more than 99 percent died as they tried to go farther upstream.

“It was bad, really bad for the Okanogan River sockeye and the Snake River sockeye,” said Ritchie Graves, a NOAA Fisheries biologists who leads the agency’s Columbia River hydropower branch. “They got a lot of miles to go, and it was hot all the time.”

A third population of sockeye that return to Lake Wenatchee spawning grounds fared somewhat better.

The analysis showed that 10 to 15 percent of those fish that made it past Bonneville reached their spawning grounds.

Graves said Columbia Basin sockeye runs are resilient and not threatened by a single bad year like 2015. But the fish would be at risk if such summers become more common due to climate change.

Graves said sockeye salmon, through natural selection, might adapt to changing conditions.

They could, for example, begin returning to spawning grounds earlier to avoid peak summer temperatures.

But it is uncertain whether they could change fast enough.

“It is kind of a race,” Graves said. “The environment is changing, and the fish are changing. Can they change fast enough?”