Washington Fish and Wildlife officials have received repeated reports of dead sturgeon this week on the Columbia River, but the exact cause of death remains a mystery.

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On a walk along the Columbia River on Wednesday afternoon, Frank Carr spotted what he thought was a log in the river.

But as he, his fiancee and her cousin got closer, they could see fins and whiskers gently waving in the water. It was a dead 7-foot sturgeon, being pushed back and forth in shallow water as the river lapped along the shore.

A little farther downriver, they saw another dead sturgeon, this one a little smaller but still more than 5 feet long.

Washington Fish and Wildlife officials have received repeated reports of dead sturgeon this week on the Columbia River.

On Wednesday, Paul Hoffarth, district biologist for the Washington State Department of Fish and Wildlife, began adding up the sightings.

His rough total came to 66 dead sturgeon in the Columbia River from McNary Dam to Boardman. More than 20 were reported upstream, from the Hanford Reach downstream to the McNary Dam.

The exact cause of the die-off of the largest freshwater fish in North America is a mystery.

Warm water temperatures and likely other factors that stressed the fish contributed to their deaths, fish biologists said.

Fisheries specialists in the Columbia River Compact plan a meeting Thursday to discuss the situation.

“I’ve fished the river for many years and periodically see dead fish, but nothing to this magnitude,” said Malcom Chunn of Richland. On a fishing trip Tuesday he counted 15 sturgeon floating belly up in the 6 or 7 miles downstream from the Irrigon boat launch.

“It was really shocking to see all this. It’s such a precious resource to see wasted like this,” he said.

Chunn said the water temperature where he saw the dead sturgeon was about 74 degrees.

The Columbia River sees temperatures in the 70s each year, and sturgeon can tolerate that. But usually that warm water is seen in late August followed by a cooler fall, rather than in mid-July, Hoffarth said.

Olaf Langness, a state fish and wildlife biologist in Vancouver, Wash., who specializes in sturgeon, ticked off several factors Wednesday that likely are stressing the monsterous fish.

Warm water, low flows and the resulting dropping dissolved oxygen levels are stressful for sturgeon, he said.

Gorging on the abundant sockeye salmon also may be messing with their metabolism as they overeat and then try to swim on hot days, he said. A similar event was seen in British Columbia in the ’90s, he said.

Hoffarth went out on the river this week to look at the dead sturgeon on a stretch below McNary Dam, sampling 16 of them, he said. He took measurements and then sliced them open to check their gender and look for obvious internal issues.

They were a mix of male and female, some floating down the river and some washed up on the edge. Most had stomachs full of sockeye.

“They appeared to be healthy fish,” he said.

Biologists also are seeing a higher number of females than usual among 60 sturgeon checked below the Bonneville Dam that should have spawned this year but did not. Instead, they are absorbing their eggs, which is another stress factor, he said.

On top of that, the prized fish are allowed to be caught and released on much of the Columbia River. The exception is Lake Wallula, where sturgeon of a certain size, about 4 to 5 feet long, may be kept through the season that ends July 31.

All fisheries could be closed to sturgeon fishing, eliminating the stress of the “catch-and-release” practice.

“We will take whatever steps possible to protect them while we determine the exact cause,” Hoffarth said.

Hoffarth said that determining a specific cause might not be possible. Scientists have been unable to determine the cause of a similar event two years ago in the Fraser River in Canada.

Most of the dead sturgeon in the Columbia River are mature fish, more than 5 feet in length.

At 5 feet, sturgeon are teenagers. More than 6 feet long, they’re in their 20s at least, Langness said. Once they grow to more than 9 feet, they are difficult to age.

An 8-foot sturgeon was found dead in Lake Washington in 2013, north of Matthews Beach.

“In general, these fish are decades old,” Langness said. Unlike salmon, which die after spawning at 4 or 5 years old, sturgeon can live for 100 years.

Dead sturgeon continue to be spotted floating down Columbia River, which may be a sign that whatever is causing their deaths is not over yet, Hoffarth said.

He has not received any reports of dead sturgeon on the Snake River, and reports of other dead species are about normal, he said.

Chunn said that in the many years he’s fished the Columbia River, he’s seen the occasional dead sturgeon, but very infrequently and only one at a time.

“They are very old fish,” he said. “They are the mature breeders of the river.”

He added that he favors halting sturgeon fishing until biologists figure out the cause of the die-off.

Carr, 20, of Sequim, and Sophia Murillo, 14, of Kennewick, waded out into the river to investigate the first dead sturgeon they saw Wednesday, as Carr’s fiancee, 19-year-old Tatiana Cardenas of Pasco, watched from shore.

“It was kind of a shock seeing how big it was and where it was,” Carr said. He and Murillo flipped it over onto its stomach, getting their hands covered with slime but finding no obvious wounds.

“It looked really gross,” but the up-close look was cool, Murillo said. She said she wants a career helping animals and was disappointed that there was nothing she could do to help.