Floating the Cedar River, Nathan Ereth and Jarrod Yates cover more than 20 miles of salmon habitat every week. The biologists with West Fork Environmental collect sockeye carcasses.

So far this season, they've picked up 1,600 of the dead fish. They mark the GPS location of each carcass and tag the fish.

The heads are then dissected for the otolith. It's a piece of the inner ear that tells scientists whether the fish are hatchery or wild sockeye. It also tells age and provides clues about environmental conditions during the sockeye's lifetime.

Scientists want to know how the hatchery sockeye affect wild salmon. The collection also shows whether the hatchery fish are spawning around all parts of the river.

"That's what these scientists have actually been able to verify," said Michelle Koehler, who works with Seattle Public Utilities on the carcass survey.

This year, they've found more sockeye dying before they ever spawn.

"That's been one of the biggest Eureka’s and surprises, but it's not a good one, because there's mystery around this disease or suite of diseases, and then problems with what we can do to counteract it," Koehler said.

The disease is likely caused by a parasite, but it could be more common with warming water temperatures. The scientists may collect 2,000 fish by the time they finish. The fish are all dead, but filled with life when it comes to clues about salmon survival.