Migrating steelhead are dying near the Hood Canal Bridge, but scientists don't know why.

When the juvenile fish migrate toward the ocean past the Hood Canal Bridge, 65 percent of them die.

"Its pontoons span 83 percent of the entire canal and go 15 feet under water. That's pretty difficult for juvenile steelhead that are only 6 inches long to navigate," said Lucas Hall, a salmon recovery projects assistant.

Acoustic sensors were lowered in the water near the bridge over the weekend. Acoustic transmitters were surgically placed in steelhead. That way, when the tagged fish swim by, scientists will get data on what happens.

"We're going to be able to understand their behavior patterns and most importantly understand where they're dying," Hall said.

The water on one side of the bridge is choppier than the other side, and little changes like that can make a big difference for fish. There might also be issues with turbidity, predation, light, temperature, and the water's chemistry.

"Steelhead have certain preferences for what they prefer as they're migrating through,” said NOAA Fisheries Biologist Megan Moore. “They're going to avoid certain conditions and have an affinity for other conditions, so those water chemistry parameters may change the way they move and where they go."

Steelhead numbers are at historic lows, and millions of dollars have focused on pollution and habitat restoration.

"If the Hood Canal Bridge is affecting all the fish coming out of there, and they’re succumbing to mortality because of what's going on at the Hood Canal Bridge, then that negates the effectiveness of all that really great work," Hall said.

The research will likely take a year or more, but scientists hope to have a better idea of what's killing steelhead sometime in 2018. If it looks like the structure of the bridge is the main problem, it could take decades to make changes.