Juvenile coho salmon are disappearing before they migrate out of South Puget Sound, so the Squaxin Island Tribe barged thousands of fish past the Tacoma Narrows Bridge to see if they fare any better.

The tribe released the juvenile coho near Vashon Island and Point No Point to determine whether they can survive at a higher rate than fish released from net pens near Olympia.

In an earlier multi-year study, the tribe fitted wild and hatchery coho with acoustic tags, tracking their migration using receivers throughout the sound. Only six of the 175 tagged coho could be found beyond the Tacoma Narrows Bridge.

“That sort of drastic drop-off is not what we would have expected,” said Daniel Kuntz, salmon biologist for the tribe. “We know that really only 5 percent of coho survive to return as adults, but there shouldn’t be this drastic of a drop so soon.”

This year, the tribe used three different coded-wire tags on their net pen coho. Two groups of 45,000 each will have different tags than the main group released from the net pens. Coded-wire tags are millimeter-sized threads of metal with an individual code engraved on them. Fisheries managers recover the tags after salmon have been caught or found on the spawning grounds. The tags aid fisheries managers by providing information such as migration patterns and marine survival.

The tribe worked with the state Department of Fish and Wildlife on the project. State support for the project came from the Puget Sound Recreational Enhancement Fund.

The low number of coho surviving the relatively short journey points to an imbalance in the local ecosystem, Kuntz said.

“We know that a lot of coho aren’t making it very far, but we don’t know why,” he said. “Possible causes could include a lack of food for the young fish or an overabundance of predators like harbor seals.”

“Coho is the backbone of our salmon fisheries,” said Andy Whitener, natural resources director for the Squaxin Island Tribe. “We know Puget Sound is ailing. We hope that with this new data we can start healing the sound and bring back our salmon.”