Sound Transit disclosed Thursday that electric current from light rail operations along the I-90/Homer M. Hadley floating bridge is suspected of killing Lake Washington fish.

An estimated 2,000 fish, mostly nonnative species, have died near the bridge. Exact counts are difficult, in part because many have decomposed, transit staff said.

Sound Transit has for months hailed its innovative engineering to control electric current across the lake, so news that fish are being killed comes as a letdown.

The Washington State Department of Transportation discovered the problem Aug. 3 and notified Sound Transit promptly, according to transit CEO Dow Constantine.

“We are still in the discovery mode. Fish biologists and engineers are now busy analyzing photos and video collected by the divers,” Constantine told a transit-board committee. “Early analysis has shown that the majority of fish are nonnative or invasive perch and bass, with a small number of salmon identified.”

The dead fish were clustered near the ends of the bridge, where cables to control electric current rested on the lake bottom, Constantine said.

WSDOT Secretary Julie Meredith, speaking remotely at Thursday’s meeting, said officials are working to find the cause and are coordinating with environmental agencies, as well as tribes that monitor fish runs.

“The bridge remains safe for the traveling public, and WSDOT and Sound Transit are going to continue to share the responsibility for guaranteeing that, both now and over the long term,” she said.

The light rail tracks, where train service began March 28, run next to the westbound lanes of I-90. The water closest to the trains — between the westbound Hadley bridge and the nearby eastbound bridge — is a maintenance zone where the public doesn’t boat, swim or kayak.

Regardless, transit staff said electric current next to floating bridges isn’t powerful enough to harm someone in the water. Scuba diving to visit old sunken I-90 pontoons is “relatively popular,” said WSDOT communications director Ryan Overton. The eastbound bridge, even without trains, also has cables where low-level current travels, “and they are not a health hazard,” he said.

Members of the board’s executive committee asked no questions during Thursday’s surprise public revelation.

The light rail line, which runs on electricity, transmits low-level electric current into the lake through metal anodes that dangle from leashes off the floating bridge. Those electrons are meant to be beneficial, by diverting stray current that might otherwise corrode the I-90 pontoons and their anchor cables.

The current-pumping anodes are part of a sophisticated system called cathodic protection, critical to building the $1 billion train corridor across the lake — the world’s only passenger rail on a floating bridge.

Since the dead fish appeared, the agencies have carried out what Constantine called an “interim solution” of pulling up certain anode strings that rested on the lake bottom, and where dead fish were concentrated. That operation was carried out Aug. 23, he said.

A total 32 anode cables were raised so they no longer contact the lake bed, spokesperson Rachelle Cunningham said later.

Sound Transit waited a month to reveal the problem. The lag in information happened because “this is an emerging issue” and as of Thursday “we now have a clearer understanding of the situation and a plan to determine and address the root cause,” Cunningham said.

Salmon kills, in particular, would be a potential crisis because several species are threatened or precarious. Last year, a state biologist estimated Lake Washington’s sockeye run at only 8,000 to 10,000 fish, down from 400,000 or more a half-century ago. In addition to habitat loss, disease and sea lions, salmon are also affected by lakes that are warming due to climate change.

State counts at the Ballard Locks show 31,302 sockeye and 10,785 Chinook salmon, as of Aug. 30. An additional 413 coho appeared, but they’ll migrate mainly in September and October, according to publicly posted state data.

As far back as 2008, WSDOT’s bridge experts identified cathodic protection as a requirement to allow light rail on I-90, so the bridge can last until 2100. Rampaging stray current could eventually rust rebar within pontoons, so they swell and crack, or weaken underwater cables that hold pontoons in position.

Fish protection from current was specifically listed among 23 technical requirements but rated “low importance.” Sound Transit attested in documents that “the amount of stray current associated with (light rail transit) will not have a significant effect on fish.”

For decades, I-90, along with many other bridges in Washington and worldwide, have employed cathodic protection. But the current-control equipment there — encompassing waterproof track-tie covers, devices inside pontoons and trackside wires to channel stray electrons — took extra months to calibrate and was the last job of the cross-lake route to be finished before the grand opening.

The cathodic protection system, as a whole, continues to show readings within normal ranges, even with some cables out of position, Cunningham said.

Biologists are taking water and soil samples this week, she said.