The state of Oregon has filed a $27.6 million lawsuit against the owners of the private Winchester Dam on the North Umpqua River near Roseburg, where an estimated 550,000 juvenile lamprey were killed during a dam repair project in August, according to the filing.

“The number of lamprey killed as a result of an inadequate fish salvage effort was significant and preventable,” the Oregon Department of Fish and Wildlife said in a news release announcing the lawsuit’s filing Friday in Douglas County Circuit Court.

The claim is among the largest ever sought by the state for the illegal killing of wildlife, according to ODFW.

The 133-year-old dam, at 450 feet wide and 17 feet high, is owned by the Winchester Water Control District, a homeowners association. It’s just upstream of Interstate 5, has a popular fish ladder with a viewing station, and creates a reservoir 1.5 miles long that once provided power and water for Roseburg but now serves primarily as a water sports lake, according to the state. The district and its contractors are named in the suit.

They might face other penalties from the Oregon Department of Environmental Quality, which has issued a pre-enforcement notice stating that the repair work had polluted the river and violated rules regarding waste discharge, unauthorized work, improper erosion controls, operating equipment that disturbed habitat and failing to report discharges, among others.

Lamprey, which resemble eels, are fish that grow up to about 33 inches long. They are indicators of healthy waterways and are used by Native Americans for food and medicine.

They are considered a “sensitive species” by the state, meaning they are facing threats to habitat or populations.

The dam has been experiencing structural decline in the past decade, according to ODFW’s online project page. Issues include water leaking through the dam, presenting a “false attraction” flow to migrating fish.

The agency authorized work on the dam from Aug. 7-28, and then in late August authorized additional work, with the stipulation that the fish ladder and area be “rewatered” by late Sept. 6. The work was authorized to take place at a time when it would least impact fish, including summer steelhead migrating upstream.

Days before the work began, the agency closed the North Umpqua River to summer steelhead fishing from July 31 to Nov. 30, due to low numbers. By the start of the repair work, most steelhead likely had migrated past the dam, according to ODFW.

It was anticipated that juvenile lamprey would be present in the substrate behind the dam, but the dam owners would be responsible for salvaging any of them left stranded and returning them to the river as soon as possible.

When the project began, and water behind the dam was dropped to perform the work on Aug. 7, it stranded juvenile lamprey in the muck. In large part, the state faulted the owners and their contractors with failing to rescue those fish.

According to the lawsuit, agency personnel on Aug. 7 notified contractor TerraFirma Foundation Repair of Roseburg about stranded fish and what appeared to be inadequate fish salvage operations involving about 10 company personnel who were collecting stranded juvenile lamprey. The next day there were about 25 personnel, but the agency deemed those efforts inadequate and instead called in agency workers from around Western Oregon on Aug. 9 in a major fish-salvage effort that was also assisted by federal and Cow Creek Tribe personnel.

Ryan Beckley, who is president of the water district and owner of TerraFirma, did not return requests for comment for this story, but told the Oregon Capital Chronicle in late September that he disputed the agency’s estimate that hundreds of thousands of fish had been killed.

“I’m incredibly impressed the design and construction teams were able to execute the project as efficiently as they did,” he said.

Beckley spoke following an ODFW presentation to the Oregon Senate Natural Resources Committee on Sept. 27. State Sen. Jeff Golden, D-Ashland, chair of the committee, asked the agency to return for another meeting on Nov. 6 or Jan. 10 with a detailed report about what happened, along with information about the condition of the dam.

“As you likely know, public perception has grown over the years that operation and maintenance of this dam have not been held to statutory and regulatory standards that similar facilities around the state have to meet,” Golden wrote.

Jim McCarthy, Southern Oregon program director for WaterWatch of Oregon, said his organization has offered to arrange for removal of the dam at no cost to its owners. He estimated demolition would cost $2 million, with the federal government picking up the tab.

He welcomed the lawsuit.

“It’s great news for North Umpqua salmon and steelhead and people who care about it,” he said.