Environmental advocates said Thursday that they have sent letters to the owners of four hydroelectric dams, threatening to sue them under the U.S. Clean Water Act unless they take action to protect Atlantic salmon in the Kennebec River.

The river’s salmon, declared an endangered species last year by the federal government, are being killed as they pass through turbine blades at dams in Waterville, Winslow, Fairfield and Skowhegan, the advocates contend. They suggest the installation of protective barriers on the turbines.

“The salmon are in a desperate situation; they’re subject to this gauntlet of turbines to get to the ocean,” said Ed Friedman, chairman of the Bowdoinham-based Friends of Merrymeeting Bay.

Friedman sent the letter, along with Douglas Watts of Augusta. The four dams they identified are the Lockwood Dam in Waterville, Hydro Kennebec in Winslow, the Shawmut Dam in Fairfield and the Weston Dam in Skowhegan.

Friedman and Watts identify the owner of Hydro Kennebec as Myllykoski North America and Madison Paper Industries, and identify NextEra Energy/FPL Energy Maine as the owner of the other three dams.

However, Russ Drechsel, president of Madison Paper Industries, said the notice inaccurately named his company and Myllykoski, which is partial owner of Madison Paper, as dam owners.

“Madison Paper and Myllykoski are not owners of any of the four dams that are mentioned,” he said. He declined further comment.

NextEra spokesman Steve Stengel couldn’t be reached for comment.

The Merrymeeting Bay group successfully petitioned to have Atlantic salmon in the Kennebec River protected under the Endangered Species Act, which happened in June 2009.

Federal law requires parties to a lawsuit to provide a 60-day notice of their intention to sue. Watts said the notice creates “an opportunity for us and the dam owners to work together, talk about the substance of this and reach a resolution, and then we don’t have to go to court.”

The citizen-lawsuit provision of the Clean Water Act was intended for such a purpose by ensuring that citizens “have a right to help ensure enforcement of the law,” said Judy Berk, spokeswoman for the Natural Resources Council of Maine.

“The court will need to decide the merits of the case, but the 60-day notice period also creates an opportunity for action by the dam owners to address the issues being raised,” Berk said. “If the dams are violating their licenses, then they should immediately bring the operation of those dams into compliance in order to help protect fish populations and a healthy Kennebec River.”

The Kennebec River was home to 100,000 Atlantic salmon before dams were installed in the early 19th century, Friedman and Watts said. Soon, there may be none left, they said.