PORTLAND – The National Marine Fisheries Service and the U.S. Fish and Wildlife Service want to expand their endangered species listing for Atlantic salmon to include three of Maine’s biggest rivers, the agencies announced Tuesday.

The plan to be published in Wednesday’s Federal Register calls for applying the endangered species designation to the Atlantic salmon population along the entire length of the Androscoggin, Kennebec and Penobscot rivers, as well as their tributaries.

Unlike Maine’s previous salmon designation in 2000, the proposed expansion would apply to large rivers that are home to industry and hydroelectric dams.

“It’s pretty daunting, really. We’re going to spend the next month with the state determining how we’re going to react,” said Pat Keliher, director of the Bureau of Sea Run Fisheries and Habitat in the Maine Department of Marine Resources.

Under the new proposal, the area in Maine encompassing a genetically distinct salmon population as defined by the Endangered Species Act would stretch from the Androscoggin River up the coast to the Dennys River in Washington County.

The endangered species designation already applies to the Dennys, along with Cove Brook and the East Machias, Machias, Pleasant, Narraguagus, Ducktrap and Sheepscot rivers.

The two federal agencies originally floated the proposal to expand the original listing in 2006, but it was shelved pending further study.

So far this year, about 2,000 salmon from the proposed expanded endangered population have returned to spawn. Though it’s a small increase, it’s just 10 percent of the number required before spawning stocks are thought to be in good condition.

Maine fought the agencies’ original decision to list as endangered wild Atlantic salmon in eight Maine rivers. The state contended the salmon on those rivers were not genetically distinct from other salmon, including salmon in eastern Canada.

The state eventually dropped its lawsuit and agreed to work with the federal agencies. But the state is not yet on board with the latest proposal.

“The governor is committed to Atlantic salmon restoration, but we’re going to weigh all of our options,” Keliher said Tuesday. Options include pursuing a declaration that the salmon are threatened instead of in danger of extinction, he said.

The big challenge under the proposed expansion is that the three rivers include so many mills and hydroelectric dams, said Mary Colligan, the fisheries service’s assistant regional administrator for protected resources.

“We’re certainly going to have to work with all of the dam owners and look at existing passages and potentially request improvement,” she said.

Andrew Goode from the Atlantic Salmon Federation said it’s unrealistic to think the dams will be eliminated, but he hopes environmentalists, conservationists and industries can work together as they did in the so-called Penobscot River Project.

Working with other parties, Pennsylvania Power and Light found that it could produce the same amount of electricity but with three fewer dams on the Penobscot, Goode said.

“It’s being looked at as a way of overcoming some of these insurmountable problems on these rivers,” he said. “We have to try to take that approach on these other rivers.”