AUGUSTA — Two federal agencies formally proposed Thursday to list Atlantic salmon in seven Maine rivers, five of them in Washington County, as threatened under the Endangered Species Act.

Gov. Angus S. King immediately said he will oppose the designation because accompanying restrictions would have a severe impact on the Down East economy. But environmentalists cheered the news, saying it was needed to build back a vulnerable population of salmon.

The threatened designation is used for plants or animals deemed likely to become endangered within the foreseeable future. Endangered status is much more restrictive and all taking of that species is prohibited.

If the listing becomes official, the state would then develop a conservation plan to be approved by the U.S. Fish and Wildlife Service and the National Marine Fisheries Service.

“This is big news and it’s good news,” said David Carle, associate executive director of RESTORE: The North Woods, a Concord, Mass.-based group. “We would like to commend the agencies for doing what’s right. We’re very pleased with this decision. This is a step in the right direction.”

Carle estimated that so far this year, a total of only about 100 salmon have returned to the seven Maine rivers.

“If 100 fish is not an endangered or threatened species, what is?” he said.

The agency action came after months of discussion following an October 1993 petition by RESTORE: The North Woods and other environmental activists. RESTORE wanted the agencies to protect salmon in all the rivers from Connecticut to the Canadian border.

But the agencies ruled that wild Atlantic salmon south of the Kennebec River were extinct by the mid-1800s, and those rivers therefore were ineligible for protection under the Endangered Species Act.

Thursday’s action affects the Sheepscot, Ducktrap, Narraguagus, Pleasant, Machias, East Machias and Dennys rivers. The agencies said there was too little information to recommend a listing for salmon in the Penobscot, Kennebec and St. Croix rivers or in Tunk Stream.

The agencies said they have been working with industries in the vicinity of the rivers, and they expect if salmon are listed as threatened, it will have little impact on the blueberry and timber industries. The federal regulators were concerned among other things about the effect of dams, deforestation, agricultural practices and pollution on salmon habitat and survival.

But the aquaculture industry may have to change some of its methods to prevent genetic contamination of wild salmon by salmon that escape from holding pens.

Steve Swartz of the Maine Aquaculture Association said Thursday his group wants to help restore wild salmon, but they don’t want to be restricted by a listing under the Endangered Species Act.

“Listing, in our opinion, will result in additional regulation of our net-pen operations, which will be costly to farms and the salmon-consuming public and accomplish nothing,” said Swartz.

Diana Weaver, a spokeswoman for the U.S. Fish and Wildlife Service, said, “We’ve been working to minimize the impacts to aquaculture, the berry industry and the timber industry. We understand how important these industries are to the state of Maine.”

Weaver said she expects there will be a public hearing on the proposed listing. There is a 90-day public comment period and the agencies can take up to a year before they decide whether to implement the rule.

In addition, Congress has imposed a moratorium on new listings, which is due to expire shortly.

King predicted that if the listing becomes final it would have a severe impact on forestry, agriculture, aquaculture and sport fishing. King said there is no biological basis for listing the seven rivers as threatened.

He said he would press for cooperation from the federal agencies in developing a conservation plan that avoids the need for listing salmon in the rivers as threatened.

“Restoring salmon stocks would be of tremendous benefit to Maine,” King said, “but I disagree with this proposal. The state of Maine is already taking action to sufficiently protect and restore salmon.

“I am confident that in the next year we will demonstrate to the federal government that there is no legal or factual basis, or practical necessity, for listing this resource as threatened or endangered under the Endangered Species Act,” he said.

Carle said RESTORE wants to build salmon numbers up to a point where anglers can catch them and take them home. Salmon fishermen in Maine now must release the fish after catching them.

As for King’s opposition, Carle said, “Unfortunately, we feel the governor misunderstands the true impact of this. It should mean very little impact to industry. The present program of salmon restoration in Maine is a failure.

“There are multiple problems offshore and in our rivers. The governor has every right to voice his opinion, but the public also has a right to be heard,” he said.

Clinton B. Townsend, a Skowhegan attorney and past president of the Maine Council of the Atlantic Salmon Federation, said, “I think it came out just the way we had hoped.”

Townsend expects that even if the threatened designation becomes official, fishermen will be able to continue catch-and-release fishing for salmon.

“Our premise is that anglers and salmon need each other,” Townsend said. “We know that Atlantic salmon are in trouble. We want anglers to be a component of the recovery plan. Certainly, there’s no question the salmon are in trouble. I think this is a very good result.”

Townsend said he thought King was overstating the impact of the designation.

Sen. Olympia Snowe and Rep. John Baldacci voiced their displeasure Thursday with the idea of listing Atlantic salmon as “threaten- ed.”

“The state, private sector and federal agencies have been working proactively since the beginning of the year to preserve the Atlantic salmon and its habitat,” Snowe said in a statement.

“With a concerted effort already under way and sensible steps being taken, this announcement seems unwarranted and truly premature,” said Baldacci in a prepared statement.

Sen. William Cohen, however, viewed the decision as an improvement from earlier calls for salmon to be listed as “endangered” throughout Maine.

“I would continue to oppose unnecessary restrictions on activities in these areas that could adversely impact the Down East economy,” he said.

Noting that the listing was only a proposal, Snowe vowed to fight the Fish and Wildlife Service.

“Mainers need no second guessing when it comes to conservation, least of all from the federal government,” said Snowe.