PORTLAND, Maine (AP) The federal government ruled Monday that the last wild Atlantic salmon in the country can coexist with hydroelectric dams on a Maine river, dealing a blow to environmentalists who have long sought to remove the dams.

The salmon, once abundant in the U.S., now return to only a few Maine rivers. One is the Kennebec River, dammed by Brookfield Renewable Energy Partners. The National Oceanic and Atmospheric Administration said Monday that the dams are not likely to jeopardize the continued existence of the salmon if conservation measures are taken.

Conservation measures along Brookfield’s four Kennebec dams are designed to improve fish passages and will require an investment of more than $100 million by Brookfield, NOAA said. The dam upgrades would allow the salmon to swim up the Kennebec from the Atlantic Ocean to freshwater inland habitats for the first time since the construction of the dams in the 19th century, the agency said.

The agency reviewed the dams because Brookfield is seeking to relicense one of them and amend the licenses for three others, said NOAA spokesperson Allison Ferreira. NOAA said in a statement that it will “oversee an extensive monitoring program to ensure that the dams meet the expectations of improved fish passage in the Kennebec River.”

NOAA’s opinion “is an important milestone in ensuring that these facilities can continue to support Maine’s clean energy future and traditional industries along the lower Kennebec River,” said David Heidrich, a spokesperson for Brookfield.

However, environmental groups said the dams threaten extinction of the salmon, and NOAA’s ruling is shortsighted. They also said the dams endanger other vulnerable species of fish, such as sturgeon.

The dams’ negative impacts on salmon are a violation of the Endangered Species Act, said the Kennebec Coalition, an alliance of several environmental groups supporting dam removal, in a statement. They argue that in addition to blocking access to key spawning habitat, the dams also create hazardous conditions for this fish.

“Removal of these dams provides the best chance to prevent Atlantic salmon from becoming extinct, while also continuing the restoration of a vibrant, healthy Kennebec River,” the statement said.

Atlantic salmon populations in the U.S. fell into the hundreds due to factors such as overfishing and habitat loss, according to NOAA. They’ve been listed under the Endangered Species Act for over two decades.

Conservationists and Native American tribes have for years made the case that Brookfield has not fulfilled its obligations to protect the remaining salmon.