The nation's only known population of freshwater seals will not get protections under the Endangered Species Act, federal officials announced on Wednesday.

The 250 to 300 harbor seals that live in Southwest Alaska's Iliamna Lake are different from harbor seals swimming the saltwater of Bristol Bay and the rest of the North Pacific marine system, but they fail to meet the criteria needed to classify as a distinct population segment, the National Oceanic and Atmospheric Administration's Marine Fisheries Service said.

With that determination, NOAA rejected a 2012 petition that sought to define the lake seals as distinct and deserving the listing.

The petition, filed by the Center for Biological Diversity, one of the most active and litigious environmental organizations in Alaska, argued Iliamna seals are "rare and unique" and at risk from several threats, including climate change and the possible development of the controversial Pebble mine project proposed for the region.

But NOAA scientists said they concluded after studying the issue that the lake seals are indeed separate from the larger North Pacific harbor seal subspecies population, but not significant enough to qualify for protection.

To be considered a distinct population segment — one of the criteria for an endangered listing — a population must be both discrete and ecologically significant from the larger group to which it belongs, said NOAA spokesperson Julie Speegle said.

"In this case, they are discrete but they are not significant," she said.

To be protected as endangered, a population must be a defined species, subspecies or distinct population segment, NOAA said.

There are about 150,000 harbor seals in Alaska comprising 12 regional stocks, the federal agency said. Bristol Bay stock, which includes the Iliamna seals, is currently estimated to total at least 28,000 animals, according to NOAA's 2015 stock assessment of Alaska harbor seals.

NOAA scientists have found at least some of the Iliamna seals persist in the lake year-round and do not breed with harbor seals in eastern Bristol Bay, Speegle said.

The state of Alaska agrees with the no-listing decision, an Alaska Department of Fish and Game official said.

"Identifying Iliamna Lake seals as a distinct population segment is not appropriate, because the population does not differ substantially from harbor seals in Bristol Bay," Bruce Dale, director of the department's Division of Wildlife Conservation, said in a statement.

Even if the lake seals did qualify as scientifically distinct from the Bristol Bay seals, their population is too healthy to warrant an endangered listing, Dale said.

"No sign of decline has been detected in animals that overwinter in Iliamna Lake, and no threats exist indicating future declines are likely. Regional subsistence hunters report the seals are healthy and more abundant now than in the past," he said in the statement.

But to the Center for Biological Diversity, potential development of the massive Pebble mine is a looming threat.

"The Pebble Project would pose a catastrophic threat to Iliamna Lake seal, as well as other wildlife and the people that depend on the fragile Bristol Bay ecosystem for their livelihoods," the center said in its 2012 petition.

Dune Lankard, the Alaska representative for the organization, said the group was disappointed by the decision, but didn't say whether it would take additional action. In other cases where federal agencies declined to list species or subspecies as endangered, the organization has gone to court.

"Alaska's Iliamna Lake is home to the country's only population of freshwater seals and the world's largest sockeye salmon run," Lankard said in a prepared statement. "The government should safeguard this important ecosystem rather than clearing the way for mining interests to industrialize it. Protecting thriving wild salmon watersheds should be an economic and environmental priority under any administration."

The Iliamna seals — which make up one of only five populations of freshwater seals in the Northern Hemisphere — have themselves been shrouded in some mystery, according to Fish and Game and other agencies.

Some research indicates the seals spend unusually long periods under lake iceusing air pockets to breathe.

Even though endangered protection is not forthcoming, the seals will continue to be safeguarded under the Marine Mammal Protection Act, Speegle said.

This story first appeared in Alaska Dispatch News and is reprinted here with permission.