The federal government today will significantly expand the critical habitat for endangered Hawaiian monk seals to include beaches and waters of the main Hawaiian Islands, officials said.

Environmentalists say the added habitat is needed to reverse the plight of the monk seals, which are among the most endangered marine mammals in the world with fewer than 1,200 remaining and their numbers declining.

Previously, the critical habitat was limited to the remote and largely uninhabited Northwestern Hawaiian Islands, where it was first established in 1986.

The enhanced designation means greater protection of seal habitat under the Endangered Species Act. It does not affect or restrict access to Hawai'i's beaches or fishing. But the designation does limit federal government activities, which must undergo reviews to ensure they do not harm the seal or the habitat.

The announcement, to be published today in the Federal Register, is a result to a petition filed a year ago by the Center for Biological Diversity, Kahea: The Hawaiian-Environmental Alliance, and Ocean Conservancy.

"It will give this species a fighting chance," said Vicki Cornish, wildlife policy director at Ocean Conservancy. "When we protect critical habitat for monk seals, we are also protecting the larger ocean ecosystem on which we all depend."

Miyoko Sakashita, an attorney with the Center for Biological Diversity, said the seals are "teetering on the edge of extinction ... and the forces are against them with rising sea levels flooding their beaches, derelict fishing gear entangling them, and foraging grounds running dry."

She called the expansion of the protection to the main islands "essential" for the recovery of the seals.

Environmental groups point to the Caribbean monk seal as proof as to what could happen to their Hawaiian relatives.

A year ago, the National Oceanic Atmospheric Administration's Fisheries Service confirmed Caribbean monk seals were extinct. They were first discovered during Christopher Columbus' second voyage in 1494 and once had a population of more than 250,000. But they became easy game for hunters because they often rested, gave birth or nursed their pups on beaches. The last confirmed sighting was in 1952.

The Hawaiian monk seal population is declining at a rate of about 4 percent annually, according to NOAA. The agency predicts the population could fall below 1,000 in the next three to four years, placing the mammal among the world's most endangered marine species.

When the numbers of any species fall to such small numbers, the population gets unstable and are more vulnerable to threats like disease.

"We cannot afford the extinction of a creature so sacred in Hawaiian culture and endemic to these islands," said Marti Townsend, Kahea's program director. "And we cannot expect to save the seals without meaningfully protecting critical habitat."

Most seals live in the Northwestern Hawaiian Islands, where they are struggling. Seal pups have only about a one-in-five chance of surviving to adulthood.

About 80 to 100 seals live in the main Hawaiian Islands, where they have higher survival rates. Scientists believe the main islands provide better foraging conditions because there are fewer seals and less competition for prey. Monk seals prefer small eels, wrasses and other prey not commonly sought by humans. Additionally, habitat in the main islands will provide a refuge for monk seals as important beaches where seal pups are born and raised have been lost in the Northwestern Hawaiian Islands due to sea-level rise and erosion.