More than 180 coral species are found on some atolls in the Pacific Remote Islands National Monument. (Photo: U.S. Fish and Wildlife Service)

President Obama on Thursday will formally establish the planet's largest no-take sanctuary for sea life, expanding the Pacific Remote Islands National Marine Monument (PRIMNM) to encompass more than 490,000 square miles. That's six times its current size and nearly three times larger than California.

Spanning a swath of Pacific Ocean south of Hawaii, PRIMNM includes seven atolls and small islands that host a wealth of wildlife. It's home to corals, fish, shellfish, marine mammals, seabirds, land birds, insects and plants found nowhere else on Earth. Many threatened, endangered and depleted species thrive there, including green and hawksbill sea turtles, pearl oysters, giant clams, reef sharks, coconut crabs, groupers, humphead and Napoleon wrasse, bumphead parrotfish, dolphins and whales.

The 82,000-square-mile preserve was created by President George W. Bush in January 2009, and earlier this year Obama proposed expanding it to nearly 782,000 square miles, the largest size possible. But Hawaiian tuna-fishing fleets balked at that idea, arguing such a vast no-take zone would cause economic hardship. Obama relented, scaling back the expansion from tenfold to sixfold.

It's still a dramatic increase, though, producing "the largest marine reserve in the world that is completely off limits to commercial resource extraction including commercial fishing," according to a White House fact sheet. It's also part of a global surge in marine conservation, including a huge new nature park in New Caledonia, the U.K.'s Chagos Islands marine preserve, a new no-take zone in Kiribati's Phoenix Islands and the 30,000-island Pacific Oceanscape, among others. A key aspect of all these, according to conservationists, is that they're being protected before their problems get out of hand.

Strawn Island, Palmyra Atoll

Palms drape over clear water at Strawn Island, part of Palmyra Atoll National Wildlife Refuge within the PRIMNM. (Photo: USFWS)

"This is a far-sighted action that will preserve for future generations a critically important reserve that can help reverse the decline of the marine environment," says Frances Beinecke, president of the Natural Resources Defense Council, in a statement about the PRIMNM decision. "The President acted expeditiously, while the area is still largely pristine and undisturbed."

The expansion means Obama has conserved more acres of federal land and ocean than any president in at least half a century — a title previously held by Bush. He was able to do this via the Antiquities Act of 1906, which gives presidents the authority to enshrine unique natural and historic features without seeking approval from Congress. First used by Theodore Roosevelt, the law has been used by 16 presidents to protect landmarks ranging from the Grand Canyon to the Statue of Liberty.

Safeguarding such a huge swath of ocean not only sets a good international example, conservationists say, but also illustrates how much U.S. wilderness still remains to be preserved.

"As we celebrate the 50th anniversary of the Wilderness Act this year," the Sierra Club's Dan Chu says in a statement, "the new monument designation for the Pacific Remote Islands is a reminder that there are still wild places in America that urgently need protection."

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.