Hawaii conservationists Tuesday welcomed a proposal by President Barack Obama to create the world’s largest marine sanctuary in the Central Pacific, calling it a significant step toward protecting diverse habitats and preventing large-scale overfishing.

But fishermen said such an ocean preserve would threaten livelihoods.

"Commercial fishermen have already been regulated up the nose," said Mc­Grew Rice, a Kona charter fisherman and member of the Western Pacific Regional Fishery Management Council. "This is going to put people out of business."

Speaking at an oceans conference in Washington on Tuesday, Obama announced that he intends to expand the Pacific Remote Islands Marine National Monument to nearly 782,000 square miles from almost 87,000 square miles.

The monument, established in 2008 by President George W. Bush, currently protects 50 square miles around each of seven small, uninhabited islands: Howland, Baker and Jarvis islands; Johnston, Wake and Palmyra atolls; and Kingman Reef.

Under Obama’s proposal the boundaries would be expanded to the 200-mile extent of the U.S. Exclusive Economic Zone, an action that would increase the monument’s size nearly tenfold and more than double the total amount of the world’s marine sanctuaries.

"Growing up in Hawaii, I learned early to appreciate the beauty and power of the ocean," Obama said in a video at the conference. "And like Presidents Clinton and Bush before me, I’m going to use my authority as president to protect some of our most precious marine landscapes, just like we do for mountains and rivers and forests."

Obama said he would use his executive authority under the Antiquities Act of 1906 to create the expanded preserve, a move that allows him to establish new environmental protections without action from Congress. He used the same authority to designate 11 new national monuments on land and protect millions of acres of wilderness.

Rick Gaffney, co-chairman of the Hawaii Fishing & Boating Association, applauded the president’s move, saying overfishing has created a dramatic decline in fish stocks across the Pacific. Too many species, he said, face an onslaught of fishing fleet vessels with no opportunity to breed and reproduce.

A sizable ocean sanctuary will give tuna and striped marlin — species under significant pressure — a place to recover, Gaffney said.

"Oceans are the breadbasket for the rest of the world," said Timothy Johns, chairman of the Northwestern Hawaiian Islands Coral Reef Ecosystem Reserve Council. "The oceans are tied to the health of the planet. The more oceans protected, the better."

At the same time, Johns said he’ll be interested to see how the sanctuary is implemented and how it will affect commercial fishing.

"Putting it on paper is admirable," he said. "But it’s the first step, not the end of the game. You need the resources, too."

Paul Dalzell, Western Pacific Regional Fishery Management Council senior scientist, said there is dubious benefit to banning fishing on the open water and that the monument’s current 50-mile off-limit zones offer plenty of protection to coral reefs and nearshore habitats.

Dalzell said the expanded conservation zones will have a major adverse effect on the bottom lines of U.S. fishermen, who have been fishing around Palmyra Atoll for decades, as well as on a sizable purse-seine fishing fleet that has scored major hauls of yellowfin tuna between Howland and Baker islands.

Previous fishing closures across large areas of open ocean have proved to have had no effect on tuna mortality elsewhere, he said. The reason is because tuna move across boundaries.

"This (sanctuary) just seems like something that is more for show than anything else," Dalzell said. "There is absolutely no reason to do that."

Edwin Ebisui, an attorney and part-time commercial fisherman, said he couldn’t support an expanded sanctuary if it cuts off fishing within its boundaries. He said U.S. fisheries are sustainable and that eliminating it in those areas would only make Hawaii more dependent on imported seafood.

Rice, the Kona fisherman, said he believes Obama is listening to environmentalists without considering all of the consequences. The truth, he said, is that a sanctuary would put fishermen out of business without accomplishing anything.

"They’re having a hard time as it is with all the cheap imported tuna coming in," he said of the tuna fishermen.

Linda Paul, president of the Hawaii Audubon Society, said she supports creation of a large preserve under the jurisdiction of the sanctuaries program of the National Oceanic and Atmospheric Administration.

"Creation of this mega-sanctuary does not mean that all fishing will be banned from the area since the National Marine Sanctuaries Act allows multiple use. It just means that fishing activities must be sustainable," she said.

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The Los Angeles Times and New York Times contributed to this report.