In a move to protect some of the most pristine coral reefs in the world, the U.S. Fish and Wildlife Service has removed three shipwrecks from Palmyra Atoll and Kingman Reef national wildlife refuges 1,000 miles south of Hono­lulu.

The $5.5 million operation was the first phase of a coral reef restoration project designed to rid the refuges of a growing "black reef" problem, a phenomenon in which a diverse coral reef is transformed into one dominated by a single, invasive species.

Scientific studies fingered the iron in the shipwrecks as the cause of the black reef, fueling the growth of invasive organisms that smothered a large amount of healthy, diverse coral, including about 15 percent of the reef at Palmyra Atoll.

"Central Pacific waters are iron-limited," explained Amanda Pollock, Palmyra Atoll and Kingman Reef refuge manager. "Metal in those waters is like putting time-release fertilizer for whatever organism can use it the best."

In this case, the interlopers are corallimorph coral at Palmyra Atoll and a filamentous algae at Kingman.

The next step, Pollock said, is to halt the progression of black reef by removing the invaders, a process that will begin this spring and take an estimated five years.

The prognosis is for full recovery in 10 to 15 years.

"Palmyra has always amazed me with its resilience," Pollock said.

More than 176 species of hard corals are found at both Palmyra and Kingman, compared with 65 species in the Hawaiian Islands and 45 species in the Florida Keys.

Global Diving and Salvage of Seattle and Curtin Maritime of Long Beach, Calif., were hired to conduct the removal from November to January, in conjunction with federal agencies and The Nature Conservancy, which owns the largest island at Palmyra and operates a research station there with the Fish and Wildlife Service and the Palmyra Atoll Research Consortium.

The consortium and other conservation groups have been urging the removal of the shipwrecks for years. The shipwrecks were:

» The 618,350-pound Taiwanese longline fishing vessel Hui Feng No. 1, which sank at Palmyra in 1991.

» A 1950s-era Department of Defense pontoon transport barge at Palmyra.

» A suspected Indonesian teak fishing vessel grounded on the Kingman Reef since 2007.

Nearly 1 million pounds of shipwreck debris will be taken to Long Beach for recycling or disposal.

"With this reef restoration project, we are delivering on the conservation promise of these refuges and the monument," Susan White, Pacific Remote Islands Marine National Monument superintendent and project leader, said in a statement.