CRAWFORD, Texas — President Bush today proposed protecting three remote island chains, launching a marine conservation effort that could be one of the largest in history.

Bush is considering conserving parts of the northern Mariana Islands, Rose Atoll in American Samoa and various islands and reefs in the Central Pacific, including Palmyra Atoll, Kingman Reef, Johnston Atoll, Jarvis Island, Howland Island, Baker Island, and Wake Island.

The president signaled his intentions in a memo to members of his Cabinet and is now awaiting their advice on how to provide additional protection to the island chains. The archipelagos are home to a diverse array of fish, birds and other marine species that are rapidly vanishing elsewhere in the world, the memo said.

"These areas are host to some of the world's most bio-diverse coral reefs and habitat, and some of the most interesting and compelling geological formations in all of our oceans," White House spokesman Tony Fratto told reporters in Crawford, Texas, where Bush is on vacation at his ranch.

The president has a range of ways to order environmental protection, with varying restrictions on development and fishing. In his memo, Bush did not indicate what type of protection would be offered for the three marine habitats under consideration.

Bush also made clear that he would protect the rights of the Department of Defense, which has active bases on two of the islands.

The conservation effort could be completed before Bush leaves office.

Environmentalists responded to the announcement as a hopeful sign.

While an assortment of activities including commercial fishing, oil and gas exploration and deep sea mining could be allowed, barring extractive activities "would be one of the most significant environmental achievements" of any president, said Joshua S. Reichert, managing director of the Pew Environment Group.

In June 2006, Bush designated a huge swath of the Northwestern Islands as a national marine monument. It is the largest conservation area in the world.