Washington ? President Bush plans today to designate an island chain spanning nearly 1,400 miles of the Pacific northwest of Hawaii as a national monument, creating the largest protected marine reserve in the world, according to sources familiar with the plan.

Establishing the Northwestern Hawaiian Islands as a strictly protected marine reserve, which Bush is slated to announce this afternoon, could prove to be the administration’s most enduring environmental legacy. The roughly 100-mile-wide area encompasses a string of uninhabited islands that support more than 7,000 marine species, at least a fourth of which are found nowhere else on Earth.

The islands include almost 70 percent of the nation’s tropical, shallow-water coral reefs, a rookery for 14 million seabirds, and the last refuge for the endangered Hawaiian monk seal and the threatened green sea turtle. The area also has an abundance of large predatory fish at a time when 90 percent of such species have disappeared from the world’s oceans.