The marine monument, a chain of islands and atolls amid more than 160 seamounts, is a trove of marine biodiversity. Environmentalists said opening the area to commercial fishing would pose a serious threat to the area’s fragile ecosystems. Mr. Trump, accompanied in the Oval Office by a fisherman from American Samoa and Aumua Amata Coleman Radewagen, the territory’s delegate to the House of Representatives, said his predecessors had deprived Pacific island communities of “fertile grounds.” “It’s so horrible and so stupid,” Mr. Trump said. “You’re talking about a massive ocean and they’re forced to go and travel four to seven days to go and fish in an area that’s not as good.”

The Trump administration is opening a national marine monument to commercial fishing, it announced Thursday. The White House issued an executive order that allows for commercial fishing within the Pacific Remote Islands Marine National Monument, a nearly 500,000 square mile area — double the size of the state of Texas — in the central Pacific Ocean. The monument, first established by former President George W. Bush and later expanded by former President Obama, contains coral reef and island habitats. Thousands of species can be found there, including coral reefs, sea turtles, whales, dolphins, sharks and manta rays. In a separate executive order, President Trump indicated more monuments could similarly be opened to commercial fishing in the future.

President Donald Trump on Thursday issued a proclamation saying he is easing federal restrictions on commercial fishing in a vast protected area of the central Pacific known as the Pacific Islands Heritage Marine National Monument. Trump said he will allow U.S.-flagged vessels to fish within 50 to 200 nautical miles of the landward boundaries of the monument, which covers some 490,000 square miles of open ocean, coral reef and island habitats. Located south and west of Hawaii, the area is home to seven national wildlife refuges. It includes some of the Earth’s last pristine maritime environments, serving as a sanctuary for species such as endangered sea turtles, sharks and migratory birds, according to marine wildlife experts.