WASHINGTON, United States (AFP) — Commercial fishing will be allowed in a gigantic marine reserve home to one of the world’s most vulnerable ecosystems in the middle of the Pacific under an executive order signed Thursday by President Donald Trump.

The Pacific Remote Islands Marine National Monument (PRIMNM) was created in 2009 by then-president George W Bush, and extended by his successor Barack Obama in 2014.

Today, it covers almost 1.3 million square kilometres (500,000 square miles) or almost twice the area of Texas, anchored around seven islands and atolls.

Commercial fishing and resource extraction activities, such as undersea mining, have been banned until now, although traditional and sport fishing were still allowed.

Trump said in his executive order Thursday that the prohibition “disadvantages honest United States (US) commercial fishermen” who were “driven… to fish further offshore in international waters to compete against poorly regulated and highly subsidised foreign fleets”.

“Appropriately managed commercial fishing would not put objects of scientific and historic interest within the PRIMNM at risk,” he added.

The PRIMNM’s waters are home to untouched coral reefs and many endangered species, including seabirds and some kinds of whales and sharks.

Among the most untouched tropical environments on the planet, the PRIMNM is especially vulnerable to climate change.

Large swathes of those waters between 50 and 200 nautical miles (93-370 kilometres) from shore will now be partially open to commercial fishing by US-flagged vessels.

Foreign ships could be granted “permits… to transship fish harvested by United States fishermen”, the executive order read.

Trump further ordered the government to “amend or repeal all burdensome regulations that restrict commercial fishing in the PRIMNM”.

The Republican president, an avowed sceptic of climate change, has since his return to the White House taken aim at environmental regulations, pushing to reduce the rules’ impact on business.