The US Merchant Marine Academy (USMMA) plans to mark 75 years in service with an important message to the entire nation.

“This isn’t just about looking back over the last 75 years,” says Captain James Tobin, president of the USMMA Alumni Association and Foundation.

“It’s to remind the nation how vital this institution is to both our national security and global commerce, and of the importance of maritime security in turbulent times.”

The Kings Point, New York, school says its 75th year arrives amid policymaker concern over China’s spending on merchant marine as the US faces a shortfall of mariners ready to serve in times of crisis.

"Without a force of fully trained merchant mariners with a mandated obligation to the US government to serve where and when needed, the United States would be forced to outsource carrying troops and materials to foreign mariners, whose national or personal interests may be at odds with US policy," the academy said.

USMMA graduates serve an eight-year armed forces reserve obligation that includes five years as officers in the merchant marine or, upon approval, serving in active military duty.

In delivering the school's 16 June commencement address, US Secretary of Defense James Mattis said the merchant marine is "absolutely essential", despite its small size today.

“It’s in every war plan I review, I guarantee you, because you’re going to be the fourth arm of defence," he added.