A little-noticed bid to exempt Puerto Rico from rules requiring imports from the U.S. be on American-built and manned ships could kill the nation's critical shipbuilding industry, including builders of naval warships, according to a new government study.

The Government Accountability Office said giving Puerto Rico the exemption could be followed by similar moves in Hawaii and Alaska which would open the door to cheaper foreign shipping companies and the death of the already beleaguered shipbuilding industry, which relies on commercial contracts since the Navy isn't buying many boats.

At issue is the Jones Act, a 1920 law that aimed to maintain a U.S. shipbuilding base by requiring inter-America shipping to be done on U.S. boats and with U.S. crews, and Puerto Rico's effort to trim shipping costs that inflate the price of goods.

The American Maritime Partnership, which is an advocate of the Jones Act, just wrote the leaders of a House seapower subcommittee to urge that exemptions not be granted, arguing that slightly higher shipping costs on U.S. boats was worth the price of maintaining an American shipping and shipbuilding industry.

They cited GAO findings that "a decline in the number of U.S.-flag vessels would result in the loss of jobs that employ skilled mariners needed to crew the U.S. military reserve and other deep-sea vessels in times of emergency."

The group, made up of shippers and shipbuilding interests, warned in their letter, "DOD and the U.S. Navy heavily rely on commercial mariners, including many from the U.S. domestic fleet, for a variety of critical national security roles," said AMP. "DOD has previously estimated that replacing the commercial maritime industry with military vessels would cost billions of dollars."