There were lots of snickers when a Chinese-Canadian man was caught trying to leave the United States with 51 turtles hidden in his sweatpants, but the case illustrated the serious threat facing native species from the booming international turtle trade, federal scientists said on Thursday.

To protect native species, the U.S. Fish and Wildlife Service proposed a rule on Wednesday that would put four kinds of freshwater turtle under the protection of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).

The common snapping turtle, the Florida softshell turtle, the smooth softshell turtle and the spiny softshell turtle, none of which are endangered yet, would be subject to a CITES rule that would let federal authorities begin collecting export data to determine whether limits on harvesters are warranted.