The U.S. National Oceanic and Atmospheric Administration (NOAA) set a 10-knot speed limit Wednesday to reducing ship strikes of whales.

Ships 65 feet or longer that travel within 20 nautical miles of major American Atlantic ports, and in areas where the North Atlantic right whale breed, feed and migrate will have to observe the speed limit starting in December.

Only 300 to 400 North Atlantic right whales still exist, NOAA said, and are highly vulnerable to ship collisions, since their migration route crosses major East Coast shipping lanes.

“Along with existing measures to prevent entanglement of right whales in fishing gear, the new ship strike reduction rule is the most comprehensive approach that NOAA has taken in its effort to help right whales recover,” the administration said in a written statement.