A North Atlantic right whale feeds on the surface of Cape Cod bay off the coast of Plymouth, Massachusetts, March 28, 2018. Credit: Michael Dwyer | AP

The federal government is again extending the use of a protected zone near Massachusetts to help rare North Atlantic right whales travel safely in the area.

The National Oceanic and Atmospheric Administration established the zone south of Nantucket to protect a group of right whales. The agency said the zone will remain in effect through April 13. Six of the whales were seen in the area on March 28.

The agency has asked mariners to travel around the area or transit through at 10 knots or less. The right whales are among the rarest large whales, as their populations were decimated generations ago during the whaling era.

The whales are vulnerable to ship strikes and entanglement in fishing gear. Slightly more than 400 of them remain.