NANTUCKET, Mass. (AP) — Federal ocean managers are extending a voluntary speed restriction zone off of Massachusetts to try to help protect endangered right whales.

The National Oceanic and Atmospheric Administration established the zone to protect a group of eight North Atlantic right whales spotted 20 nautical miles south of Nantucket on March 29. The agency says the zone will be in effect through April 12.

NOAA asks mariners to route around the area altogether or transit through it at 10 knots or less.

The right whales are one of the most endangered marine mammals in the world. They number no more than 450 and have suffered from high mortality in the past year. The whales are vulnerable to ship strikes and entanglement in fishing gear.