NANTUCKET, Mass. — The National Oceanic and Atmospheric Administration says it is placing a voluntary vessel speed restriction southwest of Nantucket to protect a group of endangered right whales in the area.

The agency says six of the whales were seen in the area 11 nautical miles southwest of Nantucket on Tuesday. It’s asking mariners to route around the area entirely or pass through it at 10 knots or less.

North Atlantic right whales are some of the most endangered marine mammals on the planet. There are about 450 of them left, and they have been dogged by high mortality and poor reproduction over the past year.

The voluntary speed restriction is in effect until April 4.