The National Oceanic and Atmospheric Administration says it’s applying the voluntary vessel speed restriction zone in an area 21 nautical miles south of Nantucket. The designation is intended to protect a group of four North Atlantic right whales seen in the area on Sunday.

NOAA says the speed restriction zone will be in effect until Dec. 3. Mariners are asked to avoid the area or go through it at 10 knots or less.

The right whales number no more than 440 and have been hurt by heavy mortality and poor reproduction in recent years.