The National Oceanic and Atmospheric Administration is asking boaters off the southern New England coast to slow down because of the presence of rare whales.

The federal agency says endangered right whales were spotted in the area July 16. The agency is extending a voluntary speed restriction order, asking mariners to avoid the area south of Nantucket, Massachusetts, or proceed at 10 knots or less until July 30.

North Atlantic right whales are one of the most endangered marine mammals in the world. The animals are vulnerable to injury and death from vessel strikes.