With three North Atlantic right whales seen south of Martha’s Vineyard this week, mariners are advised to be on the lookout for the critically endangered animals.

NOAA Fisheries announced Thursday that a voluntary vessel speed restriction zone has been established in the area after the whales were seen on 19 nautical miles south of the Vineyard on April 24. Mariners are asked to avoid the area or transit at 10 knots or less.

The restriction zone is in effect until May 9. Voluntary and mandatory speed restrictions are also in effect in other areas.

North Atlantic right whales feed in the waters off New England in the late winter and early spring. Researchers estimate that there are about 433 right whales in existence. With recent whale deaths from fishing gear entanglement and a low birth rate, scientists say the species could go extinct in the next 20 years.

In the past few weeks dozens of whales have been seen around New England, from waters east of Boston to Cape Cod Bay. An aerial survey conducted by the Center for Coastal Studies in Provincetown on April 22 showed more than 100 right whales in the bay, which is nearly a quarter of the entire population.

The Coast Guard has also issued a notice advising mariners to use caution because of whale sightings.

The whales are surface feeders, which makes it hard to see what whales as they are feeding on plankton and renders them especially vulnerable to ship strikes.

Right whales are protected by the Endangered Species Act, and it is illegal to come within 500 yards of a whale by vessel, aircraft (including drones), or any other means.