The Purple Flag: What It Means and What Happens Next

Visitors arriving on the Outer Cape for the first time see a flag they do not recognise and guess at its meaning. It is worth knowing before you are standing in front of it with your shoes in your hand.

A purple flag carrying a white shark silhouette is the Cape's shark warning. It flies at beaches where lifeguards are on duty and alerts swimmers to shark presence or to conditions warranting caution in the area.

When a shark is sighted at a lifeguarded beach, the water is temporarily closed to swimming and surfing. The lifeguards will say when people may go back in. The park does not publish a fixed closure duration, so do not plan around a number you have read somewhere; wait for the call.

At an unguarded beach, nobody closes anything. Outside the staffed season, which runs from roughly the third week of June to Labor Day, the signage at the entrance is the entire warning system.

The standing guidance from the National Park Service is short and specific. Sharks hunt seals in shallow water here, so stay close to shore where rescuers can reach you. Do not isolate yourself; swim, surf, paddle and kayak in groups. Keep well away from seals and from schools of fish. Avoid murky water and limit splashing. Follow every sign and every flag.

To report a sighting where there are no lifeguards, the Seashore publishes ranger station numbers, 508-255-2112 and 508-487-2100, and directs anyone facing an emergency to call 911. Emergency call boxes are installed at beach entrances; find yours before you go in the water.

The Sharktivity app, run by the Atlantic White Shark Conservancy with the state Division of Marine Fisheries, the National Seashore and town officials, maps sightings and tag detections, with public reports verified by the New England Aquarium. Its own disclaimer is worth quoting in substance: the app does not tell you which beaches are safe, and the only way to rule out an encounter entirely is to stay on shore.

Confirm all of the above against the park's current guidance, which is revised between seasons.