Marine Animals

Shark Sightings

Occasionally, sharks are observed from the shoreline of Nantucket beaches. It can be difficult to determine the species of the shark. Often times basking sun fish are observed and thought to be a shark by persons on the beach. However, a shark’s dorsal fin cuts through the water in a straight line; is constantly moving and normally a shark’s tail fin is exposed also. 

Experts say that people can avoid encounters with sharks by staying out of the water between dusk and dawn; staying in groups whenever possible; not swimming in sight of observed seals and not venturing too far from shore. Lifeguards are responsible to educate the beachgoer about safety to avoid encountering shark and other marine mammals. 

If a shark is sighted, lifeguards will take the following action for the safety of the people at the beach: 

  1. Determine the size of the shark. If the shark is less than 4 feet in length (sand shark or dogfish) there is no reason to get people out of the water. 
  2. Assess the shark’s behavior. If the shark is feeding, stalking or has otherwise peculiar behavior, a closure is indicated regardless of size. If a closure is initiated, the closure will be terminated two hours after the last observation of the shark if the shark is inside of the swimming area. If the shark is outside the swimming area, the closure will be terminated one hour after the last observation. 

Once a sighting has been confirmed, the Lifeguards will: 

  1. Contact Police Dispatch on Marine. Advise Police Dispatch which direction the shark is heading; how far off shore it is; and approximate size if known. 
  2. Clear the water on the protected beach and fly a double red flag. 
  3. Proceed to adjacent unprotected beaches and advise the beachgoers and swimmers on those beaches.
  4. Keep the water clear for one hour if outside the safe swim zone and two hours if inside the zone. 
  5. Check availability of an aerial marine survey team to complete a flyover of the area. 
  6. Contact Police dispatch once the closure has been terminated.

Only confirmed sightings warrant a closure of the beach to water activities. 

Only sightings from reliable sources (Lifeguards, ATV operators, Nantucket Police, Nantucket Fire or sources deemed reliable by Marine Division Supervisors) will be considered verified and confirmed. 


Nantucket is home to four species of seals. Harbor seals and gray seals are the most familiar and abundant species and can be seen in our beaches throughout the whole year. During the winter and spring months, two other species visit Nantucket waters, the harp and hooded seals. 

It is important to remember that seals are protected under federal law by the Marine Mammal Protection Act (MMPA). It is against the law to touch, feed, or harass seals. Harassment occurs when your behavior changes a seal's behavior.

How do you know when your presence is affecting a seal's behavior? 

Seals start moving back into the water; they keep looking at you; they lift their heads to watch you; stretch; wave; and yawn. 

To prevent stressful situations for seals:

  • Stay 150 FEET away from resting seals.
  • Be QUIET.
  • Kayaks, canoes and jet skis AVOID areas near seal haulouts to minimize potential collisions.