A New Apex Predator In Local Waters: Dusky Sharks Another Reason For Boaters, Swimmers To Be ‘Shark Smart’

NORTH CHATHAM – In the summer of 2023, the public beaches near Great Point, Nantucket, were closed to swimmers in early July because so many people were reporting seeing sharks feeding on seals there. Everyone assumed they were white sharks. They weren’t.

 “We had some photos sent to us through our Sharktivity app,” said Megan Winton, senior scientist at the Atlantic White Shark Conservancy. “Some people who were on the beach sent in video footage. And we took a look and realized, these aren’t white sharks.”

 They suspected that the reports might involve dusky sharks, a smaller species known to feed on fish and relatively small prey. For confirmation, researchers from the Conservancy sent a team to the area to monitor the waters with a drone. Before long, the team captured footage of several dusky sharks circling a seal. The seal makes a sprint for shore but is too slow, and a dusky shark begins eating it. 

 “Another dusky shark comes into the frame and actually steals part of the kill,” Winton said.

 “This was something that was completely new to shark science,” she said. “Up until these observations, we didn’t know that dusky sharks fed on seals.”

 The behavior likely isn’t new; it probably remained undiscovered because, as in the case with white sharks, there weren’t enough seals around for people to witness predations. For many years, government bounties on seals kept their populations low. Following passage of the Marine Mammal Protection Act, seal numbers recovered, bringing a resurgence in the sharks that hunt them.

 “This is another story that seems to be a shark species resuming an important predator-prey relationship with seals that we weren’t aware of because the seal population was knocked down for so long,” Winton said. “They seem to be making a comeback.”

 Confirming the discovery was shark expert John Chisholm, adjunct scientist at the New England Aquarium, who investigates all of the shark sightings reported through the Sharktivity app.

 “He was the first one to key in on this and kind of set all of this in motion,” Winton said. “But none of that would have happened without people sending in their sightings and sending in videos.”

 From the perspective of the beach or a boat, it’s hard to tell dusky sharks from great whites, Chisholm said.

 “White sharks get much larger and bulkier than dusky sharks,” he said. White sharks have a tail that has upper and lower lobes that are equal in size, while duskies have an upper lobe that is much longer. “Of course, most people will only ever see a dorsal fin breaking the surface, which can make it difficult to identify since both species have similar shaped dorsal fins,” he said.

 For swimmers and boaters, identifying the species isn’t particularly important anyway.

 “Pepole should treat both species with the same respect,” Chisholm said. “As we say, be ‘Shark Smart’ and follow shark safety guidelines when going in the water where these species occur.” 

 Dusky sharks have been spotted off Monomoy Island and as far north as Nauset Beach, Chisholm said, but not much farther north.

 “Cape Cod is known to be historically the northernmost extent of their range, so this is something that’s still very new, but that we’ll be investigating more in the years to come,” Winton said. Will they overtake white sharks in our region? Not likely, experts say.

 “We have not seen an appreciable increase in duskies along the Outer Cape,” state shark biologist Greg Skomal said. “I think the competition with white sharks is a losing battle for them, and so this seems to be isolated to Nantucket, particularly the eastern end of Nantucket.

 “But it’s something we’re going to continue to look at, because clearly there’s a public safety issue here with this species as well,” Skomal said.

 To reduce the risk of a shark interaction, swimmers are advised to stay close to shore, within reach of help in case of an emergency. Avoid swimming near seals and schools of fish, and avoid going into the water at dawn, dusk or nighttime, when visibility in the water is reduced. Swim in groups and avoid excessive splashing, and follow beach rules. For shark alerts, download the Sharktivity app. The best advice for safety around great whites also applies to duskies.

 “They come right up to shore the same way the white sharks do hunting for seals,” Winton said. “But as far as public safety messaging goes, it doesn’t really change it. It’s still a large shark coming in close to the beach to feed on seals.”