“This was an incredibly cool discovery, because this is a global species, but nobody throughout their global range knew they actively were able to hunt for and feed on seals,” Winton said.

Dusky sharks also feed on fish and other, smaller sharks, according to Winton.

But because their prey can be “human-sized,” dusky shark bites can be deadly to beachgoers, said John Chisholm, a scientist with the New England Aquarium’s Anderson Cabot Center for Ocean Life.

“Be shark smart. This is a large shark hunting human-sized prey in an area where people are swimming or surfing, you got to pay attention to your surroundings,” said Chisholm, who co-authored a 2025 report on dusky shark activity published by the Atlantic White Shark Conservancy.

The first confirmed instance of a dusky shark preying on a seal off southeast Cape Cod was reported in 2023, and researchers have been closely monitoring the species’ activity in the area since then, Winton said.

“We’re starting to see dusky sharks take advantage of [seals] and it’s been going on for a few years,” Chisholm said.

Winton said the dusky sharks she’s seen off Nantucket are 8- to 10-feet long, although the species can reach 12 feet in length.

Dozens of sandbar and dusky sharks gathered off the coast of northern Israel on Dec. 30, 2022.JACK GUEZ/AFP via Getty Images

Dusky sharks aren’t new to Cape Cod — or to the wider region.

The species was found in waters up and down the East Coast for centuries until they were heavily commercially fished in the 1900s, Chisholm said.

Dusky shark numbers declined “precipitously,” when the species was hunted for their long, sloped tail fins in the 1970s and ’80s, said Gavin Naylor, director of the Florida Program for Shark Research at the state’s natural history museum in Gainesville.

“Their numbers have been so depressed we haven’t seen them, but now their numbers are coming back,” said Naylor, adding that it’s “taken 40 years for them to recover.”

It took environmental protection legislation to help the sharks rebound, starting in the 2000s, experts said. The seals that the sharks rely on for food also recovered from over-hunting after the passage of Marine Mammal Protection Act in 1972, according to Chisholm.

Now, gray seal populations are growing in areas “where they used to be found,” and scientists are “seeing an overlap where the dusky sharks and the seals are occupying the same area off southeastern Cape Cod,” Chisholm said.

Because dusky sharks prefer warmer waters, they could be particularly active off Cape Cod this summer, according to Chisholm.

The sharks recovered in the Florida region long before the current resurgence in the northeast.

As the species began to rebound a couple decades ago, more sexually mature dusky sharks eventually made their way north to Cape Cod, Naylor said.

Like great white sharks, dusky sharks can cause severe injury or death to humans because they are so large, he said.

“If irritated, they could bite your arm off,” he added.

One of the most recent dusky shark attack fatalities took place off the coast of Israel in April 2025, Chisholm said.

Besides that incident, there have been only a handful of documented dusky shark attacks in recent decades, Naylor said.

Great white shark bites have caused more fatalities over the years, according to Chisholm.

This summer, dusky sharks, seals, and humans will all be in the water together off Cape Cod, Chisholm said, so beachgoers must take precautions.

“People have to respect the fact that the sharks are hunting there,” Chisholm said.

His best advice?

“Don’t swim with the bait.”

That means, if you see seals or even large schools of fish in the water, stay calm and return to the sand immediately, Chisholm said.

Dusky and great white sharks are known to swim close to shore in pursuit of seals.

“The seals are right there in the surf zone,” he said, “and that’s what they’re looking for, so they get that close.”

Claire Thornton can be reached at claire.thornton@globe.com. Follow Claire on X @claire_thornto.