CHATHAM, Mass. — The findings of a landmark population study of great white sharks off Cape Cod indicated that Massachusetts is “one of the largest shark hotspots in the world,” with hundreds upon hundreds of the apex predators visiting local waters in recent years.
A team of scientists from the Atlantic White Shark Conservancy, the University of Massachusetts Dartmouth’s School for Marine Science and Technology, and the Massachusetts Division of Marine Fisheries conducted an intensive mark-recapture survey to estimate the size of the newest white shark hotspot, which represents the first estimate of white shark abundance ever produced in the North Atlantic Ocean.
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Researchers, utilizing the mark-recapture methods, determined that about 800 sharks visited the waters off the Cape from 2015 to 2018, according to a new publication in the journal Marine Ecology Progress Series.
“Mark-recapture methods use repeated surveys of uniquely marked animals to estimate population size. In the case of white sharks, individuals can be distinguished based on unique markings and notches in their dorsal fins,” researchers noted in the study.
The old way of counting sharks was to lure them near a boat with bait — but the method was imperfect because younger sharks didn’t care to congregate with older, bigger sharks.
Megan Winton, lead author of the study, said you can actually identify individual sharks by their pigment patterns and something unique to each animal.
“There are unique patterns of notching on their dorsal fin,” she said. “It’s similar to how fingerprints are unique to every person.”
A total of 339 individual white sharks were identified by experts who combed through nearly 3,000 videos captured over the course of 137 research trips to Cape Cod beaches.
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“The estimate provides an important baseline that can be used to monitor the performance of conservation measures for the broader population, which declined by as much as 80% as fishing pressure increased in the 1970s and 1980s,” said Winton. “Cape Cod is the only area in the region where researchers can feasibly monitor the population, and our estimates suggest that the Cape is among the larger white shark hotspots worldwide, which is good news from a conservation standpoint.”
To get that count, Winton and her co-researchers used a pilot to spot white sharks off the east coast of the Cape.
“And then we essentially snuck up on them,” she said. “We had a GoPro on a pole and we collected underwater footage of them.”
The researchers also found that white shark numbers along Cape Cod peaked in the late summer and early fall, when water temperatures are warmest, and declined as sharks migrate out of the area for the winter.
While the risk of a negative encounter with a shark is low, the increased presence and population of white sharks off Cape Cod underscores the need for ongoing research, public safety initiatives, and education programs in the region, researchers stressed.
“It’s important for people to keep in mind that they are very cautious, selective predators,” Winton said.
But, she added, that doesn’t mean they can’t make a mistake.
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