ByMelanie Haiken
Published July 5, 2023
• 8 min read
It wasn’t long ago that swimmers splashed and surfers paddled along the beaches of Cape Cod with little thought that a great white shark might be on the hunt just feet away.
But today, that reality is settling in among the popular seashore towns as white sharks return to an area they disappeared from decades ago. And as communities search for solutions to keep beaches safe, one question has dominated: Just how many white sharks are out there?
That number has been hard to come by, as counting the elusive and wide-ranging predators is complicated. But researchers with the Atlantic White Shark Conservancy finally have an answer, based on an innovative combination of acoustic tracking, photographic identification, and statistical modeling.
The bombshell: Some 800 sharks—and possibly up to 900—swam the waters of Cape Cod between 2015 and 2018. In comparison, a similar white shark estimate for California’s central coast is 300, while the population for South Africa’s Dyer Island, known as Shark Alley, is thought to number between 800 and a thousand.
Cape Cod has “potentially the highest density of sharks in the world,” says Megan Winton, a fisheries scientist whose data is still in the pre-publication stage.
“We knew there were a lot more sharks out there, but this is the first time we’ve ever had a number for any portion of the species’ range in the North Atlantic, which is huge,” Winton says.
The findings are striking not only because of the number of sharks, but the fact that they’re concentrated along just 560 miles of protected coastline.
The four years of tracking also revealed the sharks, mostly adults between eight and 12 feet long, spend approximately half their time in 15 feet of water or less.
“People might know that white sharks come here, but they think they’re far offshore,” Winton says.
“We’ve seen sharks as big as 15 feet long in just four to five feet of water. And it’s incredible how camouflaged they can be. People might be right next to them and don’t even see them.”
Knowing the risk
White sharks are rebounding in Cape Cod for one simple reason: Their favorite prey, the gray seal, is back on the menu. Hunted nearly to extinction, the seal population began rebounding with the passage of the U.S. Marine Mammal Protection Act in 1972.
Today, the seal population tops 50,000. It took longer for the white sharks, considered vulnerable by the International Union for Conservation of Nature, to follow, but since receiving national protection in 1997 and Massachusetts state protection in 2005, their numbers have steadily grown off the eastern U.S.
“You really can’t think of any other location where white sharks attempting to feed on seals overlap with human activity,” says Greg Skomal, senior fisheries scientist with the Massachusetts Division of Marine Fisheries and a co-author on the forthcoming paper. (Read how to be safe while swimming in shark habitat.)
The risk of a shark bite is very small, Skomal emphasizes; swimmers are much more likely to drown. That said, five sharks have bitten people in Cape Cod since 2012, including a fatal incident in 2018, when a person was killed while bodyboarding in Wellfleet.
‘Game-changing’ science
To reduce the likelihood of shark-human encounters, scientists need to know where the sharks go and when.
To find out, the Cape Cod scientists created a catalog, or logbook, of individual sharks identified both through tagging and photo documentation of their coloration patterns and dorsal fin profiles between 2015 and 2018.
The team then conducted a three-year survey that compared the number of newly recorded individuals with those previously documented, reconstructing these encounters through statistical modeling to create a population estimate. Unlike previous surveys in South Africa, California, and elsewhere, Winton’s model considered the movements of individual sharks.
The older models assumed “all individuals act and use these areas in the same way, which can impact how good the resulting estimates are,” Winton says. “We created a new model that allowed sharks to move into and out of the area and accounted for where individual sharks like to ‘hang out’ along the coast.”
The advent of portable, high-quality underwater cameras, which the Cape Cod team used has also made identifying individual sharks easier and more accurate, says Taylor Chapple, an assistant professor at Oregon State University who wasn’t involved in the study.
“This type of study is really game-changing for species like white sharks because we can identify a really huge portion of the population, which gives us really strong confidence that our numbers are right,” says Chapple, who has studied white shark populations in California. White sharks typically live up to 70 years old.
In addition, researchers with the Cape Cod-based Center for Coastal Studies are using sonar surveys to map shark movements, which has revealed Cape Cod sharks have a hunting strategy completely unique to the region. (Learn why great whites are still a mystery to us.)
White sharks are ambush hunters, typically stalking their prey in deep water and lunging into the air to take an unsuspecting animal by surprise.
But along Cape Cod’s sandbar-lined coastline, the sharks are forced to hunt in shallow water. They do this, the scientists found, by patrolling a trough between the sandbars until a hungry seal ventures into the water to eat.
Understanding this unusual behavior will help experts predict the sharks’ movements and identify areas particularly dangerous to swimmers.
Improving public safety
Shark-detection systems are also giving scientists and the public a better sense of the predators’ activity, with the goal of improving public safety.
Since 2009, Cape Cod researchers have tagged a total of 303 sharks outfitted with acoustical transmitters. Five acoustical monitors on outer Cape beaches can detect sound pulses when a tagged shark goes by, sending this information in real time to lifeguards, beach managers, scientists, and the public via the Sharktivity app.
In 2022, the conservancy logged 193,475 shark detections. It’s not exact—one shark circling the vicinity of a monitor might set it off over and over—but it does provide a general idea, Winton says. (Read why beach warning signs are often ignored by Cape Cod swimmers.)
“Some of the lifeguards have told us the alert system has really changed how they think about the sharks,” Winton says. “They say they used to think they came by every once in awhile but were mostly offshore. Now they realize that in the summer and fall they’re here pretty much all the time.”
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