Cape Cod may have the highest density of great white sharks in the world
“It’s incredible how camouflaged they can be. People might be right next to them and don’t even see them,” one expert says.
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.
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.
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.
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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Great white sharks may change their color to sneak up on prey
First-of-their-kind experiments suggest the world’s largest predatory fish can switch between dark and light gray in a matter of hours.
As the largest predatory fish on Earth, great white sharks are already impressive, armed with up to 300 serrated teeth and weighing up to 5,000 pounds. Now, new research adds more intrigue to the oceanic beasts, suggesting that the animals can change color—perhaps as a camouflage strategy to sneak up on prey.
In new experiments off South Africa, researchers dragged a seal decoy behind a boat to entice several sharks to leap out of the water near a specially designed color board with white, gray, and black panels. The team photographed the sharks each time they jumped, repeating the experiment throughout the day.
One shark, easily identifiable because of an abscess on its jaw, appeared as both dark gray and much lighter gray at different times of day. The scientists verified this using computer software to correct for variables such as weather, light levels, and camera settings.
The researchers then humanely extracted a small piece of tissue from one of the sharks and hurried it back to a lab, where they treated it with several different types of hormones naturally occurring in sharks.
Using a time-lapse camera and a confocal microscope, the researchers watched in awe as the great white’s melanocytes—skin cells that contain pigment—contracted and turned lighter in color when doused in adrenaline. At the same time, another hormone known as MSH, or melanocyte-stimulating hormone, caused the same cells to disperse, resulting in a darker skin color. (Read about a deep-sea shark that’s one of the biggest glowing animals on Earth.)
“We wanted to trick these shark cells into thinking they were getting some kind of stimulus, like the sun or an emotional stimulus [such as seeing potential prey] to see if we could get them to change and become lighter or darker,” says Gibbs Kuguru, a shark scientist at Wageningen University and Research in the Netherlands.
“So we tested it, and not only did it work, but it was a swinging success,” says Kuguru, who is also a National Geographic Explorer and 2022 recipient of the National Geographic Wayfinder Award.
With data from a limited number of sharks, the scientists caution that the great white’s ability to alter its appearance is not yet validated, and that their research has not been published in a scientific journal. But other experts say the possibilities are tantalizing.
Unraveling secrets of the great white shark
Most of the focus on the great white in recent decades, understandably, has been on its charisma, says Ryan Johnson, a shark biologist at the Blue Wilderness Shark Research Unit in South Africa and Kuguru’s research partner.
“Their speed, their power, their size, their ability to overwhelm prey,” he says. “What excited me about this research was that we wanted to look into something incredibly subtle and microscopic.”
Anecdotally, Johnson and other scientists have noticed that great whites appear to alter the hue of their top half, or dorsal side.
This is different from countershading, which is a well-known camouflage strategy of many marine predators in which their top halves are naturally darker and their bottom halves are lighter. Countershading evolved to help predators remain inconspicuous from above and below by mimicking both the dark of the depths and the sunlight of the water’s surface.
But there’s nothing in the scientific literature suggesting that great whites can change color, which has motivated Johnson and Kuguru to keep studying the phenomenon. (Learn why great whites are still a mystery to us.
The hope is that by analyzing a larger data set, the scientists will not only be able to verify that the color change they have documented is more than a fluke, but also identify a pattern of when and why the animals go into camo-mode.
“A pretty exciting finding”
“From a publication standpoint, I don’t think anyone has tried to take on great white shark coloration like this,” says Michelle Jewell, who studies great white shark behavior at Michigan State University Museums and was not affiliated with the research.
“From personal experience, though, we definitely do notice changes in their color. But usually those changes have happened over a series of days.”
According to Jewell, the leading hypothesis for such changes is that the sharks were getting tans after spending more time in shallow water where the sun’s rays are stronger.
“We didn’t consider that this could potentially be something that they themselves are manipulating to get darker or to get lighter,” says Jewell. “But it would make a lot of evolutionary sense.”
George Probst, a photographer who has contributed to the Guadalupe Island White Shark ID Project in Mexico, says he has never noticed color changes in his work.
However, given how the extremely large animals can appear out of nowhere, even when water visibility is exceptional, it would make sense that great whites have evolved to enhance their countershading camouflage.
“It wouldn’t surprise me that they can do that just because of how freaking good they are at sneaking up on you,” says Probst, who has spent hundreds of hours in the water with more than 200 great whites. “They’re ambush predators that rely on stealth.” (See a video of a giant great white feeding on a sperm whale.)
“We know many species of fish are capable of changing their color tones,” says Skomal. “So I think it’s a pretty exciting finding.”
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This might be the first newborn great white shark ever recorded
The 5-foot great white was filmed off the coast of California covered in a strange, milky substance that could be uterine fluid.
By Jason Bittel
For as much time and money as people have spent studying and filming great white sharks, no one has ever witnessed one being born. But new footage out of California might be the next best thing.
On July 9, 2023, filmmaker Carlos Gauna and organismal biologist Phillip Sternes were following sharks with a drone off the coast of Santa Barbara. They’d already captured footage of a few larger great whites, but then something completely unexpected drifted up out of the murk.
“Toward the end of the day this peculiar-looking white shark appeared, and we were both super excited,” says Sternes, a Ph.D. student at the University of California, Riverside.
At just under five feet long with chubby, rounded fins, the duo knew they were looking at a very young great white. (For comparison, adult great whites can stretch up to 21 feet long.) But as the drone zoomed in, it caught something never seen before—a milky white film that appeared to flake off the young shark’s tail as it swam.
While no one can say for sure what the white material is, Sternes suggests it may be a sort of uterine “milk” used to nourish newborn sharks in the womb. Another possibility is that the shark has a skin condition that has never been described before.
“Both scenarios are highly significant,” says Sternes, who together with Gauna co-authored a study announcing the findings today in the journal Environmental Biology of Fishes.
“This is extremely rare. White shark birthing locations have remained extremely elusive for the scientific community,” Sternes tells National Geographic in an email. “We think we have a piece to the puzzle now. If this is a birthing location, conservation aspects must be considered.”
‘A really unique observation’
To understand just how rare a sighting of a newborn great shark is, you need to first understand that we know surprisingly little about how the world’s largest predatory shark reproduces.
(Great white sharks may change their color to sneak up on prey.)
Skomal cautioned against making any major conclusions based on the footage, however.
“It’s like any observation. You only have so much information, right? You have the imagery. You got the date, the time, the location. And the rest is speculation,” says Skomal, who is also author of Chasing Shadows: My Life Tracking the Great White Shark.
That said, Skomal did acknowledge that the facts presented would seem to point toward the shark being a newborn.
Scientists know surprisingly little about the reproduction of great white sharks—so the discovery of a birthing site would go a long way to advancing our understanding of these animals.
For instance, the area where it was spotted has been thought to be a great white nursery, as first-year sharks have been documented there before. And Skomal says the shark’s coloration, shape, size, as well as the appearance of the milky white fluid all suggest it’s extremely young.
“That’s led researchers to think it’s a newborn white shark, and it probably is,” says Skomal. “The question is, when was it born? Is it hours, or days [old]? We don’t know that.”
What this means for research
National Geographic Explorer Gibbs Kuguru, a scientist who studies the DNA of sharks, says that the sighting is “a game-changer”—particularly if the white fluid is what researchers suspect it is.
“The fact that we have a shark in the process of shedding its “amniotic fluid” is compelling evidence that we’re looking at a newborn, which is critical because it strongly suggests the shark hasn’t strayed far from its birth site,” he says. “Also, considering the decades of research about finding the elusive mating and birthing locations of Great Whites, stumbling upon a potential pupping ground like this is nothing short of radical.”
Because ocean animals spend the majority of their lives below the water’s surface, sometimes discoveries must wait until scientists happen to be in the right place at the right time. That was certainly the case when scientists witnessed a humpback giving birth for the first time in March 2021.
(Full humpback birth witnessed by scientists for first time ever. See the stunning video.)
Great white sharks are listed as vulnerable to extinction by the International Union for Conservation of Nature, with the overall population thought to be decreasing. Learning more about where they start their lives would be critical information for scientists seeking to safeguard the future of the species.
As for Sternes, he says, “I look forward to receiving feedback from the scientific community.”