'They cover a lot of ground.' About 800 white sharks visited Cape Cod over four years

At any given time in the waters off Cape Cod beaches, there are at least a couple dozen great white sharks swimming around, patrolling for seals and resting up during their migratory routes along the East Coast. In recent years, the numbers of sharks stopping by has grown.

Local shark scientists are now putting an estimated figure on just how many of these sharks are visiting. According to a study just released, 800 individual white sharks dropped in over the four year course of the survey, conducted from 2015 to 2018.

But don't be alarmed.

"The 800 number doesn't mean all 800 individuals were here off our beaches at one time," said Megan Winton, lead author of the study and a staff scientist for the Atlantic White Shark Conservancy in North Chatham, emphasizing the numbers are spread over four years.

During the study, published Thursday in the Marine Ecology Progress Series journal, the team not only generated annual estimates of the sharks, but also monthly estimates.

"They trickle in during late spring and June, and then the numbers start to ramp up. August is when there is the most activity," Winton said.

The most sharks seen on one survey day was 31

The most individual sharks they saw on a given survey day was 31. The sharks are also dispersed along the shoreline — those that are present at any given time are not all at the same beach at the same time, and different individuals come and go throughout the season.

"They cover a lot of ground," Winton explained. "And when I say they cover a lot of ground, they really do cover a lot of ground."

Demonstrating her point, she noted there was one shark the research team tagged that, based on detections by receivers in the water, swam from Cape Cod to Maine and back in less than a week.

"They're very big. They can push so much water with a flick of their tail," she said, describing the speed of their travel.

It is common for many of the sharks to range into Maine and back during the course of a summer. And during any given season, not all of the sharks in the population necessarily spend time hanging out off Cape Cod. Some individuals skip stopping by at all.

A team of scientists conducted the study

Winton was part of a team of scientists from the Atlantic White Shark Conservancy, UMass Dartmouth School for Marine Science and Technology, and the Massachusetts Division of Marine Fisheries that conducted the study, an intensive mark-recapture survey. Their goal was to estimate the size of a population in what has become known as a white shark hotspot — the only known aggregation spot on the East Coast, and the newest one on the global scene.

The study is co-authored by Gavin Fay, associate professor of Fisheries Oceanography at UMass Dartmouth and Winton's Ph.D. advisor, and Gregory Skomal with the state marine fisheries agency. It was primarily funded by the Atlantic White Shark Conservancy.

Cynthia Wigren, CEO of the organization, called the study "truly a collaborative effort."

"Our supporters and the local community dedicated a tremendous amount of time and resources to support this first-of-its-kind study in the North Atlantic," she said in a statement about the study. "We are thrilled to have led this groundbreaking work that advances our knowledge of the species and improves our ability to measure conservation efforts.”

In 2018: Researchers tag last great white shark in 5-year population study

Winton said the study is a big part of her Ph.D. research, "so this is very exciting to have it published finally." It's particularly significant, she said, because "there's never been an estimate of white shark abundance anywhere in the northwest Atlantic."

White sharks are one of the largest shark species

Reaching up to about 19 feet, white sharks are one of the largest shark species. According to the study, white sharks are "long lived, slow growing, and late to mature. Consequently, they are "highly vulnerable to overexploitation."

While the study notes that white sharks have never been the target of "large-scale directed fisheries," Winton said their population in general did decline by as much as 80% as recreational and commercial fishing pressure and shark-control programs increased in the 1970s and 1980s.

It wasn't until the 1990s that protections were put into place.

"It still blows my mind that there was no management plan for any sharks before the 1990s," Winton said.

Researchers emphasize that the results of their study "are only representative of the number of white sharks visiting Cape Cod" and are not to be taken as an estimate of the total population in the western North Atlantic Ocean. Winton said the study provides "an important baseline that can be used to monitor the performance of conservation measures for the broader population."

A perfect opportunity to understand the white shark population

Winton said it was previously challenging to estimate the population because they move around so much along the East Coast and tend to be widely scattered. But when they started aggregating around Cape Cod in the early 2000s, it was a "perfect storm" for research.

"When Cape Cod started emerging, or re-emerging, as a white shark hotspot, what also emerged was an opportunity to do a study," Winton said.

How the white shark population was estimated

To get a better understanding of the population, Winton said the team used "a lot of very innovative" techniques.

This included a mark-recapture method, which involves capturing and tagging a small number, releasing them, and then later catching another small group, tagging those that are not tagged and taking note of the number that may have been previously tagged. The smaller the population, the more likely researchers are to re-encounter them. A mathematical equation is applied to estimate the overall population.

Unique markings and notches can be identifiers

White sharks can be distinguished based on unique markings and notches in their dorsal fins, Winton explained. From almost 3,000 videos collected during 137 research trips off Cape Cod’s beaches, the team identified 393 different sharks.

Winton noted that "by comparing the number of previously identified individuals to new individuals encountered over time, we were able to estimate the number of sharks visiting the site in each month of the survey, as well as the total for the four-year period.”

The team tagged sharks and collected data off the Outer Cape twice weekly from June through October from 2014 to 2018, Winton said. Not only did they see the numbers increase seasonally, they also saw an increase in the numbers of sharks visiting Cape Cod over the four years of the study.

With climate change, white sharks could begin sticking around Cape Cod longer in the future

Besides being attracted by the rebounding seal population, a favored prey, white sharks are also thriving in the warming waters here. The Gulf of Maine, which spreads down to Cape Cod, is recognized as one of the fastest warming water bodies in the world.

Winton said East Coast scientists are noticing shifts in the ranges of many marine animals, including sharks. Most sharks are ectothermic, meaning they can't regulate their own body temperature and depend on external environment to do so. While white sharks are among a few that can tolerate cooler temperatures, they can't function when water temperature falls below 40 degrees, Winton said.

With climate change, the water is not only warming, but it's staying warm for longer periods. Because of this, said, Winton, "it's very possible in the not too distant future we could see the white sharks staying here longer." There could even come at time when they might be around all year if "the temperatures are more in their optimal thermal envelope."

The Cape Cod Times is investigating the effects of a rapidly heating planet on people who live on the Cape & Islands. This is part of the USA TODAY project Perilous Course.

Heather McCarron writes about climate change, environment, energy, science and the natural world. Reach her at hmccarron@capecodonline.com, or follow her on Twitter @HMcCarron_CCT

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This article originally appeared on Cape Cod Times: Study: Warmer temperatures in Gulf of Maine may mean year-round sharks