NORTH CHATHAM — A just-published study led by researcher Megan Winton of the Atlantic White Shark Conservancy concludes that white sharks in local waters spend nearly half of their time in waters 15 feet deep or shallower. The research signals that, while rare, encounters between white sharks and people are a regular possibility.
“These sharks are here and they spend a lot of time in shallow water when they are here,” she said. When people go to the beach, “they should realize that there is likely a shark around somewhere.”
To assess the extent of potential overlap between sharks and recreational water users, scientists from the Conservancy, Arizona State University, and the Massachusetts Division of Marine Fisheries examined tagging data to determine how much time white sharks spend in shallow water close to shore. The study, which was published in the journal Wildlife Research, found that white sharks off Cape Cod spent almost half of their time at depths of 15 feet or less. Although the overall risk posed to humans by white sharks is low, there is a high potential for overlap between white sharks and recreational water users, and the results have clear implications for shark-related public safety practices in the region.
“White sharks are regularly spotted off our coastline during the summer and fall, the peak of Cape Cod’s tourist season, but until now we didn’t know just how much time they spent in shallow water close to shore,” Winton said. She and her coauthors, James Sulikowski and Greg Skomal, analyzed data from tagging trips off the Cape during the summer and fall of 2017. The tags revealed that sharks spent 95 percent of their tracked times at depths of less than 100 feet and water temperatures ranging from 48 to 69 degrees. While they spent 47 percent of their time in 15 feet or water or less, white sharks frequently traveled further out, alternating between the surf zone and deeper offshore waters.
The sample size for the study was small, with only eight of the tags reporting usable data. “It’s always a tough thing when you’re putting really high-tech equipment out in the ocean,” Winton said, particularly if the equipment is fastened to a large, highly-mobile animal. “Funding is always a limiting constraint,” and while the tags are not inexpensive, “we were very strategic about the tags that went out,” she said.
Acoustic tags were used to alert scientists to the presence of certain sharks in local waters, and pop-up satellite archival transmitting (PSAT) tags provided detailed information about water temperature and depth. While this study focused on how much time white sharks spend in shallow waters, other research is underway to understand what they are doing and how they are hunting when they are here, Winton said.
“I like to describe it as, we’re giving smartphones to sharks,” she said. The more sophisticated tags are equipped with accelerometers that show the animal’s fine-scale movements and orientation, and have cameras to show what the sharks are doing at any given time.
“They don’t spend all of their time eating,” she said. Sharks are sometimes in local waters for a few months at a time, and the next studies should illustrate what they are doing when they are not feeding. Better understanding how white sharks interact with their environment might explain their predatory behaviors, possibly illuminating “what might be the most high-risk period for recreational water users off the Cape,” Winton said.
But as for this study, Winton said she hopes the results will simply improve beachgoers’ awareness of sharks. Since 2012, there have been four attacks by white sharks on humans in local waters, one of which claimed the life of 26-year-old Arthur Medici at Newcomb Hollow Beach in Wellfleet. Beachgoers in many Cape towns are greeted by large signs touting shark safety, and lifeguards and beach staff spend time educating visitors about sharks, but some people have trouble putting the risk into perspective.
“They realize that white sharks occur here, but they don’t realize how close to shore they occur,” Winton said. By increasing awareness, “people can proactively modify their behavior,” she said.
The study also showed that sharks were slightly more likely to occupy shallow depths at night during the new moon. That seems to reinforce the fact that sharks are opportunistic predators that rely on visual cues, she said. It underscores the need for swimmers and beachgoers to avoid being in the water at night and when there is little ambient light, or when the water clarity is poor.
The study’s findings reinforce the need for beachgoers to follow all of the shark safety rules, Atlantic White Shark Conservancy CEO Cynthia Wigren said.
“The results of this study have greatly improved our understanding of white shark behavior near swimming beaches,” she said. “It’s important to remember that the overall risk posed to humans by white sharks is low, but people should be aware that white sharks are present along Cape Cod’s beaches during the summer and fall and proactively modify their behavior to reduce their risk.”