Study Presents First Population Estimate for White Sharks off Cape Cod, Massachusetts
Newest white shark hotspot among the largest worldwide
Approximately 800 white sharks visited the waters off Cape Cod, Massachusetts, from 2015 to 2018 reports a study published today in the journal Marine Ecology Progress Series. A team of scientists from the Atlantic White Shark Conservancy, University of Massachusetts Dartmouth’s School for Marine Science and Technology (SMAST), 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.
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. From almost 3,000 videos collected during 137 research trips conducted off Cape Cod’s beaches, the researchers identified 393 individual white sharks.
Megan Winton, lead author of the study, explained 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.”
“Because white sharks travel vast distances and move into and out of the area, we needed to develop a new model that accommodates the species’ migratory behavior to generate the estimate,” noted Gavin Fay, Associate Professor of Fisheries Oceanography at SMAST and Winton’s PhD advisor. “Megan’s research applies cutting-edge statical modeling to synthesize multiple data sources.”
The researchers 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. They found that the number of white sharks visiting the site increased over the duration of the survey and estimated approximately 800 individuals visited the waters off Cape Cod from 2015 to 2018.
The researchers caution that the results are only representative of the number of white sharks visiting Cape Cod during the four-year period and are not an estimate of the total size of the white shark population in the western North Atlantic Ocean. However, Winton says “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.” She continued, “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.”
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.
Dr. Greg Skomal, who leads the Massachusetts Division of Marine Fisheries’ Shark Research Program, emphasized that it’s important for the public to know that there are not hundreds of sharks swimming off Cape beaches all at the same time. Their movements are very dynamic, they trickle in and out. Some white sharks simply stop by on their way north while others spend more time along the Cape, likely because they have success feeding on seals.”
“The study was truly a collaborative effort,” said Cynthia Wigren, CEO of Atlantic White Shark Conservancy, a Cape Cod based nonprofit that provided the majority of the funding for the survey. “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. We are thrilled to have led this groundbreaking work that advances our knowledge of the species and improves our ability to measure conservation efforts.”