Around 800 great white sharks visited the waters off Cape Cod between 2015 and 2018, according to a new study. The study, the first of its kind to assess the abundance of great white sharks in the region, suggests Cape Cod plays host to one of the largest great white shark populations in the world during the summer months, beating out comparable sites across the globe.
“Not many populations have been assessed, but for those that have been, this is a high density of sharks,” said Gregory Skomal, one of the study’s authors and a shark biologist with the Massachusetts Division of Marine Fisheries.
Estimates from the study, published in the journal Marine Ecology Progress Series, increased with each year of the study, peaking in September 2018. According to the study, there may have been more than 500 sharks in the waters off Cape Cod in that month alone.
“There’s no reason to believe [the pattern of increasing estimates] couldn’t have continued,” Skomal said, “but I imagine we may be starting to stabilize. At some point, you reach a carrying capacity for a given area.”
Though the study area didn’t cover the waters near Nantucket, many of the sharks researchers observed likely pass near the island, as great white sharks can travel thousands of miles each year. However, great white sharks mostly pass through Nantucket’s waters on their way to and from Cape Cod and don’t remain near the island in the same numbers as in the study area.
“When we compare all our areas on Cape Cod to other areas across the state, we see the preponderance of white sharks on the outer cape,” Skomal said. “[Nantucket doesn’t] really exhibit any residency in terms of white sharks. It’s more like a highway, perhaps with the exception of Great Point.”
The researchers reached their conclusions after examining almost 3,000 videos collected during 137 trips—and they think their estimate is likely too low. As they employed strict criteria for including observed sharks in the study, many potential great whites were excluded, and it's almost certain at least some of those potentials were actual great whites. At the upper limit, the study estimates as many as 1,286 great white sharks could have spent time near the east coast of Cape Cod during the study period.
“[That] does sound like a really big number, but I think we need to put that in the context that it's over a four-year period, and not all of them are here at the same time,” Skomal said. “I don’t think the public should be too alarmed.”
Shark attacks are vanishingly rare, and none have ever occurred on Nantucket. Despite significant public anxiety, sharks cause fewer deaths annually than lightning strikes or dog attacks.
“I think it should be business as usual,” Skomal said. “The reason we did this study was to give beach managers and the general public an understanding of the number of sharks that visit here.”
The study suggests conservation efforts focused on great white sharks are starting to see success. Due to their long lifespans and slow growth, great whites are vulnerable to overexploitation, and in some regions, populations fell by up to 90% before conservation measures were first implemented in the 1990s. Recently, sightings of great whites have increased near Nantucket, in part because of the recovery of local grey seal populations, a primary food source for the sharks.
The study is also notable for its unique methodology, designed to circumvent the problems that traditionally plague attempts to quantify great white shark populations. The researchers, a team of scientists from the Atlantic White Shark Conservancy, the University of Massachusetts Dartmouth’s School for Marine Science and Technology, and the Massachusetts Division of Marine Fisheries, had to develop new techniques to account for the species’ migratory patterns.
Species abundance is often estimated using a mark and recapture method, where a portion of the population is captured, marked and released, and then another sample is captured later. Ideally, if both samples are random, the portion of the second sample that is marked will be roughly equivalent to the portion of the overall population that is marked. Since great white sharks are highly migratory, moving in and out of sample areas, this method leads to biased estimates when applied to great whites. To correct for this, researchers used computer simulations to predict the migratory patterns of the sharks and other factors that might affect the results.
“What this study does is for the first time take into consideration the movement dynamics of these animals,” Skomal said. “Most other studies don’t do that, and that can create a lot of bias.”
Skomal hopes the researchers’ methodology will be reproduced in the future.
“It’s just the beginning,” he said.