When white sharks are off the coast of Cape Cod, it's clear that they're here for a single purpose: hunting seals. But tracking data shows that white sharks spend the off-season far out in the Atlantic, often in the dark depths far beyond the edge of the continental shelf. A journal article co-authored by state shark biologist Greg Skomal published Friday considers what they might be doing out there.
Analyzing data from 32 white sharks outfitted with satellite tags, the researchers concluded that white sharks don't simply migrate from north to south, as previously surmised. The article was published in the journal Marine Ecology Progress Series, and was written by Skomal, his colleague at the Mass. Division of Marine Fisheries John Chisholm, Camrin Braun of M.I.T., and Simon Thorrold of the Woods Hole Oceanographic Institution.
Skomal said previous studies of tagged white sharks indicated that the predators spent all of their time in the relatively shallow waters of the continental shelf. That's because the observations came from fishermen, who generally work close to shore, Skomal said in a talk last week at the Cape Cod Museum of Natural History. But analyzing data from satellite tags, it became clear that some white sharks venture into deep water.
“We're seeing these broad-scale movements,” he said. The 32 white sharks in the study were tracked for a total of 10,427 days, and in the summer they were located all along the coast from the Gulf of Mexico to New England. In the fall, presumably because of falling water temperatures, the tagged sharks were observed traveling farther offshore, into the Sargasso Sea and as far east as the Grand Banks. By winter, some of the sharks had ventured as far east as the Azores. The range constricted again in the springtime, with white sharks returning to the coastline in the summer.
But the increased range of the white sharks in the study was only part of the finding. Once in the deep ocean, white sharks spent a great deal of time in waters deeper than 650 feet, and periodically made dives to around 3,000 feet.
“The question is, why?” Skomal said. Researchers don't yet know the answer, though there are two schools of thought. Scientists on the West Coast theorize that white sharks are traveling to a similar area in the middle of the Pacific “that they call the 'white shark café,” he said. The theory is that the predators are feeding on some deep-water prey, “but nobody knows what's living there that they're feeding on,” Skomal said.
Other scientists, including some at Stanford University, believe the deep dives are related to mating. Humans have never observed white shark mating behavior, though mature female sharks are often seen to have bite marks on their heads, presumed to be from males. While researchers have never observed white sharks in the act of mating, “it's thought to be very violent,” Skomal said. Likewise, it's not clear where white sharks are typically born, though juveniles are often found in the New York Bight between New Jersey and Long Island.
Skomal favors the theory that white sharks are feeding in deep waters, “but what they're feeding on, I have no idea,” he said. That mystery, he hopes, will not remain a mystery forever.
“How do we figure out what a white shark is eating at 3,000 feet?” he asked. The answer will likely come from the use of autonomous underwater vehicles like the ones used in trials with the Discovery Channel off Guadalupe Island in 2012 and 2015. In those expeditions, underwater drones made by Woods Hole Oceanographic Institution successfully followed transponder-tagged sharks, capturing video images at depths of up to 2,000 feet.
“We're getting close to where I want to be with this technology,” Skomal said. “We're making baby steps. We're making progress.”
The ongoing white shark population study off Cape Cod, funded by the Atlantic White Shark Conservancy, is answering other questions about shark behavior when they are close to the shoreline. Beyond pure science, researchers want to better understand white shark movements up and down the beach during warm weather “so we can protect people who are swimming in the water every day. We think that's critically important,” he said. But understanding where sharks go in the off season is important for another reason: conservation.
The tagged sharks in the study published last week ventured far from shore, well beyond the jurisdiction of regulators in the U.S. For white sharks, which are often harvested for their fins – used for shark fin soup – unregulated, unsustainable fishing practices threaten the health of the species.
“Our findings extend the known essential habitat for the white shark in the North Atlantic beyond existing protection, with implications for future conservation,” the study reads.