Swarms of up to over a thousand basking sharks have been spotted along the northeastern U.S., puzzling experts who study the normally solitary species.

Aerial surveys meant to locate endangered North Atlantic right whales in recent decades have revealed massive groups of the world's second-largest fish. Found worldwide, these slow-moving filter feeders pose no threat to humans.

As big as basking sharks are—at 32 feet long outsized only by the whale shark—the deep-sea dwellers can be tricky to track down.

And without those opportunistic sightings, "that data was hiding away," says Leah Crowe, leader of a recent study on the phenomenon and a field biologist at the National Oceanic and Atmospheric Administration's Northeast Fisheries Science Center. "Our goal is not to do that with our research." (Read about a huge basking shark caught off Australia.)

In the study, published in the Journal of Fish Biology, Crowe and colleagues documented 10 sightings of large groups of basking sharks between 1980 and 2013 along the coast of Nova Scotia to Long Island.

The researchers uncovered about 10,000 documented sightings of basking sharks in the database, and 99 percent were of groups of seven or less.

Why Gather?

Shark experts have several theories about why basking sharks would congregate. Other shark species are known to gather for feeding, mating, and protection from predators.

A record-breaking sighting of about 1,400 sharks in November 2013 off southern New England included several young sharks, which to Crowe indicates that the group was likely feeding on zooplankton instead of mating. (See "Pictures: Biggest Whale Shark 'Swarm' Found.")

The study also suggests that the sharks could be gathering to reduce the drag caused by their open mouths during feeding, allowing them to draft off each other to conserve energy.

But aerial sightings only reveal so much.

"Seeing them from the air is interesting, but it doesn’t tell us that much about the environmental factors," such as the density of plankton, Crowe says.

Shark Mystique

Gregory Skomal, a senior fisheries scientist for the Massachusetts Division of Marine Fisheries, agreed that aerial data is limited in explaining the cause of the swarms.

Skomal has swum among a basking shark gathering, but he does not recommend sending people in to collect data, as their presence would cause the animals to act differently.

Such aggregations "build on the mystique of the animal," says Skomal, who was not involved in the research.

Shark 'Feeding Frenzy' Seen in Incredible Aerial View



Click here to read Shark 'Feeding Frenzy' Seen in Incredible Aerial View. August 24, 2017 - In a scene fit for a summer blockbuster, a group of sharks fed on schools of fish off the coast of the Hamptons, New York. The predators are most likely either sandbar or dusky sharks, and don't pose a threat to humans. The area has seen an increase in fish, as well as shark activity, making this sight not uncommon. Sharks migrate along the Atlantic coast from New Jersey to Maine, following shifting water temperatures.

But the public can help scientists in their quest via the citizen science program "Spot a Basking Shark," run by the Pacific Shark Research Center at Moss Landing Marine Laboratories. (Read about an unexpected basking shark hideout.)

Basking sharks spend about 90 percent of their time deep underwater, and only 10 percent at the surface—making any sightings valuable information, according to Dave Ebert, program director for the research center.

"Species of Concern"

Learning as much as possible about the behemoths is crucial, as some basking shark populations have fallen—the U.S. government deemed the eastern North Pacific "a species of concern" in 2010.

In the 20th century, hunting for sharks' liver oils and hides reduced their population along the U.S. West Coast, and basking shark numbers have remained low in recent decades, Ebert says.

He adds a caveat: "When I say low, we're not really sure what the baseline was to begin with."