Joseph Curl

Swarms of more than a thousand basking sharks, known as shivers, have been seen swimming off the northeast coast of the United States, and scientists don’t know why.

Normally, the massive sharks, often more than 30-feet long, remain alone throughout their lives, or at most in small groups of less than 10. Luckily for nearby residents, the basking sharks use filters like whales to feed and are no threat to humans.

Scientists have been conducting aerial surveys in search of endangered North Atlantic right whales. That’s when they saw the huge shivers of sharks, National Geographic reported.

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.

Leah Crowe, head of the recent study on the shivers and a field biologist at the National Oceanic and Atmospheric Administration’s Northeast Fisheries Science Center, said the sharks might be grouping up to feed.

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.

Crowe said the sharks could be “drafting” off each other, meaning following each other closely to reduce drag as they feed.

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.