Massive swarms of over 1,000 basking sharks spotted off the northeastern U.S. are puzzling scientists.

National Geographic reports that the shark swarms were noticed during aerial surveys to locate endangered North Atlantic Right Whales. Typically solitary creatures, the throngs of basking sharks have surprised experts.

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A research study published in the Journal of Fish Biology harnessed aerial survey data between 1980 and 2013, focusing on groups of at least 30 sharks. “These aggregations occurred in summer and autumn months and included aggregation sizes of up to 1,398 individuals, the largest aggregation ever reported for this species,” the researchers explained.

National Geographic reports that, while 10 large groups of sharks were spotted during the survey period, the record-breaking gathering of almost 1,400 sharks was particularly interesting. Spotted in November 2013 off Southern New England, the group included several young sharks. This suggests that the group was feeding on zooplankton rather than mating, according to Leah Crowe, a field biologist at NOAA’s Northeast Fisheries Science Service, and the study’s lead author.

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Although fearsome looking, the basking sharks, which can grow up to 32 feet long, are not a threat to humans.

The study suggests that the sharks may have been “drafting” off each other to save energy while feeding. The sharks may have gathered to cut down on the drag caused by their open mouths, it said.