In a recently published study, NOAA Northeast Fisheries Science Center scientists said the largest group was spotted Nov. 5, 2013, in waters off southern New England. That group contained at least 1,398 basking sharks within an 11.5-mile radius.
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It contained many young sharks in an area with abundant zooplankton, leading scientists to believe that the large group may have formed to feed. The massive basking sharks create drag in the water when they open their mouths to feed, and researchers believe they might feed in a specific formation for efficiency's sake.
Photos of that large group were not immediately available.
Otherwise, researchers write, it's not common for basking sharks to travel in such large groups. There were less than a dozen such mass sightings of the animals between June 1980 and November 2013 off waters between Nova Scotia and Long Island.
Some of the groups contained as few as 30 sharks, and the congregations were more likely to occur in the summer and fall.
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"Although the reason for these aggregations remains elusive, our ability to access a variety of survey data through the North Atlantic Right Whale Consortium Database and to compare information has provided new insight into the potential biological function of these rare events," study lead author Leah Crowe said in a news release.
The filter-feeding basking shark is the world's second-largest fish species. It can grow to be up to 32 feet long and weigh more than five tons, and it travels slowly as it filters plankton out of the seawater.