Scientists say a species of large, but harmless, shark has been gathering in atypically big groups off the coast of the northeastern United States and Canada.

The National Oceanic and Atmospheric Administration says groups of 30 to 1,400 basking sharks have been seen in waters from Nova Scotia to Long Island, N.Y. between 1980 and 2013.

The agency says sightings of basking sharks are common, but seeing them in such large groups is not.

The basking shark is the second-largest fish on the planet. It can grow more than 7.6 metres in length, lives on a plankton diet and is not aggressive or dangerous to humans.

NOAA says the reason the animals have been congregating in large groups "has not been clearly determined," but it's likely related to feeding, socializing or courtship.