Groups of up to 1,400 Sharks Gather off North American Coast, Puzzling Scientists

A basking shark off the Atlantic coast. A new study has shed light on the behaviors of this solitary species. Florian Graner/Green Fire Productions

Basking sharks are generally solitary animals, so scientists are puzzled as to why almost 1,400 of the creatures have been spotted congregating on a stretch of the North American coast.

After the whale shark, basking sharks are the world's largest fish. They can grow up to 32-feet long and weigh over five tons. Along with the whale shark and megamouth shark, basking sharks are the only species of shark that eat plankton. (And no, they don't eat humans).

For over three decades, scientists have been documenting whales in the waters stretching from Nova Scotia to Long Island. During that time, 10 sightings of what are considered to be large groups of basking sharks have been noted. In the 33 years following 1980, the size of basking shark gatherings in a 11.5-mile radius noted by experts ranged from 36 animals to 1,398 in November 2013, in what is known as a "super-aggregation."

To understand why the sharks are swarming together, researchers at the Northeast Fisheries Science Center (NEFSC) studied aerial photographs, records of basking shark behavior, and information collected by earth-orbiting satellites, and oceanographic databases. The results of the NEFSC ecosystem monitoring cruise survey, where information including water temperature and water salinity are collected, were also used.

Basking sharks were found to be aggregating in the summer and fall months, when water temperatures ranged between 55 and 75 Fahrenheit.

While scientists aren't certain why the sharks are gathering, they believe it could be down to the high levels of zooplankton on the continental shelf, particularly during the "super-aggregation" event. And due to the drag created when the animals feed with their large mouths open, grouping together could make feeding more efficient by pulling along trailing sharks, the authors said.

Judging by the behavior of other shark species, socializing and courtship are likely to play a part too, scientists believe.

The results of the study were published in the Journal of Fish Biology in March.

"Photogrammetry, the use of photographs to measure objects, has provided estimated lengths of animals at the surface and allowed us to classify animals in the aggregation as likely juveniles or mature adults," Leah Crowe, a protected species researcher at NOAA's Northeast Fisheries Science Center and lead author of the study, said in a statement.

"Although the reason for these aggregations remains elusive, our ability to access a variety of survey data though the North Atlantic Right Whale Consortium Database and to compare information has provided new insight into the potential biological function of these rare events," Crowe said.

Further research is needed to shed more light on the little-understood mating habits of basking sharks, and answer exactly why they are grouping together, and what happens during the resulting interactions.