Basking sharks, the second-largest fish in the world, are joining together by the hundreds to pass the summer and fall months in enormous groups – and marine biologists haven’t figured out why.
According to a study published in the Journal of Fish Biology, schools ranging in number from 30 to nearly 1,400 individuals have been serendipitously spotted off the northeastern coast of the US for the past several decades.
It all began in 1980, when scientists performing aerial surveys of endangered right whales first noted a puzzling aggregation of the filter-feeding sharks. Since then, nine more large grouping events have been documented.
Equipped with enormous, gill-raker-lined mouths, the low-key species gorge on zooplankton by leisurely swimming through the surface waters of both coastal and open-ocean areas. And measuring in at 6-8 meters (20-26 feet) in length, it’s fairly easy for humans to spot them from either a boat or the air.
Observing the species regularly enough to learn about their behavior or reproductive cycle, however, is quite challenging, due to the sharks’ tendency to “disappear” between seasons by diving into deep water and migrating long distances.
As such, shark researchers were keen to analyze the data gathered by right whale scientists to uncover clues as to why basking sharks are sometimes solitary and keen on large-scale get-togethers at other times.
"Aerial surveys provide a valuable perspective on aggregations and their potential functions, especially when coupled with environmental satellite and ship-based survey data," said lead author Leah Crowe, a protected species researcher at NOAA's Northeast Fisheries Science Center, in a statement.
Crowe and her team determined that the aggregation events occur during periods of warm sea surface temperatures (13-24 °C, or 55-75°F) and high concentrations of chlorophyll – meaning that large quantities of photosynthetic plankton are in the water. Tiny crustaceans called copepods, the sharks’ preferred prey, eat this plankton.
During the largest of the 10 documented aggregations, in November 2013, at least 1,398 individuals were photographed swimming within an 18.5-kilometer (11.5-mile) radius area. Water samples collected at the time confirmed that copious copepods were present.
The findings are thus in line with those from previous studies on basking sharks that suggest the animals congregate, seemingly without competition-related aggression, in areas experiencing seasonal zooplankton blooms.
Unfortunately, the researchers were unable to determine whether or not the sharks use these gatherings to engage in mating.
"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," Crowe said.
To help scientists like Crowe gather data, anyone can submit their basking shark observations to the "Spot a Basking Shark" program run by the Pacific Shark Research Center.