Basking Sharks Gathering in Swarms of Up to 1,400 Leave Scientists Confused

The Cetorhinus maximus, or basking shark, isn’t considered a threat to humans, but its sheer size and extremely rough skin is enough to merit caution. As the world’s second-largest fish, it can grow as long as 32 feet long and weigh more than five tons — an intimidating sight at the ocean’s surface, even though it’s probably just trying to scoop up a zooplankton snack. Seeing one in the wild is equivalent to stumbling along a creature the length of four Lebron Jameses. Now, imagine coming across 1,398 of them all at once.

That’s the size of the crew that National Oceanic and Atmospheric Administration researchers spotted while examining 40 years of aerial photographs taken off the northeastern United States coast. Those aerial surveys, they explain in the March edition of the Journal of Fish Biology, were intended to locate endangered North Atlantic right whales. Instead, they revealed weird patterns of basking shark activity: From June 1980 to November 2013, there were 10 large aggregations of basking sharks, ranging from a group of 36 to at least 1,398 of them within an 11.5-mile radius of the central aggregation point.

This was astounding to the researchers, not only because basking sharks are very rarely seen as a group, but because the scientists don’t really know why they would congregate in such larger numbers.

“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 to compare information has provided new insight into the potential biological function of these rare events,” lead author Leah Crowe, a protected species researcher at NOAA’s Northeast Fisheries Science Center, said in a statement on March 30. “Aerial surveys provide a valuable perspective on aggregations and their potential functions, especially when coupled with environmental satellite and ship-based survey data.”

This smorgasbord of survey techniques allowed the scientists to examine the basking shark’s breaching motions, circular swimming movements, and feeding behavior. For example, they could see that on November 5, 2013 — the day that 1,398 basking sharks gathered in southern New England waters — there was an abundance of zooplankton and many juvenile sharks. Congregating likely helped them slurp up prey because it may have allowed them to “draft off each other for more efficient feeding given the immense drag from having their mouths open.”

At the surface, they typically swim around with their mouth open for 30 to 60 seconds, a process that lets them filter 2,000 tons of seawater per hour through their large gills.

But the scientists aren’t certain that feeding is the exact reason for these shark parties. They might be socializing — or courting — which would make sense in terms of numbers alone. Basking sharks typically show strong sexual segregation, with the female to male ratio sometimes reaching 40 to 1. It’d be far easier to find a willing mate in a large group simply because there are more options. What the scientists do know is that these aggregations happened in the summer and fall, when the sea surface temperatures were between 55 and 75 degrees — meaning that basking sharks relatively like it hot.