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Massive Gathering of Mysterious Basking Sharks Found in Aerial Photos

In 2013, nearly 1,400 of the normally solitary filter feeders had a party off the coast of southern New England

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Basking sharks are the second largest living fish and tend to be solitary swimmers. But a new study suggests they may interact more than we once thought.

As Cecelia Smith-Schoenwalder reports for National Geographic, researchers teased through aerial and satellite surveys conducted in the Atlantic between 1980 and 2013. Though the surveys were originally intended to find right whales, the researchers used the data to learn more about the enigmatic interactions of basking sharks. From this analysis, they discovered that a rare event took place in November of 2013​: 1,398 of the large sharks gathered off the coast of southern New England.

Out of the nearly 11,000 sightings of basking sharks recorded in the database, only ten large aggregations (more than 30 sharks) were found in the waters between Nova Scotia and Long Island. The sharks traveled solo 89 percent of the time, and the majority of the rest were found in small groups. The research appears in the Journal of Fish Biology.

The groupings, though rare, suggest there is much more to learn about these sharks. “[Basking sharks] do aggregate at times, at least in the Pacific, but I have not observed that here or heard of it," marine conservation biologist Boris Worm of Dalhousie University, not involved in the study, tells Emma Davie at the CBC. "The population estimates, that I'm aware of, are kind of in the hundreds for this whole area. So 1,400 would be not just a large group, but a large chunk of the population.” But, he notes, there are not exact counts of these massive critters.

This big question is why these normally solitary sharks, which can reach up to 32 feet in length and weigh up to five tons, occasionally have shark jamborees.

For the most massive of these events, when nearly 1,400 creatures gathered, the researchers have a good guess. According to a press release, the researchers were able to use photogrammetry to measure the lengths of the sharks in the aerial images, classifying them as juveniles or adults. Coincidentally, a NOAA survey called EcoMon also sampled the same area for zooplankton concentrations 11 days after the shark party.

The analysis suggests the zooplankton was high in the days following the shark grouping, which had an abundance of juveniles. These findings led the researchers to conclude that the sharks were engaging in a feeding frenzy on a fall plankton bloom before beginning their annual autumn migration south. The sharks thrive on these tiny floating critters, chowing down by swimming close to the surface of the water, mouths agape, filtering the plankton in their specialized gills.

But there may be other reasons, Leah Crowe, marine biologist at the National Oceanic and Atmospheric Administration's Northeast Fisheries Science Center and lead author of the study, tells Smith-Schoenwalder. “Seeing them from the air is interesting, but it doesn’t tell us that much about the environmental factors,” she says.

It’s also possible the sharks gather for mating purposes, or even to “draft” off one another while feeding, reducing the energy costs of filter feeding. As Crowe points out, because this study was made from gleaning data from right whale surveys, it’s hard to come to definite conclusions for the source of the gatherings. She hopes that more studies focusing directly on the sharks will happen in the future, Davie reports.

The 2013 aggregation, however, is a hopeful sign that the strange, little-studied species is doing okay—at least in the North Atlantic, Worm tells Davie. During the 20th century, the sharks were intensely hunted. Their liver oil was used for lighting, their skin for leather and meat for food. Fisherman in both the north Pacific and north Atlantic took hundreds of the slow-growing sharks per year until the population collapsed and hunting them was no longer viable. Today, they are considered vulnerable and are protected in the U.S. and U.K., but threats still remain.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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