Basking Sharks Show Off Secret Underwater Life

Cameras capture synchronized swimming that may be courtship behavior.

basking sharks fin to fin
Basking sharks swim fin-to-fin.

University of Exeter and NatureScot

Although they’re the second largest fish in the world, basking sharks keep a low profile. They’re solitary animals and, until now, little was known about their mating and breeding behaviors.

But researchers recently captured these slow-moving migratory sharks swimming in groups, fin to fin, brushing up against each other in what the scientists believe could be courtship behavior. They also recorded a shark propelling itself fully out of the water in a full breach.

All of these behaviors were captured by video cameras that were temporarily attached to the sharks. The animals were recorded in the Sea of the Hebrides in the North Atlantic Ocean, west of Scotland.

Since 2012, researchers at the University of Exeter have partnered with NatureScot, Scotland’s national nature agency, to learn more about basking shark behavior and habitat use in the Sea of the Hebrides.

“This area is particularly attractive to them because their prey, zooplankton, is abundant and attracts large aggregations of sharks to feed,” study lead author Jessica Rudd of the University of Exeter tells Treehugger. “Our team has revealed just how important this area is to the sharks, which return to the same spot year upon year after long migrations.”

But the scientists believed that the sharks might be in the waters for more than just dinner. Little is known about basking shark reproduction. So researchers attached cameras to the sharks to find out what they’re up to when they’re underwater.

“We captured a range of behaviors on camera, from sharks feeding at the water’s surface, this funny worm-like undulating behavior associated with defecating, as well as our tagged sharks chasing or being chased by another shark down to the seafloor,” Rudd says.

They recorded a full breach for the first time from a shark’s perspective when one animal propelled itself from more than 70 meters (230 feet) completely out of the water then dove back down to the seabed.

“Being able to capture this incredible feat of speed in a species that doesn’t scream athleticism is absolutely amazing,” Rudd says.

The researchers were surprised to find that the sharks spend the majority of their time (88%) on the seabed. This was not expected because, as their name implies, these sharks are known for being seen at the water’s surface where they appear to be basking in the warmer waters there.

“The most exciting behavior we captured was this eerie early morning grouping behavior which has never been documented before of at least 9 sharks congregating on the seabed, following each other nose to tail, fin to fin, brushing against one another,” Rudd says.

“This kind of behavior has been observed in other shark species and is linked to pre-mating behavior and courtship displays but has never been observed in basking sharks and is the first insight into their possible breeding rituals.”

Because basking sharks are typically solitary, wandering the oceans before returning to a specific area to feed, getting together to eat may also give them an opportunity to find a mate.

The synchronized swimming behavior surprised scientists when they saw it.

“We were reviewing the footage on the boat on the way home after hours at sea retrieving the cameras and almost fell over when we saw this amazing unexpected congregation of sharks on the seabed swimming slowly side by side, touching fins,” Rudd says.

“Whilst grouping behavior can be seen on the surface, this is usually associated with feeding, with sharks trailing behind one another, mouths wide open feeding on zooplankton. These are the world’s second largest fish, reaching more than 10 m in length, so seeing so many huge animals being so tender to one another is just incredible.”

In December 2020, the Scottish government and NatureScot declared the location the first ever marine protection area to protect basking sharks. This offers protection not only to the area where they feed but also to what may be their breeding grounds, as well.

Basking sharks are found mainly in the Atlantic and Pacific oceans but live in temperate waters all over the world. They are classified as endangered by the International Union for Conservation of Nature (IUCN) Red List. They were hunted for centuries for meat, skin, cartilage, and liver oils.

Dealing with Technology

For the study, researchers attached cameras to the base of the primary dorsal fins of six basking sharks using darting poles. In the water, the camera weighed about 300 grams (10 ounces). The cameras were programmed to automatically detach after a few days and float to the surface.

The study results, which were published in the journal PLOS One, are especially interesting because so little is known about basking shark activities.

They are loners who wander the oceans most of the year, returning near shore only over the summer to feed for a few months. This makes it difficult for researchers to observe their behavior outside of those feeding occasions.

“While basking sharks provide a unique opportunity to observe their feeding habits as they forage for zooplankton close to the surface, you may spot their large dorsal fin break the water from a cliff or from a boat, these observations are restricted to daylight hours, weather conditions and being relatively close to the coast,” Rudd says.

“Sharks being fish, they do not need to come up to the surface to breath, so you essentially miss all their underwater activity and compared to more tropical sharks species living in warmer clearer waters, the dense plankton of their feeding grounds reduce the visibility combined with colder water make for less inviting snorkeling conditions and harder to observe these sharks in their habitat.”

Advancements in tracking technology have improved the understanding of what goes on below the surface, but there’s still so much to learn, researchers say.

And the logistics of tracking isn’t easy. Unless sharks are on the surface, researchers can’t spot or tag them.

“We can be stuck on land waiting out bad weather for several days or be out on the water for 17 hours searching for the tell-tale large floppy dorsal fin of basking sharks and not spot a single one for days,” Rudd says. “It’s quite frustrating thinking they might be right under our noses but with no way to see them.”

Once the camera is released from the shark, it pops up to the surface of the ocean and a radio transmitter pings its location.

“It’s like looking for a needle in a haystack searching for a red blob at sea, often in heavy swell, following the beep through headphones as it gets louder and louder as we hone into it and scoop the camera out of the sea with a large fishing net,” Rudd says.

“It then takes several weeks to watch the hundreds of hours of footage, noting down every behavior, habitat type the sharks are swimming in and any other species observed but it feels like such an immense privilege to be let into the secret life of basking sharks from a shark’s eye perspective of their surroundings.”

View Article Sources
  1. Rudd, Jessica L., et al. "Basking Shark Sub-Surface Behaviour Revealed by Animal-Towed Cameras." PLOS ONE, vol. 16, no. 7, 2021, p. e0253388., doi:10.1371/journal.pone.0253388

  2. Rigby, C.L., et al. "Basking Shark." IUCN Red List of Threatened Species, 2018, doi:10.2305/