Synchronized fin-to-fin swimming, which is probably part of courtship, has been seen for the first time in groups of basking sharks. Video cameras temporarily attached to the sharks gave scientists an unprecedented view of their previously secret underwater world.
Gentle giants are usually solitary creatures and virtually nothing is known about their reproductive behavior. The researchers also recorded a shark soaring over the water, the first time a full breach has been captured from the shark’s point of view. This can also be part of courting a partner, perhaps showing the size of the fish.
Basking sharks are found in temperate waters around the world, but are in danger of extinction after being hunted in the past for the oil from their massive livers. They are the second largest fish in the ocean, and adults usually reach 8 meters in length.
The scientists carried out their study in the Sea of the Hebrides, off the Scottish islands of Coll and Tiree. The site was known to attract fish for food in the summer and in December it was declared a marine protection zone, the first in the world to be specifically designated to protect basking sharks.
“One of the most exciting moments in my career with basking sharks was seeing images of them all grouped together on the seafloor,” said Matthew Witt, from the University of Exeter in south-west England. “It was absolutely phenomenal, you just don’t think of them doing that.”
“It has been really fascinating to have this incredible information,” said Jessica Rudd, also from the University of Exeter and who led the field work. “It feels like a privilege to have a shark’s eye on what they do. There were large congregations of sharks, swimming very slowly, side by side or one on top of the other, or swimming from nose to tail, fins in contact and in groups of up to 13 “.
Copulation was not captured on camera, but Witt said that based on behaviors seen in other sharks, these congregations and social behaviors are often what precedes mating.
“Could it be that feeding on these [food] Hotspots also provide the opportunity for these solitary sharks to meet other sharks, ”said Rudd. Scientists have been tagging animals for a long time, he said, but typically the data is recorded once a day or only when a satellite tag surfaces.
“With video cameras, it’s essentially 24 hours a day,” he said. This means that scientists can not only determine where a shark is, but also why it is there.
Breaking takes a lot of energy, especially for giants like basking sharks. Various reasons for the behavior have been suggested, such as getting rid of parasites or even just for fun, but courtship is another explanation.
“It’s a really creepy video, where the shark emerges from a depth of 77 meters and then comes to the surface in 70 seconds and breaks,” said Rudd. “We can see it completely out of the water and these are sharks that weigh up to more than a ton.”
Other research from the group, which used tags similar to Fitbit, showed that basking sharks can break four times in 45 seconds.
The researchers were also surprised to find that sharks spent up to 88% of daylight hours near the seabed, rather than near the surface where they primarily foraged. This information could be useful when considering restricting fishing activity, such as bottom trawling.
The investigation was published in Plos One magazine and followed six sharks for a cumulative total of 123 hours. The cameras were attached to the sharks with a dart stick and weighed only 300g in the water. The sharks quickly resumed their normal activity within minutes and the cameras automatically separated and floated to the surface after a few days. More cameras will be attached to sharks this summer.
Suz Henderson from NatureScot said: “The group behaviors described in this paper, as well as the habitats with which the behaviors are associated, may well be important in answering the key conservation question of where these sharks breed.”
George is Digismak’s reported cum editor with 13 years of experience in Journalism