Mysterious circles of basking sharks explained - Marine Biological Association

an aerial view of basking sharks circling in the water
(Irish Basking Shark Group)

Rarely observed circling behaviours of endangered basking sharks have now been explained as ‘shark speed dating’ courtship displays, thanks to a new study.

Marine biologists from the Marine Biological Association (MBA), the Irish Basking Shark Group and colleagues have led ground-breaking research which reveals the circles of basking sharks seen off western Ireland are engaged in annual reproductive behaviour, the first place in the world where this has been verified.

Circling formations have been documented on a few occasions over the past 40 years in the north-west Atlantic off Canada and the USA. Although basking sharks are often seen filter-feeding plankton in UK and Irish coastal waters in the summer, the circling formations were rarely seen, and until now, scientists could not explain the behaviour.

Scientists captured footage of 19 circling groups using underwater cameras and aerial drones off County Clare, Ireland, from 2016 to 2021. They found each group comprised between 6 and 23 sharks swimming slowly at the surface, with others below them deeper down, in a three dimensional ring structure the researchers termed a ‘torus’.

The team found that the sharks in circle formations were equal numbers of sexually mature male and females and were not filter-feeding. Some females had a paler body colour than males, a difference seen during courtship and mating behaviour in other shark species.

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Posted On: 12/09/2022

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