Basking sharks diving changes with the seasons

Basking Shark Underwater Feeding
iStock/Rebecca-Belleni-Photography

Studies show that the basking shark dives much deeper for food in the winter months and stays at the surface in the summer.

Researchers believe that the seasonal variations in the basking shark’s feeding behaviour is caused by the environmental conditions of the changing seasons. The shark is believed to explore different depths in order to deal with changes in food abundance.

Basking sharks are the second largest living shark species on the planet. These non-aggressive, plankton-eaters have a liver that weighs 25% of it’s body weight. Basking sharks are typically used for food, animal feed, shark fin and shark liver oil.

The basking shark use a diving method, called ‘yo-yo diving’ in the late winter and early spring. This style of diving is typically a rapid and repeated movement between the surface and the depths of the ocean. Researchers saw basking sharks dive as deep as 1500m in these ‘yo-yo’ dives.

Dr Phil Doherty, a postdoctoral researcher at the University of Exeter’s Penryn Campus and lead author of the study, said: “We do not know exactly why the sharks are performing these dives. They may be sampling the water column in efforts to detect prey or attempting to re-orientate themselves for navigation purposes.”

Dr Doherty and his colleagues from the University of Exeter collaborated with Scottish Natural Heritage, MarAlliance, Manx Basking Shark Watch and Wave Action to study how the movements and diving behaviour of basking sharks change throughout the year.

The Exeter team tagged 32 of these gentle giants off the coast of Scotland in order to monitor the depth, temperature, and light levels of the locations that they visit. The researchers discovered that the basking shark not only alters the depth of its dive depending on the season, but the sharks studied presented depth changes at different times of the day.

“We found that sharks spent most of the summer near the surface of the water, occupying the top few metres during the day, moving down to depths of 10-25 m at night. But in winter, they did the opposite, spending the majority of time between 50 and 250 m, but more often shallower during the night,” said Dr Doherty.

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