How seasons and time of day impact movements of basking sharks

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How seasons and time of day impact movements of basking sharks

September 29, 2019 - 11:05
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It appears that basking sharks tend to stay close to the surface in summer and dive to the ocean depths in winter, according to a study.

Data collected by satellite tags attached to 32 basking sharks has yielded some interesting facts, indicating that these sharks spent most of the summer months at the ocean’s surface, and descended to deeper waters during winter, according to a study.

No, the sharks are not taking advantage of the sunny weather (at least, we don't think so). Rather, their movements appear to be caused by having to deal with seasonal changes in food abundance.

Towards late winter and early spring, the sharks were also observed doing “yo-yo” dives. These are rapid and repeated vertical movements between deep and surface waters.

The reason for these dives remain unknown. Lead author Phil Doherty, a postdoctoral researcher at the University of Exeter’s Penryn Campus, suggested that the sharks "may be sampling the water column in efforts to detect prey, or attempting to re-orientate themselves for navigation purposes.”

The researchers from the University of Exeter collaborated with Scottish Natural Heritage, MarAlliance, Manx Basking Shark Watch and Wave Action for the study. Their findings was published in the October 2019 issue of the Marine Biology journal.

The time of day also had an impact on the sharks' movements. "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 metres at night. But in winter, they did the opposite, spending the majority of time between 50 and 250 metres, but more often shallower during the night," said Dr Doherty.

The research team hopes that their study can serve as a starting point for studying the "depth hotspots" of the basking shark. Such information may be used to lower the risk of incidental catch of these sharks by commercial fisheries.

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