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Last Updated: Thursday, 21 July, 2005, 15:19 GMT 16:19 UK
Basking sharks 'canny hunters'
A basking shark, image courtesy of David Sims/MBA
It was thought basking sharks followed a rigid pattern. Image courtesy of David Sims/MBA
Basking sharks are cannier hunters than previously thought, a report in the Journal of Animal Ecology has claimed.

A tagging study has shown that the massive fish adapt their diving behaviour to foil the escape strategies of their main prey, zooplankton.

It is the first time these sharks have been witnessed deviating from their seemingly rigid routines.

The researchers hope to inform conservation initiatives as well as illuminate basking shark behaviour.

"This has big implications for conservation," said co-author David Sims from the Marine Biological Association (MBA), UK. "With this information [ecologists] can build in the probabilistic estimates for how often you should see a shark in [UK] waters."

Benign giants

At the moment hardly anything is known about these animals' life history
David Sims, MBA
Basking sharks (Cetorhinus maximus) are the world's second largest fish, frequently reaching a whopping 10m in length. They are benign giants, surviving by filtering plankton from the water.

It was hitherto assumed that the sharks followed a regular pattern of swimming behaviour, remaining near the surface through the night and diving into deep waters at dawn. This timetable mirrors the activity of Calanus, a certain zooplankton that basking sharks feed on.

Previously scientists believed the sharks kept to this pattern religiously, perhaps to maintain specific levels of light within their environment.

It was thought "light tracking" might be a sensible strategy because the plankton, which like to avoid visual predators, probably do the same thing.

In an ambitious tagging study, marine researchers managed to track basking sharks in waters around the UK.

They found that sharks in deep waters exhibited so-called "normal" diving behaviour; in other words, they spent the night near the surface and the day at greater depths.

However Dr Sims and his colleagues found, to their surprise, that in tidal areas the sharks followed a reverse pattern: they went down at night and up in the day.

Flexible predators

This finding rules out the theory that sharks are following light levels. Instead, Dr Sims thinks they are actually following the plankton, which are themselves more flexible than previously thought.

A basking shark being tagged, image courtesy of David Sims/MBA
The team tagged sharks to monitor their movements. Image courtesy of David Sims/MBA
It seems that in certain places, such as the western English Channel, Calanus are troubled by a particular predator called an arrow worm. Arrow worms, like most plankton and basking sharks, rise at night and dive during the day. So the plankton reverse their dive pattern to avoid the arrow worms.

And, it now seems, the basking sharks also reverse their dive pattern in these areas to stick with their prey.

"The plankton were exhibiting short term changes but the sharks were able to modify their behaviour," Dr Sims told the BBC News Website.

Although it might seem intuitive that sharks would follow their prey, it has never been observed before. In fact, very little is known about these mammoth predators at all.

Dr Sims hopes this research will help shed some light on the enigmatic creatures, and help with conservation efforts.

"It will help us understand how many sharks there are and how they are affected by fisheries," he said. "At the moment hardly anything is known about these animals' life history."




SEE ALSO:
Divers seek new pictures of shark
01 Jun 05 |  Cornwall
Great white wins more protection
12 Oct 04 |  Science/Nature
Scotland tops basking shark sweep
14 Sep 04 |  Scotland


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