But over the last four years sightings in the South West have dropped by 66%, while they have shot up by an almost equal amount, 65%, in Scottish waters.

The sharks are also arriving earlier in Scotland, with the first sightings this year in May.

They are not usually seen in large numbers in northern waters except during the peak of the summer in July and August.

The jury is out on whether the shift in populations can be interpreted as a sign of climate change, however.

The Marine Conservation Society (MCS), which coordinates a nationwide tally of the massive sharks, believes it may play a role.

"After 17 years of public participation, Basking Shark Watch is now providing exciting insights into the large-scale and long-term movements of these spectacular creatures," said Dr Jean-Luc Solandt, MCS biodiversity policy officer.

"While our analysis continues, the results so far indicate that Britain's basking sharks may be responding to climate change.

There is already undisputed evidence that rising sea temperatures are affecting the distribution of the plankton on which the sharks feed, with increasing blooms in the north.

"That may in fact be making Scottish seas more favourable for the sharks," said Dr Solandt.

But some argue that the data collected by the MCS's survey is of limited scientific value, as not all sightings are reported and an increase or loss of public enthusiasm for the survey in a particular area could skew the results.

It is also possible that the results simply reflect the natural ebb and flow of populations in particular areas, witnessed by scientists over generations.

By Sam Bond

 

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