The large, harmless sharks are normally a common sight off the west coast at this time of year, particularly in the outer Firth of Clyde and off Coll, Tiree and Canna.
However, the Marine Conservation Society (MCS) has received hardly any reports of sightings so far this year, and believes this could be down to the cooler-than- average sea in May.
Jean-Luc Solandt, the society's senior biodiversity officer, said divers were reporting water temperatures of 10˚C or 11˚C when it should be nearer 13˚C.
Dr Solandt said: "This means the plankton, which is the basking shark's favourite food and the reason they come to our waters, are not blooming in the usual quantities so basking sharks are staying in warmer seas to feed."
The basking shark – which can grow to the size of a double-decker bus and is identifiable by its large, broad dorsal fin and sweeping tail fin – is the world's second-largest fish after the whale shark, but relatively little is known about the species.
Adult females give birth about once every four years to five or six pups, and their three-year gestation period makes the populations extremely vulnerable to over-fishing.
Last year more than 170 sightings off British waters were reported to the MCS. A project by Scottish Natural Heritage to tag the species in Scottish waters has just been extended and has found the sharks are travelling in some cases up to 1900 miles before reaching the water off the west coast.
News about the delay in the arrival of the basking sharks follows reports in April that Scottish chefs could not find enough lobster because the cold weather meant they were still hibernating.
The MCS said it did expect to see the numbers of basking sharks increase in the com- ing months. The society runs Basking Shark Watch, a database of publicly reported sightings.
Dr Solandt said the society was also keen to hear of any sightings made by members of the public but warned that, despite their size, the creatures could be hard to spot and recognise.
"Although the basking shark is the world's second-largest fish after the whale shark, if you haven't seen one before it can be quite tricky to work out if what you've just spotted is in fact a basker," said Dr Solandt. He said there were certain key signs to look out for – the large, broad dorsal fin and sweeping tail fin breaking the surface, the snout often breaking the surface when feeding and the wide, circular gaping mouth that is clearly visible when feeding.
"With so many people carrying smart phones these days, it's easy to go straight to www.mcsuk.org, record your sighting and take a picture and upload it directly to our Facebook or Twitter pages, all within moments of seeing a basking shark."