The numbers of basking sharks so far recorded this year by the Hebridean Whale and Dolphin Trust are believed to be the lowest ever for the same period.
So far just 14 sightings - most of them individuals - have been recorded by the trust, though a group of 37 was seen west of Barra on Sunday by Tobermory-based Sea Life Surveys.
In the same period from January to mid-July last year there were 42 sightings.
The trust admits it is baffled by the apparent disappearance of the world's second biggest fish from Scotland's west coast, one of its most popular haunts.
Changes in the pattern of the distribution of plankton - which the sharks feed on - could be the cause.
"If the plankton is further offshore then the basking sharks will follow it making sightings less likely. Because there are fewer sightings so far this year does not necessarily mean there are less basking sharks about. It is just that they are harder to see," said Conor Ryan, sightings officer with the HWDT.
"It is a bit of a mystery why there have been so few sightings this year. The conditions are quite favourable for them and we have had good sightings of other species like minke whales and various dolphins. August is the peak month for sightings of basking sharks so we are hoping to get more records then."
Only last month a major dating and mating site for basking sharks was revealed by scientists.
Seas between the islands of Skye and Mull on Scotland's west coast are highly important as a place for basking sharks to find a mate, according to a report published by Scottish Natural Heritage. Each year large numbers of basking sharks are seen in an area of the Sea of the Hebrides which is currently being assessed as part of the Scottish Marine Protected Areas Project.