Marine experts say the UK’s first officially recorded basking shark sightings of the year have come in from Cornwall.
Scientists believe this could be due to a recently reported 50-mile (80km) long algal bloom off the Cornish coastline.
Although this is not their direct food source, zooplankton at the site could be attracting the sharks.
Divers spotted the gentle giants off Newlyn and holidaymakers on the Roseland also reported a sighting.
Marine biologist Professor Monty Priede, from the University of Aberdeen, said: “We know from tracking studies on the sharks that they are very effective at finding patches of zooplankton and, if around this algal bloom the zooplankton are there, then that is where the sharks will be.
“The plankton bloom may have been triggered by the fact that it was a rather stormy winter which stirs up more nutrients in the water, which can kick off the spring bloom to be much richer than it might otherwise be.”
He added that basking sharks tended to follow their food source into deeper water during the winter months and appeared on calm days in the midsummer when they fed near the surface.
Hotelier Tim Brocklebank watched a shark from the beach at Portscatho.
“I’ve never seen them this early, the sea is still pretty cold and its not really until June or July that we expect to see them,” he said.
“He was rising up out of the water a little bit so we saw him quite easily.”
Ruth Williams, Cornwall Wildlife Trust marine manager, said divers were often the first to spot the sharks.
“It’s not unusual for divers to see them cruising at that depth,” she said.
“The first sightings can come in this early, the bigger numbers start to come in April or may, when the waters really start to warm up.”