Higher than average sea temperatures could explain the slow start to this year’s basking shark season, according to analysts.
Despite a spell of settled weather and good visibility, even the most keen-eyed wildlife watchers have been looking out across curiously empty seas around the island.
Manx Basking Shark Watch (MBSW) co-ordinator Jackie Hall told the Examiner: ‘We’re now halfway through our season and we’ve had only 38 sighting reports, and never more than four sharks at a time. By this time last year we’d had 146. ‘
The numbers are a far cry from 2009, the best year on record, where there had been an incredible 448 sightings at this point in the season.
The key to the mystery is plankton, the microscopic waterborne organisms that basking sharks feed on by filtering seawater.
There are two distinct types of plankton: phytoplankton is plant material, while zooplankton, the basking shark’s preferred food, is animal.
‘Simply put, the plankton’s completely wrong – there’s no food on the table for sharks,’ explained Jackie. ‘There is lots of plant plankton out there, but the sharks want the animal plankton and it’s not here.
‘These are migratory animals and if the food isn’t there, they’ll simply move on.’
High sea water temperatures around the Isle of Man could account for the lack of zooplankton, according to Dr Kev Kennington, marine monitoring officer at the Department of Environment, Food and Agriculture (DEFA).
The sea temperature in June was a degree Celcius higher than the 30 year average, possibly due to the mild winter and warm spring.
The two types of plankton bloom separately, with plant plankton responding to light and zooplankton linked to sea temperature.
Dr Kennington suggested that the higher temperature may have triggered an early bloom and subsequent collapse of the population.
Meanwhile, levels of plant plankton are higher than have ever been recorded in 50 years of monitoring by the DEFA.
This year’s figures might be startling but Jackie confirmed that the number of sharks can vary. She said: ‘When we started the MBSW 10 years ago, local mariners told us that in some years the sea was full of sharks – they said there were so many you could almost jump from one to the other – but in other years there would be virtually none. It looks like we’re having a year where conditions are wrong, even though the weather has been perfect for sightings.
But I’m optimistic – I think at some stage this summer it’ll come good for a few days.’
Jackie and her team of volunteers aboard Happy Jack, the project’s research vessel, may only need a few good days to complete their mission: attaching three satellite tags to sharks to monitor their movements over the coming months, and creating more shark ‘passports’ with photo and video footage to help with future identification.
The improved satellite tags can now keep transmitting for months, possibly years, and the public can follow the tagged sharks online at www.wildlifetracking.org
MBSW have renewed their appeal to the public to report any sightings: ‘Reports from the public have always been a key part of our work, and this year they might be more important than ever. We ask that people keep looking, especially in the south and west of the island, and report any sightings to us at www.manxbaskingsharkwatch.com as soon as possible.’