LONDON - Marine experts are calling on the public to report sightings of basking sharks in UK waters this summer.
The sharks are drawn to warm, plankton-rich surface waters off the west coast of Great Britain and Ireland.
These huge sharks are harmless, but experts are also asking people to "keep a respectful distance and enjoy the spectacle".
Basking sharks are protected under European and UK law, so it is illegal to disturb or harass them.
Basking sharks are the second largest fish in the sea; they have a jaw one metre wide and can filter plankton from 1,500 cubic litres of sea water every hour.
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"They're here for most of the summer," said Dr David Gibson, managing director of the National Marine Aquarium in Plymouth. "We're asking people to let us know whenever they see one of these fantastic animals.
Basking sharks are the ocean's second biggest fish, measuring up to seven metres in length.
A large adult male can have a dorsal fin up to 1.5m high, which protrudes from the water when the fish are feeding at the surface.
"We'd also like people to take photographs if they can," said Dr Gibson.
"These animals live for between 30 and 40 years, so [with photos] we might be able to identify individuals that are returning to UK waters."
Researchers at the aquarium also use photographs to spot any signs of damage to sharks' fins that could indicate where the fish might be "coming into conflict" with fisheries.
The sharks visit British and Irish shores as the warming sea surface creates "a healthy soup of nutrient-rich seawater around our coastline". Miniature plants that bloom in the sun-warmed water attract tiny marine animals, or zooplankton. This, in turn, attracts the basking sharks.
It is difficult to predict exactly when and where these plankton banquets will occur, but experts say that hot-spots for shark sightings are the south-west of England, the Isle of Man, south and west Ireland and the Firth of Clyde on the west coast of Scotland.
To allow people to observe the sharks without disturbing them, the Shark Trust has published a code of conduct to be followed in any basking shark encounter.
There are separate checklists for swimmers, boat-users and kayakers, but the key points to note are:
Keep your distance: keep at least four metres between you and the shark so as not to startle it. If you are swimming with other people, stay in a group, but don't invite others over to take a look.
If you're in a boat, turn off your engine (boat propellers are a major cause of serious injury to basking sharks feeding near the surface)
If you have a camera handy, take lots of photos of the dorsal fin and any distinguishable features on the shark, as this may help the researchers identify the individual
Move away gently and quietly and report your sighting to the Shark Trust
A basking shark feeds by filtering 1,000 tonnes of seawater filled with microscopic plankton every hour.
Ali Hood, director of conservation at the Shark Trust, told BBC Nature: "Basking shark are not aggressive, but a fish of that size (mature basking sharks can weigh well over four tonnes) could cause serious injury.
"Basking sharks are extremely strong and surprisingly able to breach clear of the water.
"If people get too close and the shark makes a rapid movement, it could cause harm to both the shark and the person.
"The code of conduct is there to allow people to comply with the law and enjoy seeing these magnificent animals."
But Dr Gibson says he hopes people will take pleasure in witnessing the ocean's second largest fish and says the data gathered from reported sightings is invaluable for marine conservation work.
"Understanding when they first appear and when they leave shows us year on year trends of the plankton blooms.
"This has already given us evidence that habitats are shifting in response to climate change, and that basking sharks are moving north."
People can report basking shark sightings to the Shark Trust via its website.
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