A glistening black fin slices through the choppy surface of the water and a cry goes up from the yacht's bow: "Shark ahoy!" After hours of waiting it's a welcome sight. Skipper Colin Speedie coaxes our vessel closer to the shark, explaining that it's almost certainly come here to feed - thankfully not on us.
We're aboard Forever Changes, cruising off the west coast of Scotland in tidal waters that race between the Hebridean islands of Coll and Tiree.
Fast currents churn up nutrients and plankton, providing a rich soup for hungry basking sharks. Unlike their man-eating cousins, they are filter feeders, sucking up as much as 20 to 25 kilograms of plankton an hour.
"These kinds of places, where the tide races over rocks and pinnacles on the bottom, are shark paradises," says Mr Speedie.
Around 30ft long when mature, basking sharks are the largest fish found in British waters, but most people don't know they're there. With financial support and volunteers from conservation charities such as the Born Free Foundation and Earthwatch, Mr Speedie spends his summers patrolling the UK coast surveying their numbers and behaviour.
Since 1998, Earthwatch has been organising annual environment awards for teachers; the prize in 2005 was one of four places on Mr Speedie's shark-surveying trip. The aim is to give educationists an experience to share with pupils back in the classroom, an unforgettable story to tell the conservationists of tomorrow.
A fortnight after my trip, the four winners, all science teachers - Jane Meadows from Barnsley college; Claire-Louise Evans from the Colonel Frank Seely school in Nottingham; Jon Hall of Woodlands school in Chester; and Peter Stanley of Coleg Powys in Wales - headed for Scotland's north-west coast to undertake their mission.
Over the following days they sailed around the islands of Skye, Mull and Eigg recording data. Every half hour they noted time, location, vessel bearing, wind direction and speed, sea depth and temperature, tide direction and speed, choppiness, swell height, and weather. "We survey nearly 4,000km a year," says Colin Speedie. "By last year we had 1,440 sharks on our database."
The teachers spotted one basking shark, a minke whale, several harbour porpoise and seals, plus lots of jelly fish. They also used a mesh device to sample plankton, analysing the tiny organisms under a microscope.
The weather was not entirely kind, however, and at one point the yacht had to seek refuge from a force 12 gale. Even in the relative shelter of Arisaig harbour on the mainland, gusts of force 10 strength battered the yacht and its crew. "It was like being inside a tumble dryer," says Jane Meadows. "I was sleeping at the front of the yacht and a couple of times I had to put my hand up to stop me hitting the ceiling because I was being lifted out of my bed." She knows exactly what it means to batten down the hatches.
A few weeks later, Ms Meadows invited me to see how she applied experience in an environmental science class at Barnsley college.
The class begins with pupils discussing why conservation is important.
Their concerns range from ecological stability and sustainability, to loss of culture and ecotourism, which gives Ms Meadows her cue to cite the basking shark as an example of an endangered species. She explains that the sharks are listed in the Convention on International Trade in Endangered Species of Wild Fauna and Flora, which means they are not yet threatened by extinction but will become so if their trade is not subjected to strong regulations.
She highlights her points with photographs and anecdotes from her trip. The class concludes with the students each selecting an endangered species to study and prepare a presentation on. "It was so good to go back to that kind of experience after 25 years," Ms Meadows says in the staffroom afterwards. "It's rekindled my enthusiasm and made me more excited about what I'm doing."
As well as offering the four teachers a chance to carry out unusual research, Earthwatch gave each a grant to help them initiate a conservation project in their areas. Barnsley college gave Ms Meadows permission to work on a piece of derelict land and, with the help of her students, she has already begun turning it into a conservation area.
"Being involved with the shark project has helped me realise just how important conservation is," she says. "I want us to do something in college and then in the local community. We have links with the Countryside Restoration Trust, which has a farm and woodland area near here and which needs volunteers to help with tree planting and dry-stone walling. I want to give the pupils who are keen as much hands-on experience as possible.
Ultimately I want them to see that what they do at a local level can have an impact at a global level."