“There’s one!” Came a voice from the back of the boat, “Ah… sorry, just a dolphin.” I turned around to see the keen-eyed fellow grinning. Normally I’d be pretty pleased with dolphin sightings. In the lea of the low islands of Coll and Tiree the sea was flat calm. We were on the lookout for the fins and snouts of basking sharks (Cetorhinus maximus), breaking the surface as they fed, filtering and growing fat on the rich plankton. We’d left Oban early that morning, via the colourful harbour of Tobermory on Mull for coffee and pastries before heading west. In the clear, calm air we could see all the way to the Cuillin on Skye; as we headed further west the distant bulk of the Highlands came into view.

We were accompanied by Manx shearwaters and guillemots, and we had a spot of a white-tailed eagle as we left Mull. The guillemots were not helping. Parent and chick would float a few metres apart on the surface, two distant blobs in the binoculars that could be a basking shark.

Facebook Twitter Pinterest In the rather chilly waters ... Photograph: Richard Aspinall

I had long wanted to see one of these fish. I’ve dived with sharks in tropical waters, but to swim with one of these giants had long been on my list. “We’ve got one,” said one of the crew with well-practiced eyes. The skipper turned the boat around, killed the throttle and allowed the current to take us towards the fish, keen not to disturb it. The shark was cruising up and down, where the tide had concentrated the plankton into a long line, Its half-metre-tall dorsal fin scything through the water. Countless millions of life forms - from tiny crustaceans like copepods to fish larvae and jellyfish - were giving away their presence through a thin oily film on the surface that hinted at the valuable nutrition slowly being concentrated in a band running parallel to the rocky coastline.

We kitted up slowly, not rushing. Once the sharks find a good feeding spot they tend to stay for a while and are seemingly not discouraged by the boat. They have few natural predators when fully grown, after all, but they can be spooked. Responsible sight-seers and tour operators will take care to avoid disturbance and collision. The water was a bracing fifteen degrees and I felt the chill through my thick wetsuit, but as we finned towards the fish, following directions from the boat, I quickly warmed up. And then, out of the murk the basking shark appeared.

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The water was cloudy with life, small jellyfish easily made out amidst the far smaller beings. The visibility was only three or four metres and there, mouth wide open, its gill rakers pure white, the shark approached - and at quite a speed! I managed to get a few photographs, hoping to get a sharp image. The water was hazy with plankton; indeed, the shark wouldn’t be here if it wasn’t. I kept firing the shutter as the five-metre beauty cruised past. Encounters with the sharks are best when they swim past you, and chasing the fish is not recommended. I suspect I couldn’t have kept up with it if I tried: its slow movements were deceptively powerful and it was gone, but in that brief moment I had my shot.

According to my guide, our shark was a small example: they’ve been seen almost twice as large in these waters, and at one time represented a substantial fishery, with many thousands being landed in the 1940s. Taken for their oily livers, which can make up a quarter of the fish’s weight, basking shark numbers dwindled. A single shark could provide hundreds of gallons of oil for lamps and industrial lubricants and cosmetics - their skin was even used for leather. The fish’s flesh was of little value for eating, giving off a strong smell of ammonia, and was best used for fishmeal.

Facebook Twitter Pinterest Out of the murk, the basking shark appeared. Photograph: Richard Aspinall

Hunting took its toll, though as the value of the oil fell the fishery declined. In an echo of the whale oil industry, petrochemicals eventually offered cheaper alternatives. The fish were legally hunted until the 1990s, and their numbers have yet to recover in UK waters. As a wide-ranging species, they are not protected in all the waters they travel through, and the Northeast Atlantic subpopulation of basking sharks is listed as endangered by the IUCN. The sale of basking shark products has been illegal in Europe since 2007, but this slow-to-mature species is very much at risk from entanglement in fishing nets and being taken for the shark fin market.

Like their tropical relatives, basking sharks are now forming part of the eco-tourism business. From The Western Isles of Scotland to the Isle of Man and down to Cornwall, feeding hotspots where the sharks gather also attract tourists.

Organisations such as the Shark Trust have worked to produce a code of conduct to prevent harassment and disturbance by the shark-spotting industry. Whilst the fish are apparently tolerant, and many conservation agencies recognise the value of ecotourism to shark and wider marine conservation, tourists are encouraged to choose tour operators accredited under the Wildlife Safe (WiSe) scheme. The scheme provides tour operators with training on how best to interact with basking sharks and cetaceans.

Basking sharks may live for fifty years, and what little is known of their reproductive history suggests it may take many years to determine a rise in their numbers. They may not reach sexual maturity until they are twenty years old. To help with conservation efforts, sightings of basking sharks and photographs of dorsal fins can be uploaded to the Shark Trust’s Basking Shark Project website.