In those first moments when the fishermen spot the unmistakable signs of a tuna shoal, everything changes. The inky entity that is the Indian Ocean suddenly reveals the life beneath its surface. Yellowfin tuna, the third largest in the tuna family after bluefin and big eye, are usually accompanied by dolphins. We see their dark backs curving in and out of the water about 100 yards away, and the boat turns towards them. Birds are also circling the area, another sure indication that there are tuna below.

On the 90ft dhoni (fishing boat) manned by 17 fishermen, led by skipper or ‘keyolhu’ Adam Mohammed, there is a rush of activity. Live bait – trigger fish, sprats and mackerel, plus some unfamiliar fish local to the Maldives – are scooped out from a large tank beneath the boat, hooked on each fisherman’s line and dropped over the side. There are no rods or reels. The fishermen don gloves and rubber socks. If a fish is caught, it will be pulled in by hand and killed when rolled on to the boat. But this morning there’s no need. The yellowfin are not biting.

We had left Hanimadhoo harbour at 6am searching for both yellowfin and the smaller species, skipjack. Hanimadhoo Island is in the undeveloped far north, an hour’s flight from the capital, Male, and nearby coral islands with their paradise hotels and incumbent honeymoon couples. But it shares an extreme beauty – the astonishing turquoise of the shallow lagoons, white sand and green coconut palms. Many islands in this area are uninhabited or devoted to boat-building and fish-processing.

Tuna itself is revered by the Maldivian people. Skipjack is eaten with every meal, either salted and dried (known as ‘Maldives fish’) or curried. It is the islands’ only plentiful source of animal protein, and along with coconut one of the few foods the country produces. The 1,192 islands of the Maldives amount to only about 180 square miles of land, little of which can be cultivated. Most of the islands’ food is imported.

There are two Maldivian fishery bosses on board the dhoni: Nashid Rafeeu of Big Fish, and Yasir Waheed from Cyprea Marine Foods. ‘The yellowfin and skipjack tuna fisheries are integral to the Maldives,’ Waheed says. ‘It is a tradition passed down through families; we have never changed the way we fish: on lines with live bait.’ There is much to protect; fishing represents 30 per cent of industry here. Hi-tech methods, which damage fish stocks, have never been permitted within the 200-mile exclusion zone around the island, protecting its resources.

I had travelled to the Maldives with the British seafood importer Fred Stroyan and Paul Willgoss, the technical director of Marks & Spencer. Stroyan supplies the chain’s food halls with fresh yellowfin tuna, and M&S also sources canned Maldivian skipjack tuna. Willgoss oversees 68 of the 100 M&S ‘Plan A’ initiatives for sustainability, which include recycling waste, ethical trading and animal welfare, plus a sustainable sourcing policy for fish. In 2009 M&S was the first British company to sign up to the World Wildlife Fund’s seafood charter, committing to source all seafood sustainably by 2012 – so far the chain has a good record, sourcing white fish, organic tiger prawns, gurnard and MSC-certified wild Alaskan salmon. Plan A’s objective is a very tall order, watched with much interest by other chains, environment experts and the fishing industry.

The involvement with Fred Stroyan’s company, New England Seafood International (NESI), is a wise one. Stroyan, a keen fisherman himself, has 10 years’ experience working with sustainable fisheries and importing to Britain, notably fresh tuna (since 2003) and MSC-certified wild Alaskan salmon. ‘I had seen what happened with UK and European fish stocks,’ says Stroyan, who spends more than five months a year visiting fisheries that supply NESI. ‘Being a fisherman myself I was passionate about this and we have always worked in tuna fishing areas that are artisanal. It is always better-quality fish as a result.’

Tuna made headlines last year with the release of the film The End of the Line. Its focus was on the safety of the bluefin, the favourite sashimi and sushi fish of the Japanese. Bluefin is classed as endangered. At the time some press reports implied that all tuna were bluefin, canned, in sushi, in sandwiches. But this tuna is almost always skipjack or yellowfin, both available from sustainable sources.

Yellowfin is the viable alternative to fresh bluefin. Reaching weights of up to 440lb, yellowfin are found in all tropical and subtropical waters, but not in the Mediterranean. The appetite for fresh tuna in Western countries has encouraged fishermen to hunt using hi-tech methods that are not permitted in the Maldives. Most notorious are the purse seine nets, up to three miles long, used to encircle and ‘bag up’ huge numbers of fish.

‘It can take up to three hours to draw in a purse seine net,’ says Cesar Basalo, who audits the quality of fish for NESI. ‘The fishing boats pull the net tighter and tighter, crowding the fish, which will be fighting on top of each other. Some die as they fight; the surface water will be red with blood and full of floating body parts.’

This method is also indiscriminate, killing more than one species. Such fishing results in tuna of a much lower grade

‘It is pretty horrific when hundreds of tons are caught, and these boats are capable of doing this three or four times in a day,’ Stroyan says. This method is also indiscriminate, killing more than one species. Such fishing results in tuna of a much lower grade. ‘Tuna must be killed quickly or they produce lactic acid in the muscle,’ Basalo says. ‘The meat turns brown with a rainbow sheen and cooked appearance.’ In the international waters outside the protected fishing grounds, a bizarre protection from the purse seiners has sprung up in the form of Somali pirates, renowned kidnappers and boat thieves.

Yasir Waheed and Nashid Rafeeu run separate fishing companies but work together and are also good friends. They share processing facilities in the Maldives and operate boats. The dhoni are low and wide, built from fibreglass, with a vast tank underneath to carry the live bait. The water inside the dhoni gives the vessel an uncomfortable gait and it rocks like a moving hula-hoop on the Indian Ocean. We are 15 miles offshore, not an atoll in sight. We had breakfast shortly after leaving; a dish made by the fishermen containing grated coconut, cooked skipjack, lime and chilli, served with roti (flatbreads) and hard-boiled eggs. It was one of the most delicious tuna dishes, and breakfasts, I have had.

There is a shoal of skipjack ahead and two boats have already arrived on the scene. In the Maldives, the smaller skipjack are caught by a different method to the large yellowfin: pole and line. As the boat slows the fishermen gather at the back of the boat and turn two water sprays on the water’s surface. Two of the crew begin to throw bucketfuls of live sprats over a wide area. ‘They are creating a feeding frenzy,’ Stroyan says, picking up a 12ft bamboo pole with a small barb-less hook and a feather attractor. When the fish, confused by all the activity in the water, bite, the fishermen yank the poles over their shoulders and the fish, not more than 12-20in long, slip off the hooks and are flicked on to the boat. Each time the poles are lowered back into the water, more fish bite. ‘They could fish here for hours, catch several tons of fish and still make an impact on only 10 per cent of the shoal,’ Stroyan says.

Our day ends without the sight of a fisherman playing a yellowfin on his hand line, testament to the minimal impact of fisheries on the tuna population. There are mutterings about women bringing bad luck to boats, but forgiveness when the crew settles down on the journey back to sing, drumming water bottles. ‘They are singing about their wives, who are unfaithful when they are away,’ Rafeeu says.

On the landing stage of another island with a processing plant, a skipper waits in suspense as 20 yellowfin are taken from his boat’s ice boxes, then weighed, temperature-tested and graded. Basalo inserts a sashibo, a slim tool that takes a sample of flesh. ‘Clarity and good colour earn the fish an A or B grade; a fish that has not been landed quickly, which has lactic acid in the flesh, is a C. The flesh will be like this one, opaque and pale,’ he says. Fishermen are paid less for low-grade fish – one third of the full price. C-grade fish are rejected for the British market.

‘In the Maldives the methods are sustainable but more care is needed when landing the fish on the boats. It needs to be done quickly, yet not change the tradition of hand-lining.’ Stroyan is keen to see the introduction of electronic reels to the Maldives, to boost the number of fish they can export. ‘This is very important, it means they can bring in a fish without a struggle and it will be on ice in no time.’

The quality fish are divided into loins inside a state-of-the-art, well-scrubbed plant. Vacuum-packed, they are dispatched to Britain via BA passenger planes – returning honeymooners sit above next week’s tuna niçoise. ‘Fish that is caught on a Wednesday will be in M&S stores within four days,’ Stroyan says, ‘and all is traceable back to the boat.’ He estimates he is now bringing 700 tons of yellowfin from the Maldives each year.

The British market has become essential to the Maldivian economy. This is the cottage industry that grew up. ‘The Maldives have an opportunity to become iconic in the way they manage their fishing,’ Paul Willgoss says. ‘It is up to us to help them increase their returns and take the earnings back to the people of these islands.’

Fresh yellowfin tuna is available from Marks & Spencer; all M&S canned skipjack tuna is pole-and-line-caught from the Maldives

Tags: big fish, bigeye, bluefin, m&s, maldives, marks and spencer, skipjack, stroyan, tuna