Bluefin tuna - the staple of sushi boxes the world over - is on the brink of extinction, with its plight highlighted in a new film. Sandwich seller Pret A Manger is changing its stance on tuna, but should the rest of us follow suit?
The bluefin tuna is one of the world's most impressive fish - a predatory cruiser of the open ocean that can zoom through the sea at the rate of 40mph and grow to the size of a small car.
The bluefin has become the unwitting star of the campaign to end overfishing. The End of the Line, a documentary based on a book by journalist Charles Clover, examines the mismanagement of bluefin fisheries as a microcosm of how we are hoovering the seas of their fish.
The bluefin is particularly prized in Japan. And the explosion in the market for sushi - which 25 years ago was a relative rarity outside Japan - has created a global market for the three species of bluefin, as has the taste for omega-3-rich tuna steaks to be pan-fried or thrown on a barbecue.
"Bluefin tuna has become the poster boy for the overfishing campaign. It's on the buffers - it's really on the slide down now," Clover says.
"There are no large tuna anymore. There were bluefins of 250lb in Japanese fish markets when I went there four years ago - there are none now. A third of the catch is undersize."
1. Fish shoals are encircled by a large "wall" of net
2. Net is closed off at base, like a purse
3. Fish captured along with a bycatch of non-target species, including bluefin and dolphins
The backlash against those selling bluefin has already begun - a raft of celebrities, including actress Charlize Theron and Sting, have barracked high-end fusion restaurant chain Nobu for still offering the critically endangered bluefin - some have likened it to offering a dish made of tiger or white rhino.
Let's be clear - bluefin is not the tuna you add to your jacket potato or rustle up a tuna pasta bake with. The tuna that finds its way into cans is usually the much more common skipjack variety.
But even the catching of skipjack impacts on the bluefin. Skipjacks are caught using purse-seine nets, which encircle an area before being pulled together in the shape of a purse.
Rod and line
"The trouble is," Clover says, "the skipjack run with all these other tuna species, like bigeye and bluefin. The skipjack are close to the surface and the bluefins swim further down, so there is often bluefin bycatch."
The solution may be to buy canned tuna which is caught by pole and line - fishing vessels with rods and lines. This method is much less wasteful, with fewer fish dying from bycatch - and underage fish can be returned to the sea.
Clover's film, which had its premiere in the UK on Monday, has already had an effect on another high-street name, the Pret A Manger sandwich chain. Co-founder Julian Metcalfe was so incensed when he saw the film he made the chain change its policy on tuna.
Out went the yellowfin tuna from the sushi - less endangered than bluefin, but still overfished. The tuna and cucumber sandwiches are - from July - to use only supplies from a sustainable rod-and-line fishery from the Maldives, says the chain's head of sustainability, Nicki Fisher.
"Our skipjack was dolphin-friendly but was still caught by purse seine nets. The dolphins could get out of the nets, but the sharks, the turtles and the bluefin couldn't. Seeing the film has really galvanised us into action."
Other supermarkets have also been taking action.
Supermarket changes
Marks & Spencer made its big announcement on Tuesday, boasting it is the first of the supermarkets to switch to pole-and-line-caught tuna, not just with fresh and canned but also in sandwiches and salads. The retailer sells 20,000 tuna sandwiches per day.
Sainsburys switched a while back to using pole-and-line-caught for all own brand canned and fresh tuna, while Morrisons says all own brand fresh, and some canned, is pole-and-line. Waitrose is introducing an own brand pole-and-line canned range and Co-op has 60% of its own brand as pole-and-line, Greenpeace says.
But the environmental campaign group has criticised the two big tuna suppliers Princes and John West for not making the switch.
"John West tuna is largely made of skipjack tuna caught using a combination of pole-and-line and purse seine fishing. Global stocks of skipjack are scientifically recognised as being healthy and are not overfished," says John West board member Adolfo Valsecchi.
Mr Valsecchi suggests the firm does its utmost to minimise bycatch and promote sustainability.
Moving to pole-and-line across the skipjack market would help eliminate the issue of bluefin killed because of bycatch, but it would not get rid of the wider problem.
Efforts to regulate the fishery in the Mediterranean - where the Atlantic population of bluefin migrates to spawn - have failed because the Sicilian and Neapolitan mafias have a stake in the fisheries, and have little compunction about putting profit ahead of sustainability. Illegal tuna worth more than $6bn (£3.2bn) is thought to have made its way to Japanese markets in the 20 years up to 2006.
Also, tuna spend their life on the move, and range over a number of territorial waters to and from spawning grounds, which has made the management of the fishery by the likes of the European Union so difficult.
Giles Bartlett, the fisheries officer for the Worldwide Fund for Nature (WWF), admits that while the WWF was able to make an icon of some endangered animals, like the panda, a fish may be a more difficult sell.
"That's the real challenge we face. What we try and do is link it to everything else in the sea, like turtles and whales and big sharks," he says. "It's not just about sustainable fishing, it's about how we look after the oceans as well."
What Mr Bartlett says will save the bluefin is a better fisheries management system - at the moment the Mediterranean fishery continues to fish thousands of tonnes more each year than even the International Commission for the Conservation of Atlantic Tunas (ICCAT) scientists say is allowable.
He says centralised management, with quotas revised once a year, will not work. Fishermen have to agree quotas, and have the flexibility to manage them over five or 10 years.
For the moment, though, sushi lovers cannot look forward to a guilt-free portion of bluefin.