Sunrise is at least an hour away when Atsushi Sasaki steers his fishing boat out of Oma and into the notorious straits separating Japan's mainland from its northernmost island, Hokkaido.

By the time he reaches the open water of the Tsugaru Strait, the wind has turned into a gale and the waves grow higher with every assault on the bow of his boat.

But Sasaki, a wiry 61-year old with a crewcut and the teak complexion of an inveterate fisherman, is unfazed: even the discovery that the coolbox containing his lunch is now flooded with seawater is accepted with a shrug. For now, his concern is directed solely at his prey: the bluefin tuna.

Global stocks of the highly prized fish have plummeted by 90% in the last 30 years, and much of the blame rests with Japan, by far the world's biggest consumer. Every year the Japanese get through about three-quarters of the world's bluefin catch; 80% of tuna caught in the Mediterranean ends up on the Japanese market.

Faced with the imminent collapse of bluefin stocks, fisheries officials from 45 countries are meeting in Morocco this week to discuss bluefin quotas for the Atlantic and Mediterranean next year. Conservationists want a moratorium, but Japan is reportedly about to support a scientific panel's recommendation that the quota be set at 15,000 tonnes, about half the current level.

But while attempts are being made to rescue bluefin tuna populations in seas thousands of miles away, nothing is being done to prevent Japan's appetite for tuna sushi and sashimi from ripping through stocks along its own coastline.

But Sasaki is not part of Japan's overfishing problem. Rather, he could be the solution. There are no trawler nets or lines coiled in heaps on his boat (named, with incidental irony, Man'yu, or Ten Thousand Tuna). He is one of barely 200 ippon-zuri fishermen around Japan, who catch tuna sustainably using a combination of a rod and line, a basic sonar and occasional luck.

The former salaryman, who quit his office job 20 years ago to lead the life of an itinerant fisherman, is a regular visitor to Oma, one of just three places in Japan where the method survives.

In an attempt to prevent the tradition from dying out and to protect local stocks from being fished into oblivion, the local authorities have assigned the Tsugaru Strait for the exclusive use of Oma's 60 rod-and-line fishermen.

The move has met with mixed results. The ippon-zuri have become embroiled in a row with longline fishermen who violate the exclusion zone by using baited lines often several miles long. Elsewhere, trawlers, equipped with sophisticated sonar, plunder coastal waters, aided by the absence of official quotas and collusion between politicians and the powerful fishing lobby.

High fuel prices, lower profit margins and stricter quotas in other parts of the world have created an irresistible urge for Japanese boats to take more bluefin from their own waters. And all the time demand is growing, not only in Japan, the US and Europe, but increasingly in China and Russia.

"Japan's fisheries have no idea how many tuna they are catching or what size they are," says Sasaki, in the smoke-filled cabin of the Man'yu. "The smaller tuna have all been caught, along with the fish they feed on, and unregulated fishing with trawlers is to blame."

Faced with official diffidence and scant popular enthusiasm for conservationism, Sasaki is spurred on by relatively low operating costs and the knowledge that he is playing a small part in a nascent interest among the Japanese in sustainable sushi.

"We need proper stock management," he says. "Collapse is just around the corner."

The bluefin tuna caught off Oma, a town of 6,000 people on the northern coast of Aomori prefecture, are seen as the tastiest in Japan and typically fetch twice as much as imported fish at auction. In 2001, a 202kg (445lbs) Oma bluefin sold for a record ¥20.2m (£141,400).

The yearly average catch for Oma is 2,500 tuna, worth about ¥1.6bn (£11m) to the local economy. This is tiny compared with a few decades ago, says Hirofumi Hamabata, head of the town's fishing cooperative. "After the war, each boat returned with about half a dozen tuna every day," he says. "They were so cheap you'd have to sell 4kg of fish just to be able to afford a pack of cigarettes."

Akihiro Furukawa, a longline fisherman for 13 years, admits he fears for the future: "My son wants to follow in my footsteps, but by the time he's old enough to go to sea, there won't be any fish left to catch."

In the Tsugaru Strait it is usual to see 150 boats fishing for tuna. Today, though, the weather has put most fishermen off. And after several hours at sea on an empty stomach, Sasaki is ready to call it a day. As darkness descends on Oma, another ippon-zuri fisherman who has had better luck returns. Watched by groups of children, six tuna weighing up to 100kg are unloaded and packed into wooden vats of crushed ice, ready be driven to the Tsukiji market in Tokyo before dawn. The fish may well fall under the gaze of Toichiro Iida, a wholesale trader who seeks out Oma tuna at auction. His family firm, Hicho, has been in business for almost 150 years.

He says many of his fellow traders know nothing about the provenance of their tuna."They're just happy to buy the cheaper fish and make easy profits, but to do that they have to buy tuna that has come off a trawler," says Iida, who counts Tokyo's best sushi chefs among his clients. "Even some sushi restaurateurs don't know if their tuna is caught using nets or by more sustainable methods," Iida says. "It is about time they learned."

Backstory

The Japanese eat 600,000 tonnes of tuna a year - about a third of the total fished worldwide, and about three-quarters of the total bluefin fished worldwide. In 2006, Japan mported 44,000 tonnes of bluefin, just over half of it from the east Atlantic and Mediterranean. According to the International Commission for the Conservation of Atlantic Tuna, which meets in Marrakech this week, about 61,000 tonnes of bluefin tuna were caught in these seas last year - more than double the permitted catch of 29,500 tonnes. The commission has set a target of 25,500 tonnes by 2010, but many experts believe this should be nearer 15,000 tonnes. The Blue Ocean Institute's guide says bluefin tuna should be avoided altogether. Some restaurants, such as the Moshi Moshi chain in the UK, have removed bluefin from their menus.

Justin McCurry