OMA, Japan — Fishermen here call it “black gold,” referring to the dark red flesh of the Pacific bluefin tuna that is so prized in this sashimi-loving nation that just one of these sleek fish, which can weigh a half-ton, can earn tens of thousands of dollars.

The cold waters here once yielded such an abundance of bluefin, with such thick layers of tasty rich fat, that this tiny wind-swept seaport became Japan’s answer to California’s Napa Valley or the Brie cheese-producing region of France: a geographic location that is nearly synonymous with one of its nation’s premier foods.

So strong is the allure of Oma’s tuna that during the autumn fishing season, tens of thousands of hungry visitors descend on this remote fishing town, located on the northernmost tip of Japan’s main island of Honshu. On a recent Sunday, dozens of tourists, filmed by no fewer than three local television crews, crowded into an old refrigerated warehouse on a pier where Oma’s mayor presided over a ceremony to slice up a 220-pound bluefin into brick-size blocks for sale.

“This is a pleasure you can only have a few times in your life,” said Toshiko Maki, 51, a homemaker from suburban Tokyo, as she popped a ruby-red cube of sashimi into her mouth.

But now the town faces a looming threat, as the number of tuna has begun dropping precipitously in recent years because of overfishing. This has given Oma another, less celebrated distinction, as a community that has stood out by calling for greater regulation of catches in a nation that has adamantly opposed global efforts to save badly depleted tuna populations.

Just a decade or two ago, each boat here could routinely catch three or four tuna a day, fishermen say. Now, they say Oma’s entire fleet of 30 to 40 boats is lucky to bring in a combined total of a half-dozen tuna in a day.

The problem, they say, is that all the fish are being taken by big trawlers that come from elsewhere in Japan, or farther out to sea from Taiwan or China. Some of these ships even use helicopters to spot schools of tuna, which they scoop up in vast nets or catch en masse with long lines of baited hooks. According to local newspapers, there have been repeated incidents of small fishing boats from Oma and other ports intentionally cutting such trawl lines.

“I’m furious at Tokyo’s bureaucrats for failing to protect our tuna,” said Hirofumi Hamahata, 69, the president of the Oma fishermen’s co-op, who has worked as a commercial fisherman since age 15. “They don’t lift a finger against the industrial fishing that just sweeps the ocean clean.”

Such flares of temper are rare in normally reserved Japan, and especially in conservative fishing communities like this one. But this is a town fiercely proud not only of its tuna, but also of how it catches them: in two-man open boats, using hand-held lines and live bait like squid.

Mr. Hamahata described catching tuna in this traditional way as a battle of wits against a clever predator that he called “the lion of the sea.” After hooking one, the contest becomes a battle of strength: he said it typically took one or two hours to pull a big tuna close enough to the boat that it could be stunned with an electric charge.

In one Hemingwayesque battle, Mr. Hamahata said he fought for 12 hours with a huge bluefin that finally broke free.

Despite such difficulties, Oma’s fishermen said they preferred their generations-old fishing method because it allowed them to catch just large, adult fish, leaving the smaller young ones to sustain local stocks.

Fishing experts say the overfishing is a result of a broader failure by the Tokyo authorities to impose effective limits on catches in its waters. Indeed, Japan, which consumes some 80 percent of the 60,000 tons of top-grade tuna caught worldwide, has lobbied hard against efforts to limit tuna catches, such as are now being proposed by European countries for the Atlantic Ocean.

“There are too many entrenched interests whose objective is maximizing profit, not sustainable use,” said Masayuki Komatsu, an expert on the fishing industry at Tokyo’s National Graduate Institute for Policy Studies.

In Oma, catching a big tuna has become rare enough — and the market price high enough — to be cause for celebration. On a recent evening, family members rushed to the pier to greet one boat that had caught a 410-pound bluefin, whose tear-shaped body had to be hoisted off the boat’s deck with a forklift.

Moving quickly to gut and ice the fish to preserve its value, workers from the fishing co-op presented the footlong dorsal fin as a trophy to the captain’s wife, who said it was the first catch in 10 days. The workers said the fish would fetch more than $10,000 at Tokyo’s Tsukiji Fish Market.

“Catching a tuna is like winning the lottery,” said another fisherman, 23-year-old Takeshi Izumi, who said his boat had yet to catch a tuna this season.

To maximize prices, Oma has registered its name as a trademark that can be used only with tuna brought ashore here. This has made Oma a brand that is gaining recognition even outside Japan. In March, a sushi chef from Hong Kong paid some $50,000 to buy half of a 280-pound Oma bluefin.

The prices can be even higher: In 2001, a Japanese buyer paid a record $220,000 for a 444-pound Oma bluefin.

One unfortunate side effect, said the town’s mayor, Mitsuharu Kanazawa, was that few of Oma’s 6,200 residents can now afford their own town’s tuna. However, he said the fish have been a boon to the town’s economy, pumping in some $15 million a year from fishing and tuna-related tourism.

After a popular 2000 TV drama featured Oma, the town increased tourism by starting a three-day tuna festival every year in mid-October, which now draws 15,000 visitors a day, as well as hordes from the Japanese media, Mr. Kanazawa said.

“We Japanese have a weakness for brands,” said Ryuko Nishimura, 43, a homemaker from Kuroishi, a three-hour drive away. “It makes the tuna taste two or three times more delicious.”

But with tuna now in danger of perhaps disappearing, the mayor said the town was struggling to find another local product to keep the tourists coming.

“We tried kelp and abalone,” Mr. Kanazawa said, “but nothing has the appeal of tuna.”

By MARTIN FACKLER