“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.

Image A fisherman unloading an increasingly rare tuna catch. Credit... Ko Sasaki for The New York Times

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.”