The problem is the growing appetite for sushi and sashimi outside Japan, not only in the United States but also in countries with new wealth, like Russia, South Korea and China. And the problem will not go away. Fishing experts say that the shortages and rising prices will only become more severe as the population of bluefin tuna — the big, slow-maturing type most favored in sushi — fails to keep up with worldwide demand.

Last year, dozens of nations responded by agreeing to reduce annual tuna catches in the eastern Atlantic and Mediterranean oceans by 20 percent in an effort to stabilize populations. But the decision only seemed to crystallize growing fears in Japan about tuna shortages, helping to push up prices of the three species of bluefin — northern, Pacific and southern — that are considered the best tuna to eat raw.

Since the start of last year, the average price of imported frozen northern and Pacific bluefin has risen more than a third, to $13 a pound, according to Japan’s Fisheries Agency.

Wholesalers say that competition from foreign fishing fleets and buyers has made the top-quality tuna increasingly hard to come by here. Tadashi Oono, who sells big red slabs of tuna from a stall in the sprawling Tsukiji fish market of Tokyo, said that three years ago, he routinely sold two or three top-grade bluefin every day. This year, he said, he sometimes finds only two or three tuna of that quality to sell in a month.