But because of health concerns and growing demand, 50 to 60 percent of sushi in the United States is frozen at some point in its journey from the ocean, according to wholesalers. And rare is the sushi restaurant that tells customers upfront that they may be eating fish that has been in deep freeze for up to two years. Most would be even more surprised to learn that if the sushi has not been frozen, it is illegal to serve it in the United States.

Food and Drug Administration regulations stipulate that fish to be eaten raw — whether as sushi, sashimi, seviche, or tartare — must be frozen first, to kill parasites. “I would desperately hope that all the sushi we eat is frozen,” said George Hoskin, a director of the agency’s Office of Seafood. Tuna, a deep-sea fish with exceptionally clean flesh, is the only exception to the rule.
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“In Japan,” Mr. Kawauchi said, “50 percent of the sushi and sashimi is frozen. Only my American customers are so concerned with fresh fish.”
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The Food and Drug Administration does not enforce the frozen-fish rule, leaving that to local health officials. The agency says sushi fish can be frozen either by the wholesaler or in the restaurant, and each party likes to believe that the other is taking care of it.
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But “superfrozen” fish seems to be in a category by itself. Many top sushi chefs are finding that fish frozen to about 70 degrees below zero, instead of the commercial standard, usually 10 below, can stand up to their rigorous standards.