Sushi/sashimi (doesn't matter - it's still uncooked fish) is always frozen to a very deep temperature to kill parasites, including tapeworms which would be an almost sure-fire 'souvenir' from the restaurant if it wasn't.

There is a very high end sushi bar in New York which has its own deep freeze blast freezer which keeps their fish at incredibly low temperatures and perfectly 'fresh' (tasting/texture) for a couple of years so they can "buy low, sell high" as availability and prices fluctuate. Apparently they buy the fish from Fulton St when it's perfectly fresh and hours old, then chill it to some crazy low temperature in this machine - when it comes out and thaws, the taste and texture matches the fish when it went into the freezer.

Even blast frozen, if the thawed fish is not properly cared for by the restaurant, it becomes infested with bacteria that will cause another 'problem' after dining - a reason why you should never dine in a sushi restaurant that has an obvious 'fishy' odor, or the fish is mushy when served.