Which fish is traditionally used in sushi more than any other?

Tuna, and it is a modern taste rather than a traditional one. Raw tuna was low-status in Japan into the twentieth century - the fatty belly cuts now most prized were often discarded or used as cat food, because without refrigeration the oily flesh spoiled fast and the fish was considered inferior to white-fleshed alternatives.

Two things changed it. Freezing technology made the fat an asset rather than a liability, and in the 1970s Japan Airlines began filling empty cargo holds on return flights from North America with Atlantic bluefin that nobody there wanted - creating a supply chain and, with it, a market. Global demand followed Japanese cuisine outward.

The consequence is a fishery in serious trouble. Atlantic bluefin can exceed 400 kilograms, are warm-blooded to a degree unusual in fish, cross oceans, and take years to mature - all of which makes them easy to overfish and slow to recover. Populations fell by enormous margins before quotas were tightened, some have since improved, and a single fish sold at Tokyo's new year auction has gone for over a million pounds, which is a price signal pointing in exactly the wrong direction.