Perhaps the single most extensively used wild-caught fish in sushi, on a global scale, is the Bluefin Tuna. It is so overfished, in fact, that it is very threatened as a species, which in turn is considered a major sustainability risk to the world oceans as a system. A Greenpeace activist once told me that there are more frozen Bluefin tunas than live ones. For this reason, a lot of people, including myself, have started to avoid Bluefin more and more. Now having said that, as long as you’re not eating it everyday, I would be the last person to cast aspersions.