Generally speaking, if a fish smells fishy, it has started to go bad. That said, Epicurious points out that a fish will start to smell fishy in the fridge, even if it’s technically not bad. To prevent this, make sure that you eat fresh yellowtail if you’re going to have it as sushi. If you plan on cooking, make sure that you cook it right away. If the fish has been flash-frozen, you do have a bit of time, but usually, it’s best to eat it right away. If you’re not completely sure, try soaking the fish in a milk bath for about 15 to 20 minutes before you cook it. The compounds in milk will pull the smelly compounds off the fish. If it’s still edible, the fish’s smell will change after the milk bath. It’ll take on a sweet, fresh scent again. The flavor should follow suit. Lemon juice has been known to neutralize this issue as well, though the fish will take on a lemony taste following a treatment with lemons.