Tuna Pastel Drawing by Chef Kaz Matsune
(Originally posted on Quora.com)
Let's start looking at the whole fish - Yellowfin, Bigeye, and Bluefin
What's marked in green is suitable for Sashimi.
Tuna uses its tail to swim, so the tail meat is the leanest, containing the least amount of fat, therefore tastes less flavorful compared to the belly.
Here is a top view and some side views of Yellowfin tuna.
You see white tendon left side of this loin (just outside of the green line,) This part is too tough to chew. Because Yellowfin does not grow big enough to store fat as Bluefin does, this part is unsuitable for either Sashimi or Nigiri.
It, however, can be used for Spicy Tuna after scraping.
The best part is the top part (pointy triangular shape) meat. It is tender and has a very mild tuna flavor. Great for Sashimi!
Things are a bit different with Bluefin.
If you look at the whole fish, tail meat would be unsuitable for Sashimi just like Yellowfin.
However, because the belly meat contains a lot of fat, generally known as Toro, you can pretty much eat any part of Blue Fin Tuna as Sashimi, as marked below in the green line.