Why does tuna have to be pink in the middle?

when you cook mahi mahi,I see some recipes that leave the middle pink,why is that?

Why does tuna have to be pink in the middle?
Certain types of tuna steaks are really good when only seared on the outside. This way you get some of the flavor of the cooked fish and some of the flavor as if it were served as sashimi. It adds another level to the flavor.
Reply:You cook it to suit your own tastes. It's like beef: rare, medium, and well-done. You don't have to leave it pink in the center if you don't want to. I don't!
Reply:I personally do not like my fish "pink" in the middle. This is the latest rage "Searing Fish". I think it started because it is so easy to over cooked.
Reply:It doesn't have to be, some people just like rare tuna.
Reply:A lot of people like the taste and texture of VERY rare or raw tuna
Reply:It doesn't, Albacore tuna is white after it's cooked.
Reply:Smother it with mayo and ya won't know the difference.
Reply:Tuna is a lean fish. The more you cook it the drier it is. They begin to flake the more you cook, and don't stay quite together or look nice or taste good. Always use the highest grade when cooking tuna, since you are going to eat a part of it raw. Just like ahi tuna that is the bright red tuna, sushimi grade.
Reply:I dont know...personally I prefer my fish (as well as other meats) cooked all the way through. Its kind of like picking a fish up out of the water and taking a bite of it. Except it is 'fresher' that way. lol It is just a trend...if you want to cook it all the way through you can.