The differences between bigeye tuna and yellowfin tuna, the two species of ahi, are that bigeye has extremely large eyes, a plump body and a larger head than yellowfin. Yellowfin tuna has bright yellow finlets with soft anal and dorsal fins. Bigeye flesh color is reddish-pink with a fat content usually higher than yellowfin, because bigeye tuna live in cooler, deeper water. Small yellowfin tuna have pink flesh, with colors ranging to deep red flesh in large yellowfin.
Sashimi experts favor the mild flavor and firm texture of bigeye tuna over yellowfin, but the average consumer may not notice the difference. Bigeye is a tuna that cooks can grill easily due to its high fat content. A 4-ounce serving of fresh bigeye tuna contains about 500 milligrams of omega-3 fatty acids while the same size serving of yellowfin contains about 350 milligrams. Both ahi tunas are high in lean protein, but low in sodium and saturated fat. The tunas contain vitamin B6, vitamin B12 and niacin.
Bigeye and yellowfin tuna provide essential minerals including phosphorus, selenium and magnesium, and bigeye provides iodine. October through April is peak season for bigeye tuna, while yellowfin season peaks in May through September. Both species of ahi tuna grow to more than 200 pounds.