Yellowfin tuna (Thunnus albacares), sometimes called ahi or shibi, have a range through most tropical and subtropical waters of the world. Size is highly variable. The smallest tuna caught may be only three pounds (1.36 kg), but some sports fisherman have caught yellowfin tuna as large as 200 pounds (90.72 kg) in weight.
The yellowfin tuna derives its name from the pronounced yellow color of its anal and dorsal fins. It also has a row of finlets; tiny fins that proceed down the back of the fish and are often yellow in color. Larger fish have much larger fins, and the rest of the fish is a steely gray to blue-black in color. Sports fisherman tend to enjoy catching the largest yellowfin tuna they can find because they boast large and impressive yellow fins, and they are fast and strong swimmers.
Yellowfin tuna is especially prized in raw seafood dishes. It may be used to make sashimi, tuna carpaccio, or can be lightly pan seared. When cooked is usually served rare and exhibits a dark red flesh. Smaller yellowfin tuna may have a slightly lighter red or even pink flesh, which is a normal and expected variant.
Big eye tuna may be used in place of yellowfin tuna, and in fact both are called ahi. It should be noted that yellowfin tuna has a slightly shorter shelf life than does either bigeye or albacore. It is also noted for its firm flesh, but the tuna caught late in the season may be more watery and have less vibrant color. This is often called tuna burning or simply burnt tuna. It’s considered to be lower in quality than yellowfin tuna caught in peak season, and will be rejected as sashimi. Most people don’t notice a taste difference when the tuna is cooked.
Most of the yellowfin tuna available in the US is caught near Hawaii where the fish proliferate. Preferred commercial catching method is through netting the tuna. When surplus exists, a market springs up for smoked and dried yellowfin tuna.
Since the yellowfin tuna tends to degrade in quality after being caught, emphasis should always be on finding the freshest fish. Most that come from Hawaii are minimally dressed because if they are filleted, they begin to oxidate and loose color. For sashimi, only the freshest fish should be used, but you can certainly use yellowfin tuna that has been frozen when you plan to cook it.