While similar in appearance, bigeye (top) and yellowfin tuna are easy to differentiate.

Some people will try to tell you that juvenile bigeye and yellowfin tuna are difficult to tell apart. This statement is completely false and is usually perpetuated by fishery scientists and fishermen who have not seen enough of both species to qualify them to make any statements on fish identification.

The two fish are “similar” in appearance but have several distinguishing internal and external characteristics that will allow the careful observer to identify each species in a range of conditions, from fresh to frozen.

This subject is not a trivial matter of academic interest. Bigeye and yellowfin tuna are two of the most important commercial species for canned and fresh tuna markets worldwide, which process hundreds of thousands of tons of raw material per year. The correct identification of both species in commercial landings is of primary importance for stock assessment and management purposes.

In Hawaii, bigeye and yellowfin tuna are the first and third most important species by dollar value in commercial landings, and yellowfin are of primary interest to sport and charter fishermen. Unfortunately, a significant proportion of the reported catch of yellowfin in Hawaii may actually be bigeye tuna.

Due to the importance of both species to Hawaii-based commercial and recreational fisheries, the Hawaii Tuna Tagging Project was funded to learn more about the yellowfin and bigeye resources of the Central Pacific region from which we all benefit.

Below is a guide for the identification of yellowfin and bigeye tuna.

Internal Characteristics – Tuna livers are large, conspicuous organs that lie closest to the belly or bottom of a fish when the body cavity is opened. The organs are simply “liver-colored” and are composed of three lobes joined at the anterior (front) end. Yellowfin livers are composed of two lobes of equal size and one long, thin lobe. The liver of a bigeye tuna is made up of three lobes of similar length with the central lobe slightly enlarged.

External Characteristics – Live or fresh yellowfin have a metallic blue/black back with a bright gold yellow or iridescent blue side band, changing to silvery sides and belly, being more elongated. Bigeye have a large blue/black back that blends into a peculiar iridescent cobalt/metallic blue region followed by a golden/yellow side band above silvery gray sides and a white belly. Most of these colors fade soon after death. The body is more football-shaped.

Tail Fin – The characteristics of yellowfin and bigeye caudal (tail) fins are distinct and can serve as a quick means of identification if the tails are in good condition. This characteristic is most useful when identifying fish larger than 15 pounds. Yellowfin tails are yellow/golden, while bigeye tails are darker in color, being purplish-black that fades to a dusky black at death.

The central portion of the trailing edge of a yellowfin tail forms a distinctive “V” shape. Two raised bumps, or mounds, are apparent on either side of the fin, behind the large side keel; these are absent in bigeye. The center of the trailing edge of a bigeye tail has no “V” notch, but forms a flat or slightly crescent-shaped cup.

Bigeye and yellowfin are similar in many respects, but the bigeye’s second dorsal and anal fin never grows as long as those of the yellowfin do.

These features are difficult to describe – but easy to see – if you have a yellowfin and bigeye side by side.

Now you should be able to tell the two species apart, so please don’t lump them together on your catch report forms. Accurate and complete catch report forms give the scientists the information necessary to better access and manage local fisheries.

Please do not label all small yellowfin and bigeye as “shibi” or the larger ones as “ahi.”