Correctly identifying a large yellowfin or small bigeye can sometimes be a rather daunting proposition; we hope this guide – courtesy of the International Game Fish Association (IGFA) – will help you better understand the species of tuna that has just entered your boat.
The first place to look is the ventral finlets; if they are silver or dark then you have just boated either a longfin, blackfin, skipjack or false albacore. In most cases the finlets will be yellow indicating the fish on the deck is either a bluefin, yellowfin or bigeye tuna. Next it’s time to look at the pectoral fin; if that fin does not reach the second dorsal, then you are looking at a bluefin tuna. If the pectoral fin does reach the second dorsal fin, now you’re looking at either a yellowfin or a bigeye.
This is where the identification becomes slightly more challenging. If you can see a pattern of alternating light lines and dots on the belly region then the fish is a yellowfin tuna as bigeye do not show this pattern. The second method of identification would be examining the liver of the fish. In bigeye tuna the margin of the liver is striated, and the right lobe is about the same size as the left lobe, in yellowfin tuna the liver is smooth, and the right lobe is clearly longer than either the left or the middle lobe.
Bluefin Tuna – Bluefin are the largest of the tuna species and one of the ocean’s largest true bony fish. Bluefin can be distinguished from almost all other tuna species by its rather short pectoral fins which extend only as far back as the 11th or 12th spine in the first dorsal fin. There are 12 to 14 spines in the first dorsal and 13 to 15 rays in the second; the anal fin has 11 to 15 rays. Bluefin have the highest gill raker count of any tuna species with 34 to 43 on the first arch. The ventral surface of the liver is striated, and the middle lobe is usually the largest. The anal fin and the finlets are dusky yellow edged with black, and the lateral keel is black in adults.
Yellowfin Tuna – Yellowfin are often considered the most colorful of all tunas, with a blue-black back that fades to silver on the lower flanks and belly. A golden yellow or iridescent blue stripe runs from the eye to the tail, though this is not always prominent. All of the fins and finlets are golden yellow though in some very large specimens the elongated dorsal and anal fins may be silver, edged with yellow. The finlets have black edges, and the belly frequently shows as many as 20 vertical rows of white spots. Most large yellowfin have overextended second dorsal and anal fins that may reach more than halfway back to the tail base. In smaller yellowfin, under about 60 pounds, and in some very large specimens, this may not be an accurate distinguishing factor since the fins do not appear as long in all.
Bigeye Tuna – A bigeye’s pectoral fins may reach to the second dorsal fin, however the second dorsal and anal fins never reach back as far as those of large yellowfin tuna. The two dorsal fins are close set, the first having 13 to 14 spines and the second, 14 to 16 rays. On either side of the caudal peduncle there is a strong lateral keel between two small keels located slightly farther back on the tail. The first dorsal fin is a deep yellow, the second and the anal fin are blackish brown or yellow and may be edged with black. The finlets are bright yellow with narrow black edges and the tail does not have a white trailing edge.
Bigeye and yellowfin are similar in many respects, but the bigeye’s second dorsal and anal fins never grow as long as those of the yellowfin. In bigeye the margin of the liver is striated and the right lobe is about the same size as the left, while the yellowfin tuna liver is smooth and the right lobe is clearly longer than either the left or the middle lobe.
