How to differentiate bigeye and yellowfin tuna
The scientific name for the bigeye, Tunnus obesus, simply – and perhaps unflatteringly – means “fat tuna.” However, if you’ve ever seen one, you’ll know it is an apt description for this football-shaped speedster.
The bigeye and yellowfin tuna are sometimes difficult to tell apart, especially at smaller sizes. Nevertheless, there are a number of features which are identifiable and which should enable anyone to identify bigeye tuna with relative ease.
The live color of a yellowfin has a metallic black back, and bright yellow band along a band above silver/white sides and belly.
A bigeye has a dark blue/black back, blending to cobalt metallic blue, to a golden band above silvery/grey sides and white belly.
The tails are very distinctive. In yellowfin, the tail has a yellow/golden tinge. The middle part of the trailing edge is indented into a distinct “V” with two raised ridges on either side.
A bigeye’s tail is dark, showing little if any yellow. It is dusty black after death. It does not have a distinct “V” at the middle of the tail, with no raised ridges but more of a dish look to it.
In bait-sized tuna, 4-10 pounds, a yellowfin has many silvery vertical bands (striations) covering the lower rear half of the body. The bands alternate between solid ones and lines consisting of rows of spots. The body is relatively elongated and slim. The pectoral fins (side fins) reach to the length of the second dorsal fins (top and bottom fins lower third of body) and are often rounded at the tip.
In bigeye, 4-10 pounds, it has widely spaced irregular vertical stripes, mostly solid, with darker areas in between. The head is large compared with the body. The body is deep and rotund. The top and bottom edges, from head to tail, form a continuous curve. The pectoral fins are fine tipped and are as long or longer than the yellowfin.
In larger tuna, 10-45 pounds, the shape of a yellowfin’s pectoral fins when viewed from above are blade-like, with the tips pointing at about 45 degrees to the line of the body.
A bigeye’s pectoral fins are very long, reaching to the rear of the second anal fin, with a thin, flexible tip. The shape of the fin, when viewed from above, is crescent-shaped, with the tips pointing backward.
In tuna over 50 pounds, a yellowfin’s body is still elongated, resulting in a relatively small head compared with the total body length. Its pectoral fins are relatively short and stiff. The second dorsal and anal fins (sickles) began to elongate – the main identifying feature.
A bigeye’s body is very rotund, taking on the shape of a football. The head is large with a relatively short tail section. Its pectoral fins are still quite long, with importantly, the second dorsal and anal fins remaining short.