How to Identify Bigeye Tuna

To identify Bigeye Tuna (Thunnus obesus), look at these key features.

Physical Characteristics

Body shape

torpedo

Coloration

Dark metallic blue to black on the dorsal side, fading to silvery-white on the belly. The first dorsal fin is dark, the second dorsal and anal fins are yellowish, and the finlets are bright yellow with black margins. A faint iridescent blue band may be present along the flanks of live specimens.

Scale type

cycloid

Mouth position

terminal

Distinctive features

Very large eyes, proportionally larger than other tuna species, adapted for low light., Long pectoral fins, typically extending beyond the origin of the second dorsal fin, sometimes reaching the anal fin., Relatively robust body compared to Yellowfin Tuna., Small, smooth scales covering the body., Seven to ten dorsal finlets and seven to ten anal finlets.

Dorsal fin

Two dorsal fins; the first is spiny and depressible into a groove, the second is soft-rayed and followed by 7-10 finlets.

Tail fin

Deeply forked caudal fin (tail fin), characteristic of fast-swimming pelagic fish.

Pectoral fin

Long and falcate (sickle-shaped), typically extending past the origin of the second dorsal fin.

Distinctive fins

Small, yellow finlets with black margins following the second dorsal and anal fins.

Identification Tips

1. 1

   Eyes

   Proportionally larger eyes compared to other tuna species, especially Yellowfin Tuna. This is the most distinguishing feature.

2. 2

   Pectoral Fins

   Long pectoral fins that typically extend past the origin of the second dorsal fin, often reaching the anal fin. Yellowfin Tuna pectoral fins are usually shorter, rarely reaching past the second dorsal fin origin in adults.

3. 3

   Body Shape

   Generally more robust and stocky than Yellowfin Tuna of similar size, particularly in the anterior part of the body.

4. 4

   Finlet Coloration

   Bright yellow finlets with distinct black margins, a feature shared with Yellowfin Tuna but can be a useful general identification point.

5. 5

   Ventral Striations

   Live or freshly caught specimens may show faint, broken, vertical white striations on the lower flanks, which are less prominent or absent in Yellowfin Tuna.

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