How to Identify Longtail Tuna

To identify Longtail Tuna (Thunnus tonggol), look at these key features.

Physical Characteristics

Body shape

torpedo

Coloration

Dorsal side is dark metallic blue to black, fading to silvery white on the flanks and belly. Often has faint, broken, oblique stripes or rows of spots on the lower sides, which may disappear after death. Finlets are typically yellow with dusky margins.

Scale type

cycloid

Mouth position

terminal

Distinctive features

Relatively long pectoral fins, extending beyond the anterior margin of the second dorsal fin, but not reaching the end of the second dorsal fin base., Small, conical teeth in a single row., Absence of striations on the liver (a key internal diagnostic feature)., Swim bladder present., First dorsal fin has 13-16 spines; second dorsal fin followed by 8-10 finlets; anal fin followed by 7-9 finlets.

Dorsal fin

Two dorsal fins; the first is spiny and triangular, the second is soft-rayed and similar in shape to the anal fin. Followed by 8-10 dorsal finlets.

Tail fin

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

Pectoral fin

Long, slender pectoral fins, typically extending past the origin of the second dorsal fin.

Distinctive fins

The length of the pectoral fin is a primary distinguishing feature from other Thunnus species.

Identification Tips

1. 1

   Pectoral Fin Length

   The most reliable external feature: pectoral fins are noticeably long, extending past the origin of the second dorsal fin, but not reaching the end of its base. This distinguishes it from Yellowfin Tuna (shorter pectorals) and Bigeye Tuna (much longer pectorals).

2. 2

   Body Coloration

   Dark blue/black dorsally, silvery white ventrally. Faint, broken, oblique stripes or rows of spots may be present on the lower sides, which are less distinct than those on Skipjack Tuna.

3. 3

   Finlets

   Yellow finlets with dusky margins are characteristic.

4. 4

   Absence of Liver Striations

   Internally, the liver lacks the characteristic striations found in other Thunnus species like Yellowfin and Bigeye Tuna. This requires dissection for confirmation.

5. 5

   Gill Rakers

   Has 20-26 gill rakers on the first arch, a count that can help differentiate it from some other tuna species.

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