Thunnus obesus

Osteichthyes Scombridae Genus Thunnus Thunnus obesus
Bigeye tuna BET
~50 cm FL. Source: © D. Itano.
Characteristic features:
Colour:

Dark blue-black above edged with bright iridescent blue. Fresh specimens display gold bands running along all or part of the lateral line. A series of white/silver vertical, broken and incomplete lines mostly below the mid lateral line and confined to the posterior half of the body. Fins yellow to yellowish, anal fin often tinged with silver, caudal fin often dusky black. Finlets bright yellow with black edges.

Size:

Up to 250 cm TL and up to 210 kg in weight.1

Important conditions and life stages:

Small/juvenile (less than ~40 cm FL)
~34 cm FL *Thunnus obesus*
© D. Itano. ~34 cm FL Thunnus obesus
~32.5 cm FL *Thunnus obesus*
© D. Itano. ~32.5 cm FL Thunnus obesus

Small or juvenile Thunnus obesus less than ~40 cm FL can be confidently identified using a combination of the following external features:

 

  1. White/silver body markings in irregularly spaced, complete and broken vertical lines
  2. Diffuse demarcation between marked and unmarked region below pectoral fin base
  3. Eyes large and elliptical in shape relative to other Thunnus of the same size
  4. Pectoral fins long and pointed at tip
  5. Body deep, its profile forming a rounded arc from snout to caudal peduncle
  6. Caudal fin never yellowish (as sometimes seen in Thunnus albacares)

Fresh condition
~35 cm FL *Thunnus obesus*
© D. Itano. ~35 cm FL Thunnus obesus
~50 cm FL *Thunnus obesus*
© D. Itano. ~50 cm FL Thunnus obesus

Fresh condition Thunnus albacares can be confidently identified using a combination of the following external features:

 

  1. White/silver body markings in irregularly spaced, complete and broken vertical lines
  2. Fresh specimens may display a gold band along all or part of the lateral line
  3. Diffuse demarcation between marked and unmarked region below pectoral fin base
  4. Eyes large and elliptical in shape
  5. Pectoral fins long, extending beyond second dorsal fin (in adults greater than ~40 cm FL) pointed and flexible at the tip
  6. Body deep, its profile forming a rounded arc from snout to caudal peduncle
  7. No distinct notch in central portion of trailing edge of caudal fin

Less than fresh to poor condition
~40 cm FL *Thunnus obesus*
© D. Itano. ~40 cm FL Thunnus obesus
~51 cm FL *Thunnus obesus*
© D. Itano. ~51 cm FL Thunnus obesus
~70 cm FL *Thunnus obesus*
© D. Itano. ~70 cm FL Thunnus obesus
~46 cm FL *Thunnus obesus*
© D. Itano. ~46 cm FL Thunnus obesus

Thunnus obesus in less than fresh condition to poor condition may display faded, partial or absent markings and/or colouration. They may also display damaged fins and/or disfigured shape due to freezing, transport and storage. These individuals should be identified by using a combination of the following external features, and also internal features if external are unavailable:

 

  1. Remnants of white/silver body markings in irregularly spaced, complete and broken vertical lines (if visible)
  2. Eye Large and elliptical in shape (if intact) relative to other Thunnus of the same size
  3. Pectoral fins long, extending beyond second dorsal fin (in adults greater than ~40 cm FL) pointed and flexible at the tip (if intact)
  4. No distinct notch in central portion of trailing edge of caudal fin (if intact)
  5. Body deep, its profile forming a rounded arc from snout to caudal peduncle
  6. Ventral surface of liver without striations, right lobe elongated
  7. Swimbladder usually inflated, occupying almost entire body cavity
Distribution:

Tropical and temperate seas, circumglobally.

View FAO distribution map

Habitat:

Pelagic and oceanic. Found in temperatures ranging between 13–29 °C and at depths from the surface to 250 m.

Biology:

Feeds on a variety of fish, squid and crustaceans. Schools by size in same species or multi species groups such as with Katsuwonus pelamis or Thunnus albacares. Age at first maturity varies by region, in the Eastern Indian Ocean first maturity is estimated at 2–3 years.2, 3 Length at first maturity is estimated from 80 cm FL with an estimated 50% becoming mature between 102–135 cm, corresponding to 3.5 years.4 Bigeye tuna are multiple spawners, spawning every 1–2 days over a period of months throughout the year. Spawning occurs in tropical waters and coincides with periods of a full moon5 across their entire range, although usually in the eastern Pacific. This species is highly productive, producing an estimated 2.9–6.3 million eggs each spawning session.4 Maximum age varies by region; 5 years in the Eastern Pacific6 and 16 years in the Western Pacific.7

Indonesian fisheries:

Caught predominantly by longlining. Smaller juveniles are caught by purse seining.

Similar species:

Thunnus alalunga
Albacore

Thunnus alalunga

Thunnus alalunga differs in having a smaller head length and depth (vs. greater) and smaller eye diameter (vs. larger) for a given FL; body slender and elongate (vs. deep and rounded); pectoral fins very long, tips bluntly rounded (vs. very long, tips tapering to a thin flexible point (in adults)) and white/silver body markings as horizontal to oblique, broken stripes in stressed live, and freshly dead specimens only, otherwise no markings (vs. white/silver irregularly spaced vertical, often broken lines).

to Thunnus alalunga

Thunnus albacares
Yellowfin tuna

Thunnus albacares

Thunnus albacares differs in having a smaller head length and depth (vs. greater) and smaller eye diameter (vs. larger) for a given FL; body more slender and elongate (vs. deep and rounded); pectoral fins moderately long extending to base of second dorsal fin, broader and stiff (vs. very long, extending to second dorsal finlet, tips tapering to a thin flexible point (in adults)); pectoral fins straight when viewed from above (vs. arc shaped); white/silver body markings in regularly spaced, vertical lines and alternating lines of spots in a ‘chevron’ pattern (vs. white/silver body markings in irregularly spaced, vertical, sometimes broken lines only); finlets yellow (vs. yellow edged with black); a central portion of trailing edge of caudal fin with a distinct ‘V’ or ‘M’ shaped notch (vs. smooth crescent shaped) and ventral surface of liver not striated, right lobe elongate (vs. striated, lobes roughly equal in length).

to Thunnus albacares

Thunnus maccoyii
Southern bluefin tuna

Thunnus maccoyii

Thunnus maccoyii differs in having a smaller head length and depth (vs. greater) and smaller eye diameter (vs. larger) for a given fork length; pectoral fins short, not reaching past interdorsal space (vs. very long, extending beyond second dorsal fin); a series of white/silver body markings as irregularly spaced, often broken or incomplete vertical stripes present in stressed live specimens only, otherwise silver/white below (vs. white/silver markings irregularly spaced, vertical, sometimes broken lines); 31–34 gill rakers on first gill arch (vs. 23–31) and a yellow median keel in adults (vs. dark).

to Thunnus maccoyii

Thunnus orientalis
Pacific bluefin tuna

Thunnus orientalis

Thunnus orientalis differs in having pectoral fins short, not reaching dorsal interspace (vs. very long, extending beyond second dorsal fin); a series of white/silver markings as vertical lines and alternating lines of spots, mostly confined to lower portion of body (vs. white/silver irregularly spaced, vertical, sometimes broken lines).

to Thunnus orientalis

Thunnus tonggol
Longtail tuna

Thunnus tonggol

Thunnus tonggol differs in having pectoral fins moderate in length, not reaching beyond second dorsal-fin origin (vs. very long, extending to second dorsal finlet); posterior portion of body (from deepest point to caudal peduncle) long relative to FL (vs. moderate); horizontal rows of white/silver elongate spots on belly (vs. white/silver body markings in irregularly spaced, vertical, often broken lines); ventral surface of liver smooth with elongated right lobe (vs. striated, lobes roughly equal in length) and 19–27 gill rakers on first gill arch (vs. 23–31).

to Thunnus tonggol
Internal links:


Thunnus obesus gallery
Thunnus obesus quiz

External links:

FishBase
The IUCN Red List of Threatened SpeciesTM

References:
  1. Frimodt C. Multilingual illustrated guide to the world’s commercial warmwater fish. Fishing News Books Ltd.; 1995.
  2. Nootmorn P. Reproductive biology of bigeye tuna in the eastern Indian Ocean. In: IOTC proceedings. Citeseer; 2004. p. 1–5.
  3. Calkins TP. Synopsis of biological data on the bigeye tuna, Thunnus obesus (Lowe, 1839), in the Pacific Ocean. Special Report. 1980;2:213–60.
  4. Collette BB, Cole K. Reproduction and development in epipelagic fishes. Reproduction and sexuality in marine fishes: patterns and processes University of California Press, Berkeley. 2010;21–63.
  5. Kailola PJ, Opnai J. Fisheries Resources Profiles: Papua New Guinea. In Forum Fisheries Agency; 1995.
  6. Schaefer KM, Fuller Daniel W. Estimates of age and growth of bigeye tuna (Thunnus obesus) in the eastern Pacific Ocean based on otolith increments and tagging data. Inter-American Tropical Tuna Commission; 2006.
  7. Farley JH, Clear NP, Leroy B, Davis TL, McPherson G. Age, growth and preliminary estimates of maturity of bigeye tuna, Thunnus obesus, in the Australian region. Marine and freshwater Research. 2006;57(7):713–24.