The bigeye tuna, Thunnus obesus, is an important food fish and prized recreational game fish. It is a true tuna of the genus Thunnus, belonging to the wider mackerel family Scombridae. Bigeye tuna are found in the open waters of all tropical and temperate oceans, but not the Mediterranean Sea.
The pectoral fins may reach to the second dorsal fin. The second dorsal and anal fins never reach back as far as those of large yellowfin tuna (Thunnus albacares). It has a total of 23 31 gill rakers on the first arch. The margin of the liver is striated. The two dorsal fins are close set, the first having 13 14 spines and the second, 14 16 rays. The anal fin has 11 15 rays. On either side of the caudal peduncle there is a strong lateral keel between two small keels that are located slightly farther back on the tail. The scales are small except on the anterior corselet. The vent is oval or teardrop shaped, not round as in the albacore. The first dorsal fin is deep yellow. The second dorsal fin and the anal fin are blackish brown or yellow and may be edged with black. The finlets are bright yellow with narrow black edges. The tail does not have a white trailing edge like that of the albacore. Generally, there are no special markings on the body, but some specimens may have vertical rows of whitish spots on the venter.
At one time it was not recognized as a separate species but considered a variation of the yellowfin tuna. They are similar in many respects, but the bigeye second dorsal and anal fins never grow as long as those of the yellowfin. In the bigeye tuna the margin of the liver is striated and the right lobe is about the same size as the left lobe, in the yellowfin tuna the liver is smooth and the right lobe is clearly longer than either the left or the middle lobe.
Longer-lived than the closely related yellowfin tuna, the bigeye has a lifespan of up to 12 years, with sexual maturity at age four. Spawning takes place in June and July in the northwestern tropical Atlantic, and in January and February in the Gulf of Guinea, which is the only known Atlantic nursery area.
Satellite tagging showed that bigeye tuna often spend prolonged periods diving deep below the surface during the daytime, sometimes reaching 500 metres (1,600 ft). Bigeye have been tracked entering water as cold as 5 °C (41 °F). These movements are thought to be in response to vertical migrations of prey organisms in the deep scattering layer.
Feed items include both epipelagic and mesopelagic species, with deep diving behaviour during the day thought to be related to the seeking of prey. Juvenile bigeye tuna associate closely with floating objects such as logs, buoys and other flotsam.
Its diet includes squid, crustaceans, mullet, sardines, small mackerels and some deep water species. Fishing methods are trolling deep with squid, mullet or other small baits, or artificial lures and live bait fishing in deep waters with similar baits. It is an excellent food or sport fish, an important commercial species.
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