Bigeye Tuna

The bigeye tuna (Thunnus obesus) is a large, warm-blooded tuna species in the family Scombridae, found in tropical and subtropical waters of the Atlantic, Indian, and Pacific Oceans. It is distinguished from other tunas by its unusually large eyes, an adaptation that allows it to hunt effectively at depth in low-light conditions. Bigeye tuna regularly dive several hundred meters below the surface during the day, following the deep scattering layer of prey, then return to shallower waters at night. It is one of the major commercially targeted tuna species worldwide and is currently classified as Vulnerable by conservation assessments due to fishing pressure across its range.

Bigeye tuna resemble other large tunas but have several distinguishing features.

• Eyes: notably large in proportion to the head, giving the species its name
• Body: robust, torpedo-shaped, slightly more compressed than yellowfin
• Color: dark metallic blue back, silvery flanks and belly, no strong yellow lateral stripe
• Finlets: bright yellow edged in black along the back and belly near the tail
• Pectoral fins: moderately long, reaching past the second dorsal fin origin
• Size: commonly 1.8-2.5 m, among the larger tuna species

Bigeye is best separated from yellowfin tuna by its larger eye, shorter second dorsal and anal fins, and a slightly stockier body shape.

Bigeye tuna occupy tropical and subtropical waters across the Atlantic, Indian, and Pacific Oceans, generally between about 45°N and 45°S. They are highly migratory and occupy a wide vertical range, from the surface down to depths beyond 500 m, with adults regularly diving into cooler, oxygen-poor waters during the day to feed on the deep scattering layer before rising toward the surface at night. This vertical migration pattern distinguishes them ecologically from more surface-oriented tunas. They favor waters near seamounts, current boundaries, and drop-offs where deep prey layers concentrate.

Bigeye tuna exhibit a distinctive daily vertical migration, spending daylight hours hundreds of meters deep hunting squid, fish, and crustaceans in the dim mesopelagic zone, then rising closer to the surface at night. They often form loose schools, sometimes mixing with yellowfin tuna or associating with floating objects and seamounts. As apex predators, they play an important role linking deep mesopelagic prey communities with surface ecosystems. Spawning occurs in warm tropical waters year-round, with peak activity in summer months, and females release millions of pelagic eggs that hatch within a day or two.