Bigeye Tuna

The bigeye tuna (Thunnus obesus) is one of the most commercially significant members of the genus Thunnus, belonging to the broader mackerel family Scombridae. Named for the distinctively large eyes that help it see in the low-light depths where it spends much of its day, the species is a prized target for both commercial longline fisheries and recreational anglers across the world's tropical and temperate oceans.

Where are bigeye tuna found?

Bigeye tuna inhabit the open waters of all major tropical and temperate ocean basins — the Atlantic, Pacific, and Indian oceans — but they are notably absent from the Mediterranean Sea. Unlike yellowfin tuna, which tend to stay near the surface, bigeye tuna are strong vertical migrants. During the day they descend to considerable depths, where cooler, oxygen-rich water holds the prey fish and squid they feed on. At night they rise toward shallower, warmer layers to feed more actively.

In Sri Lankan and wider Indian Ocean waters, bigeye tuna appear throughout the year, making the island's deep offshore grounds a productive area for deep sea fishing expeditions targeting this species.

How big do bigeye tuna grow?

Bigeye tuna are large, deep-bodied fish with a streamlined torpedo shape well suited to sustained high-speed swimming. Adults can reach up to 250 cm (roughly 8 feet) in length. Maximum individual weight is thought to exceed 180 kg (around 400 lb), and the all-tackle angling world record stands at 178 kg (392 lb) — a figure that underlines why the species commands serious attention among sport fishers.

Key physical features include:

• Unusually large eyes relative to head size, adapted for dim-light vision at depth
• Very long pectoral fins that extend beyond the start of the second dorsal fin in juveniles, and reach the space between the first and second dorsal fin in adults
• 13 or 14 dorsal spines
• A metallic blue-black back fading to silvery white on the belly

How long do bigeye tuna live?

Conventional tagging studies and analysis of growth increments in otoliths — the calcified "ear bones" used to age fish — have recorded a maximum lifespan of around 16 years. This is somewhat longer than many other commercially fished tuna species, which has important implications for stock management: slower-maturing, longer-lived populations recover more slowly when heavily exploited.

When do bigeye tuna reach sexual maturity?

The age and size at which bigeye tuna mature varies geographically. Research suggests that 50 percent of fish in the eastern Pacific Ocean are sexually mature at around 135 cm in length, whereas in the western Pacific the corresponding figure is 102–105 cm. This translates to a maturity age of roughly two to four years. Differences in methodology between studies partly explain this variability, and ongoing research continues to refine these estimates.

When and where do bigeye tuna spawn?

Spawning behaviour differs significantly by region:

• Tropical Pacific: Spawning can occur across most months of the year in tropical latitudes, becoming more seasonal at higher latitudes when sea surface temperatures climb above 24 °C.
• Northwestern tropical Atlantic: Spawning is concentrated in June and July.
• Gulf of Guinea: Spawning occurs mainly in January and February. The Gulf of Guinea is the only confirmed nursery area for Atlantic bigeye tuna stocks.
• Indian Ocean: Spawning activity has been recorded in equatorial and near-equatorial waters, broadly consistent with the warm-water preference seen in other ocean basins.

How is bigeye tuna different from yellowfin tuna?

The two species are frequently confused, particularly in Hawaiian fisheries where both are called ahi. At sea, the most reliable distinctions are the bigeye's notably larger eye, longer pectoral fin (especially in adults), and its tendency to occur at greater depths. In the market, bigeye tuna is often regarded as having a higher fat content — particularly in larger, older fish — making it a preferred choice for high-grade sashimi and sushi. Yellowfin tends to be leaner and is more commonly sold as steaks or canned product.

What is the conservation status of bigeye tuna?

The bigeye tuna is listed as Vulnerable on the IUCN Red List, primarily due to pressure from industrial longline fishing targeting its deeper feeding grounds. Purse-seine fisheries that set nets around fish-aggregating devices (FADs) also catch large numbers of juvenile bigeye as bycatch, which affects stock replenishment. Several regional fisheries management organisations (RFMOs) — including the Indian Ocean Tuna Commission (IOTC), which covers Sri Lankan waters — implement catch limits and monitoring programmes aimed at preventing further stock decline.

Can you fish for bigeye tuna in Sri Lanka?

Sri Lanka's deep offshore waters in the Indian Ocean offer genuine opportunities to encounter bigeye tuna, particularly on longer deep-sea charters that target the thermocline layers where the species concentrates. The southern and western coasts, with easy access to deep blue water, are the most practical departure points. Trolling with rigged baits or large lures at depth, and dropping live bait below the surface layer, are established techniques. Bigeye caught recreationally can be prepared as sashimi-grade sushi — its high fat content at depth makes it arguably the finest eating of all the tunas.

Bigeye tuna in Sri Lankan cuisine and trade

Sri Lanka has a long tradition of tuna fishing, and bigeye tuna — along with yellowfin and skipjack — forms part of the commercial catch landed at ports including Mirissa, Negombo, and Trincomalee. Locally, fresh tuna is grilled, curried, or prepared as ambulthiyal (sour fish curry), a distinctly Sri Lankan dish that uses goraka (Garcinia cambogia) as a souring and preserving agent. Canned tuna is also a staple pantry item across the island.