The Bigeye tuna (Thunnus obesus) is a species of true tuna of the genus Thunnus, belonging to the wider mackerel family Scombridae.
In Hawaiian, it is one of two species known as ʻahi, the other being yellowfin tuna. Bigeye tuna are found in the open waters of all tropical and temperate oceans, but not the Mediterranean Sea.
Bigeye tuna can grow up to 250 centimetre (98 inches) or 8 feet, in length. Maximum weight of individuals probably exceeds 180 kg (400 lb), with the all-tackle angling record standing at 178 kg (392 lb). They are large, deep-bodied, streamlined fish with large heads and eyes. The pectoral fins are very long, reaching back beyond the start of the second dorsal fin in juveniles and the space between the first and second dorsal fin in adults. They have 13 or 14 dorsal spines.
Conventional tagging data and counts of growth increments in otoliths (ear bones) of bigeye tuna have recorded a maximum age of 16 years.Recorded lengths at which sexual maturity is attained varies geographically with a length at which 50% of fishes sampled are mature of 135 cm in the eastern Pacific Ocean and 102–105 cm in the western Pacific Ocean.This translates to an age of maturity of 2 – 4 years. Differences in methods of studies may contribute to this variability. Spawning takes place across most months of the year in tropical regions of the Pacific Ocean, becoming seasonal at higher latitudes when sea surface temperatures are above 24 °C. In the northwestern tropical Atlantic spawning occurs in June and July, and in January and February in the Gulf of Guinea, which is the only known Atlantic nursery area.