Species Description 

Bigeye tuna are similar in general appearance to yellowfin tuna (especially as juveniles) and even share the same name, "ahi", in Hawaiian. Both bigeye and yellowfin tuna have bright yellow dorsal fins, but bigeye can be distinguished by its plumper body, its larger head and its unusually large eyes!

Bigeye tuna have a dark, metallic blue colouration on the top of their body, and are white on the underside, which provides camouflage in the ocean when hunting for prey. Prey looking up below the tuna will not be able to distinguish the fish from the glare of the surface water. Likewise, prey from above will not be able to see the tuna as the dark blue colour on the fish blends in with the darkness from the deep. This is also known as countershading, and is seen across many marine animals.

Bigeye tuna can be found in the warm, tropical waters in the Atlantic, Pacific and Indian Oceans, and are migratory, meaning they can travel long distances. They are a relatively fast-growing tuna species, reaching up to 2.5 m (8.2 feet) in length, though they average at 1.8 m (5.9 feet), and can live longer than 10 years. There is an interesting disparity in the size of bigeye tunas caught using different fishing methods. For longliners, the average bigeye caught is 55kg while purse seiners encounter mostly juveniles in their catch, averaging around 6kg. The less selective method of purse seining catches these juveniles mostly in association with skipjack tuna schools. They are all sexually mature by an age of three years and typically spawn in the summer months, releasing between three and six million eggs each time.

While bigeye tuna are near the top of the food chain, feeding primarily on fish and squid, they are the prey for top predators such as sharks, larger tuna and toothed whales.