The Bigeye tuna (Thynnus obesus) is one of the most challenge sportfishing species available to anglers fishing offshore from Australia’s east and west coasts.
This species occurs worldwide in the tropical and sub-tropical oceanic waters of the Atlantic, Indian and Pacific Oceans, being encountered virtually anywhere where surface water temperatures exceed 17°C, except for the Mediterranean Sea. Commercial longline data shows bigeye can be found at a wide range of water temperatures (13-29°C), but the optimum surface temperature range to target them is between 17°C and 22°C near ocean features such as current fronts, floating objects and thermoclines.
Indeed, this species is one of the deepest diving of the tunas, often feeding in the thermocline over 200 metres deep, but large adults can dive deeper than 500 metres if the water is not too cold and sufficient oxygen is available.
Growing to nearly 200kg, (the all tackle world record of 197kg and 230 + cm was taken off Peru in 1957) bigeye are one of the largest tuna species. As juveniles less than around 50 cm long they look superficially similar to yellowfin tuna in many respects, due to the yellow colour of the dorsal fins and the finlets near the tail. However, at this size they are deeper in the body compared to juvenile yellowfin, and also have relatively long pectoral fins, similar to those seen in albacore. As they grow bigger, however, they begin to look more like a bluefin tuna in general appearance, but with a much stouter body, a relatively large eye as well as 23 to 31 gillrakers on the first arch.
This species spawns at night over full moon periods in tropical waters between 15°N and 15°S during months when sea surface temperatures exceed 24°C. The average mature female spawns between 2 and 6 million eggs. Like other tuna, bigeye grow incredibly fast, reaching around 70 cm long in their first year and 100 cm in their second year, maturing in their second to third year of life at between 100 and 130 cm long. They feed on a wide range of fish, squids and crustaceans both during the day and at night. Maximum age for bigeye tuna is between 10 and 16 years old, with their main predators being billfish, toothed whales, and of course, humans.
There are four stocks that are globally managed for this species, all of them being heavily fished by commercial purse seiners and longliners. The stocks in the Atlantic and Indian Oceans are both considered to be fully exploited, while the stocks in the Eastern Pacific and Western Pacific are considered to be overfished. Schools of juvenile and sub-adult bigeye less than one year old are often found mixed in with other tuna species (yellowfin, skipjack) near floating objects.
Research using electronic tagging has confirmed how man made fish attraction devices (FADs) alter the behaviour of juvenile bigeye tuna, with fish less than around 70cm long staying associated with a FAD for long periods. This behaviour has been exploited by commercial purse seiners in particular, who deploy their own “Smart” FADs equipped with GPS and other electronic gear including sounders that monitor the number of fish nearby. When the fish numbers reach a certain level, they relocate the FAD and run the purse seine around it, an efficient fishing method which is how many juvenile bigeye tuna meet their demise these days.
Other interesting information found during electronic tagging studies of bigeye include afternoon dive behaviour, where fish dive down to the thermocline in the afternoon to feed on fish and crustaceans, with both tuna and their prey rising back to the surface as the light fades into the evening.