Tuna: Bigeye

Ahi, Maguro, Po’onui

Bigeye tuna have an enormous range, being distributed throughout all the world’s tropical and subtropical oceans. Bigeye hunt in relatively deep, dark water, preying on animals that associate with the water’s deep scattering layer. Their diet includes squid, fish, and crustaceans. The largest bigeye on record weighed 333 kg, although the average weight of individuals caught is 15-20 kg and cm in length. They are considered to sexually mature at 3-4 years and 100 cm in length, although there is uncertainty about these figures. Little work has been done to determine a validated maximum age for this species; the figure of 5-7 years cited by some industry sources is probably a serious underestimate.

Juvenile bigeye form mixed schools with yellowfin and skipjack tunas. These schools often associate with drifting objects, which makes juvenile bigeye vulnerable to purse seiners using fish aggregating devices (FADs). As bigeye mature, their attraction to floating objects appears to lessen.

Spawning takes place in tropical waters, and young fish migrate into temperate waters as they grow. Bigeye are known to breed in the tropical center of the Pacific and in the northern Indian Ocean. In 2001, management scientists began research to confirm that Atlantic bigeye breed in the Gulf of Guinea, near Ivory Coast.

Bigeye mature at 3-4 years of age with a reported maximum age of 5-7 years (probably an under-estimate). Overall they are considered to have a low inherent vulnerability to fishing pressure.

For all regions there is a moderate degree of stock status uncertainty. Overfishing is occurring or likely occurring in all fisheries with the exception those in the Atlantic. Declining long and short-term population trends are characteristic of all stocks. Overall stock status is considered poor (including Eastern Pacific, Western and Central Pacific and Indian Ocean – stocks representing 87% of catch) while stocks in the Atlantic (representing 17% of catch) are rebuilding and represent a moderate conservation concern.

Fishing for bigeye tuna by troll is considered to have a low level of bycatch and is of low conservation concern.

Trolling for bigeye tuna does not have any significant habitat effects or large scale ecosystem impacts.

ATLANTIC: The International Convention for the Conservation of Atlantic Tunas (ICCAT) currently manages Atlantic bigeye tuna. The number of undersized fish catch has stabilized since ICCAT established a time/area closure for purse seining in the eastern tropical Atlantic, but still remains a concern. To prevent further decline of the stock ICCAT’s Standing Committee on Research and Statistics (SCRS) has recommended a reduction in the catch. Monitoring includes reporting of boat positions and catches and the storage of fishing gear when the boat is outside of authorized fishing grounds.

PACIFIC: The Inter-American Tropical Tuna Commission (IATTC), established by international convention in 1950, is responsible for the conservation and management of bigeye tuna fisheries in the Pacific. The National Marine Fisheries Service regulates Hawaii-based bigeye fisheries under the Western Pacific Pelagics Fisheries Management Plan (FMP). Monitoring is the same as for Atlantic bigeye.

INDIAN OCEAN: The Indian Ocean Tuna Commission (IOTC) has not yet produced a definitive survey. There is no international management plan in place. No coordinated monitoring program in place. Little information is available about enforcement.

Enforcing international tuna fishing law is a challenge, given boats operate on the high seas. Some nations do various amounts of at-sea inspection and enforcement using surface vessels.

Overall, the management of bigeye tuna has moderate conservation concerns.

How was it caught?

Consumption advisory due to mercury.

No.