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.

ATLANTIC: About 25% of the world’s bigeye tuna is caught in the Atlantic Ocean. Between 1997 and 2000, the fishery transitioned from “not overfished” to “overfished” in NMFS’ ratings. Short-term abundance trends are encouraging with the spawning biomass increasing. Of all the Atlantic bigeye harvested since 1981, approx. 55% are smaller than the ICCAT minimum size of 3.2kg.
EASTERN PACIFIC: Approximately 37% of the world’s bigeye tuna catch is from the eastern Pacific. Bigeye biomass in the eastern Pacific is below that which produces maximum sustainable yield, but well above the minimum stock size threshold. Eastern Pacific bigeye have been in decline for two decades but there has been a short-term upswing in spawning biomass and abundant juveniles reaching spawning age. However, IATTC also notes that there has been a substantial increase in the take of immature bigeye since 1993. Thus stock rebuilding is not yet required but there is a need to reduce fishing mortality.
WESTERN PACIFIC—23% of the world’s bigeye. Abundance as a whole has been steady at about 70% of unfished biomass since the 1960s. There is a long-term declining trend in the weight of bigeye landed, related to the increased take of juveniles in FAD purse-seine fisheries. Recruitment was good the past few years making for short-term increases in abundance. For Western Pacific bigeye, overfishing is now occurring.
US Catch: NMFS’ review of trends in longline CPUE indicate that the biomass of bigeye fished by US West coast fleets has declined since the 1970s.
INDIAN OCEAN-25-33% of the world’s tuna catch. Artisanal fisheries take as much as industrial fisheries. Stock status is not well understood.
Overall, the stock status of bigeye tuna is 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 stabilised 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 authorised 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.