Bigeye tuna

Scientific name: Thunnus obesus

Family: Scombridae

Other names: Bigeye

Description: Bigeye tuna have torpedo-shaped bodies with dark metallic blue backs, and whitish lower sides and belly. The dorsal and anal fins are yellow. The finlets are bright yellow with a black margin. Live fish have an iridescent blue lateral band running along the sides. The eye is relatively large compared with that of other tunas.

Size (length and weight): Up to 2.5 metres in length and 200 kg. Commonly found at 1.8 metres in length and 100 kg.

Life span: Up to 11 years, depending on location.

Habitat: Bigeye tuna occur in the tropical waters of the Pacific, Atlantic and Indian Oceans. Bigeye tuna are highly migratory and travel long distances. They are a pelagic species that can be found to depths of 250 metres. Juveniles and sub-adults usually school at or near the surface with other tuna species. Schools may also occur in association with floating objects.

Prey: Various fishes, squid and crustaceans.

Predators: Larger tunas, billfish and toothed whales.

Reproduction: Most bigeye tuna reach reproductive maturity at about two years of age. Spawning occurs throughout the year in tropical waters, although the majority of spawning appears to occur in the eastern Pacific Ocean. Peak spawning periods in the southern hemisphere are between summer and autumn. Females may spawn every 2‑3 days during the spawning season. Spawning primarily occurs at night. Females produce 2.9‑6.3 million eggs per spawning event.

Other notes: Bigeye tuna are classified as Vulnerable by the IUCN Red List of Threatened Species.

The Commonwealth catch of bigeye tuna is managed by quota. Which means the catch of this fish by commercial fishers is restricted by weight. AFMA also restricts the type of gear that can be used to fish for bigeye tuna.

Commercial fishermen are required to fill in records of their catches, during each fishing trip and when they land their catch in a port. This helps us keep records of how much is being caught.

AFMA decide on the amount that can be caught each year from expert advice and recommendations from fisheries managers, industry members, scientist and researchers.

The commissions meets every year to review the latest scientific information and to set global catch limits for key tuna species such as bigeye tuna. They also specify what each member country must do to manage their catch of the tropical tuna species, such as carrying observers, sharing fishing information and tracking fishing vessels by satellite.

AFMA decide on the amount that can be caught each year from expert advice and recommendations from fisheries managers, industry members, scientist and researchers.

When managing the Australian catch of bigeye tuna, AFMA has to follow the decisions made by both commissions.

Bigeye tuna is a highly migratory species, swimming continuously over large distances. They are found commonly in eastern Australian waters, but they are also found in the waters of other countries within the Pacific and Indian Oceans.

In Australia, bigeye tuna are caught anywhere along both the east and west coasts and is a targeted species by fishers in both the Eastern Tuna and Billfish Fishery and the Western Tuna and Billfish Fishery. Bigeye tuna is a tropical tuna species and prefers warmer oceanic waters.

There are generally low levels of bycatch, or other species, caught in the Eastern Tuna and Billfish Fishery, but some sharks, seabirds and other species may be caught when fishing for bigeye tuna using longline gear.

Pelagic longline fishing has minimal impact on the marine environment as they hang in the water column not touching the seafloor. Sometimes seabirds, turtles and sharks are caught accidentally. The capture of these species is monitored by on-board AFMA observers and any interactions with these species must be reported to AFMA and the Department of the Environment. In recent years, AFMA in conjunction with the Department of the Environment and the fishing industry have worked hard to reduce seabird interactions with longline gear.

Ecological risk assessments are done for the Eastern Tuna and Billfish Fishery and all other fisheries to find out if the fishing of bigeye tuna is harming other species. The most recent assessment was done in 2019 and the risk to other species was assessed as low.

Gear

• Pelagic longline