Taxonomy
- The bigeye tuna, Thunnus obesus, is a true tuna of the genus Thunnus, belonging to the wider mackerel family Scombridae.
Species of Commercial Interest
- The designation “Bigeye” refers to a single species of tuna, Thunnus obesus.
Distribution
- Like most tuna species, Bigeye are wide-ranging, open-water fish. They occur in all tropical and temperate waters (but not the Mediterranean) with temperatures ranging from 13–29°C, but the optimum is between 17°C and 22°C. Variation in occurrence is closely related to seasonal and climatic changes in surface temperature. This species is mostly found above 500 m, but can dive deeper. With respect to relevant fisheries, there are four stocks of bigeye: the western and central Pacific (WCPO), eastern Pacific (EPO), Atlantic, and Indian Ocean.
Biology
- Bigeye matures at an early age, has a moderate lifespan, is highly fecund, and is widely distributed, making it inherently resilient to fishing pressure. Juveniles and small adults school at the surface in monospecific groups or mixed with other tunas, and may be associated with floating objects. Adults stay in deeper waters. Longevity is determined to be between 5-16 years depending on region. This species feeds on a wide variety of fishes, cephalopods and crustaceans during the day and at night.
- Age at first maturity is estimated to be about two years. This species is a multiple spawner that may spawn every one or two days over several months. They spawn over periods of the full moon, and spawn throughout the year in tropical waters. Spawning is primarily at night between 1900 and 0400 hr. The average mature female spawned every 2.6 days. The number of eggs per spawning has been estimated at 2.9–6.3 million. Eggs and larvae are pelagic.
Sustainability
- IUCN Red List Status: Vulnerable (VU)
- MBA Seafood Watch:
- Bigeye is caught with troll, pole-and-line and longline gear. There is little or no bycatch when bigeye is caught with troll or pole-and-line gear. However, longlines, the most common method, results in large bycatch, including threatened or endangered species such as sea turtles, sharks and seabirds. Since there are no international laws to reduce bycatch, these longline fleets are contributing heavily to the long-term decline of some of these species.
- Most of the world's bigeye populations have been depleted due to longline fisheries and longline-caught bigeye is ranked as "Avoid." One notable exception is longline-caught bigeye from the U.S. Atlantic, where strict bycatch regulations and more abundant populations result in a "Good Alternative" ranking. When possible, look for bigeye caught with troll or pole-and-line which has very low levels of bycatch.
Potential Health Concerns
- Environmental Defense Fund has issued a health advisory for longline-caught bigeye tuna due to elevated levels of mercury. (No consumption advisories are listed for troll- or pole-and-line-caught bigeye as these methods catch younger tuna with lower mercury levels.)
