Tuna, bigeye

Thunnus obesus

Method of production — Caught at sea
Capture method — Longline
Capture area — Indian Ocean, Western (FAO 51) and Eastern ( FAO 57)
Stock area — Indian Ocean
Stock detail — All Areas
Picture of Tuna, bigeye

Sustainability rating three info

Sustainability overview

The latest stock assessment in 2016 indicated with high likelihoods that overfishing is not occurring and the stock is not in an overfished state. The provisional catch in 2017 is 90,050 t, lower than the 2013-2017 average of 95,997 t and below the estimated Maximum Sustainable Yield (MSY) of 104,000t. There are no specific management measures for IOTC bigeye tuna although interim target and limit reference points relating to MSY are in place. Some countries in the IOTC do not report fishery data which is important for stock assessment and management and in 2018, the IOTC introduced a new measure aimed at improving reporting on direct and incidental catches, including prohibiting a country from retaining a species if they fail to report catches for that species. About 54% of bigeye catch is taken in pelagic longline fisheries and there is concern for bycatch of sharks, turtles and seabirds in these fisheries. It is likely that longline fleets are contributing to decline of these species. Monitoring and mitigation of vulnerable bycatch species is generally insufficient and observer coverage is relatively low at 5% for vessels over 24m and for vessels fishing on the High Seas. 20% coverage has been recommended in other longline fisheries.

Commercial buyers should establish what measures the flag state and fleet relating to their source is taking to reduce impacts to and improve reporting of interactions with vulnerable species. Large buyers should consider supporting such improvements. Some longline fleets operating within this area are in a Fishery Improvement Project (FIP) which is making progress to improve some issues of environmental concern. More info about these FIPs is available from fisheryprogress.org.


Tuna belong to the family Scombridae. They are large, oceanic fish and are seasonally migratory, some making trans-oceanic journeys. Bigeye tuna is a tropical and subtropical species found from the surface down to 250 m in the Atlantic, Indian and Pacific Oceans. It is slower growing than skipjack or yellowfin tuna, maturing at about 3 years old and reaching a maximum size of 250cm in length and 200kg in weight, with a maximum age of 11 years. Bigeye are considered moderately resilient to exploitation.

Stock information

Criterion score: 0 info

Stock Area

Indian Ocean

Stock information

Indian Ocean stocks are managed by the Indian Ocean Tuna Commission (IOTC). Catches steadily increased from around 1950 to peak in 1999 at roughly 160,000 t, and have since gradually declined. The provisional catch in 2017 is 90,050 t, lower than the 2013-2017 average of 95,997 t and below the estimated Maximum Sustainable Yield (MSY) of 104,000t. Declines in longline effort since 2007, particularly from the Japanese, Taiwan, China and Rep. of Korea longline fleets have lowered the pressure on the Indian Ocean bigeye tuna stock, indicating that current fishing mortality would not reduce the population to an overfished state in the near future.

The latest assessment for bigeye tuna was undertaken in 2016, with less optimistic results than the previous assessment in 2013. It indicates with high likelihood that overfishing is not occurring (Fishing mortality, F, = 0.76Fmsy) and the stock is not in an overfished state (spawning biomass, SB, = 1.29SBmsy). Spawning stock biomass in 2015 was estimated to be 38% of the unfished levels.

Projections show that there is a low risk of stock status changing by 2025 if catches are maintained at 2015 levels. However, increased catch or increases in the mortality on immature fish will likely increase the probabilities of the stock becoming overfished or undergoing overfishing in the future. While there is confidence in this assessment, continued monitoring and improvement in data collection, reporting and analysis is recommended to reduce the uncertainty in assessments.


Criterion score: 0.75 info

Most tuna stocks range across and are accessed by numerous coastal states, making harmonised and effective management of these individual stocks very difficult. To try and achieve this, Intergovernmental Regional Fisheries Management Organisations (RFMOs) have been established; for this stock it is the Indian Ocean Tuna Commission (IOTC). There are five main tuna RFMOs worldwide and it is their responsibility to carry out data collection, scientific monitoring and management of these fisheries. Whilst the RFMOs are responsible for the development of management and conservation measures, the degree to which they are implemented, monitored and enforced still varies significantly between coastal states. For this reason, it is important to choose tuna that has been caught by vessels that are well regulated by their flag state.

There are persistent failures by some countries to report to the commission annually, including reporting catch data, and other issues with lack of data and poor quality data persist. In 2018 IOTC introduced a new measure aimed at improving reporting on direct and incidental catches, including prohibiting a country from retaining a species if they fail to report catches for that species.

IOTC has set targets and thresholds for fishing effort and spawning stock biomass for the species it manages. There is no harvest control rule for the stock, although in 2016 a Technical Committee on Management Procedures was established to help guide the Commission on developing one. However, there are no specific management measures in place for bigeye.

Other IOTC conservation and management measures of note include:
A ban on the discarding of bigeye, yellowfin and skipjack tunas by purse seine vessels which from 2018 will extend to non target species such as other tunas and billfish.
A ban on the use of aircrafts and unmanned aerial vehicles as fishing aids, which significantly contribute to fishing effort by helping to detect fish.
A ban on surface or submerged artificial lights for the purpose of aggregating tuna and tuna-like species beyond territorial waters.
In 2012 IOTC banned the use of driftnets on the high seas. In 2022 this will be extended to the entire IOTC area (i.e. within countries’ EEZs as well).
Regarding the use of drifting Fish Aggregation Devices (FADs): The maximum number of drifting FADs that can be in use at any one time by each purse seiner was reduced from 550 in 2015 to 425 in 2016 and has been set at 350 since 2017. The maximum that can be acquired each year was reduced from 1100 in 2015 to 850 in 2016 and has been 500 since 2017. Countries that use FADs must report regularly to the Commission and submit FAD management plans outlining how they will minimise mortality of juvenile yellowfin and bigeye tuna and vulnerable non-target species such as sharks, turtles and rays. To reduce the entanglement of sharks, marine turtles or any other species, the design and deployment of FADs must be based on certain principles: The surface of the FAD should not be covered, or only covered with non-meshed material; If a sub-surface component is used, it should not be made from netting but from non-meshed materials such as ropes or canvas sheets; To reduce the amount of synthetic marine debris, the use of natural or biodegradable materials should be promoted. From 2016, each FAD must be marked with a unique identification number.
There is a freeze on capacity to 2006 levels which extends to vessels greater than 24m in length, or vessels under this length operating in international waters. This is to be reviewed in 2018.
5% regional observer coverage is required for all vessels over 24m and for vessels under 24m fishing outside of their Exclusive Economic Zone (EEZ).
To help address IUU, the IOTC maintains an active vessel register and an IUU Vessel List and prohibits transhipments for large scale vessels at sea unless they are pre-approved, monitored by an observer and the vessel uses a Vessel Monitoring System (VMS).

In 2016 IOTC introduced a number of resolutions to improve the poor compliance with existing management measures, e.g. observer coverage, catch and effort reporting, support for countries to implement measures.

Capture Information

Criterion score: 0.75 info

54% of bigeye catch is made by longlining. Several mitigation measures are in place (sharks, turtles, sea birds) but monitoring is deficient. Large bigeye tuna (averaging just above 40 kg) are primarily caught by deep setting longliners. Since 2007, catches of bigeye tuna by longliners have been relatively low as a result of piracy in the Indian Ocean. Since 2012 however, catches appear to show signs of recovery as a result of a reduction in the threat of piracy and return of fleets that appear to be resuming fishing activities in their main fishing grounds in the north-west Indian Ocean. Longlining targets large mature fish compared with purse seining, yet is associated with significant incidental capture and mortality of vulnerable sharks, turtles and sea birds.

A number of threatened seabirds can interact with longline fisheries in the Indian Ocean, including the critically endangered Amsterdam albatross (which lives on Amsterdam Island in the southern Indian Ocean), shy, black-browed, and wandering albatrosses. As with most tuna RFMOs, the IOTC requires two seabird bycatch mitigation measures from a list of three options (weighted branch line, bird scaring lines, and night setting), but recommended best practice is for these three measures to be applied simultaneously. Monitoring and reporting is deficient, and there has not been enough information available to fully review the effectiveness of the applied mitigation measures.

There is currently too little data to carry out stock assessments for shark species, so the scientific committee recommends better monitoring and a precautionary approach to their management. Full utilisation of sharks is required (i.e. no fin removal), unwanted sharks must be released live wherever possible and shark catches must be reported annually. Hooking mortality is apparently very high for bigeye and pelagic threshers, therefore the prohibition on retaining of any part of thresher sharks onboard and promoting live release of thresher shark may be largely ineffective for species conservation. In 2018 IOTC introduced a new measure on management of blue shark stocks, requiring better data collection on catches and discards and paving the way to consider additional management measures in 2021. Transshipment of oceanic whitetips and thresher sharks is prohibited. Countries must develop conservation and management measures for vulnerable shark species. A number of countries are currently incorporating a ban on the retention of oceanic whitetip sharks into national legislation in accordance with IOTC resolutions, but it is too early to evaluate the impact of this.

The status of all turtle species in the Indian Ocean is concerning. The scientific committee advises that maintaining or increasing fishing effort in the Indian Ocean without appropriate mitigation measures in place will likely result in further declines in biomass, and recommends that appropriate mechanisms are developed to ensure compliance with data collection and reporting requirements. Turtles must be released wherever possible and countries are requested to research other mitigation techniques. Longliners must carry cutters or de-hookers to aid with this but gear modification, such as circle hooks, is not required.

Interactions with all vulnerable non-target species should be recorded. Several countries have failed to implement national plans for sharks, seabirds and turtles as required (although the shark plan is not binding in India as they have objected to the measure). Click here to see which countries had and had not fully implemented plans and actions for seabirds, sharks and marine turtles in 2016: http://www.iotc.org/documents/status-development-and-implementation-npoas-seabirds-and-sharks-and-implementation-foa


Based on method of production, fish type, and consumer rating: only fish rated 2 and below are included as an alternative in the list below. Click on a name to show the sustainable options available.

Anchovy, anchovies
Arctic char
Herring or sild
Salmon, Atlantic (Farmed)
Salmon, Chinook, King Salmon
Salmon, Chum, Keta, Calico or Dog salmon
Salmon, Coho , Silver, White
Salmon, Pink, Spring , humpback
Salmon, Sockeye , Red Salmon, Bluebacks, Redfish
Sprat, whitebait
Trout, Rainbow
Tuna, albacore
Tuna, bigeye
Tuna, skipjack
Tuna, yellowfin


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Fishery Progress, 2018. Fishery Improvement Project Directory. Available at https://fisheryprogress.org/ [Accessed on 05.12.2018].

IOTC, 2017. Report of the 13th Working Party on Ecosystems and Bycatch, IOTC-2017-WPEB13-R, for the Indian Ocean Tuna Commission, 4-8 September 2017, San Sebastian, Spain. 124pp. Available at http://www.iotc.org/meetings/13th-working-party-ecosystems-and-bycatch-wpeb13 [Accessed 21.11.2017].

IOTC, 2018. Compendium of Active Conservation and Management Measures for the Indian Ocean Tuna Commission, 04 October 2018. Avaiable at http://www.iotc.org/cmms [Accessed on 6.12.2018].

IOTC, 2018. Draft Resource Stock Status Summary Bigeye Tuna, Status of the Indian Ocean bigeye tuna (BET: Thunnus obesus) resource, IOTC-2018-SC21-ES02 for the 21st Meeting of the Scientifc Committee of the Indian Ocean Tuna Commission, 3-7 December 2018, Mahe, Seychelles. 3 pp. Available at http://www.iotc.org/meetings/21st-scientific-committee-sc21 [Accessed on 04.12.2018].

IOTC, 2018. Outcomes of the 22nd Session of the Indian Ocean Tuna Commission, IOTC-2018-SC21-16 [E], 21-25 May 2018, Bangkok, Thailand. 4 pp. Available at http://www.iotc.org/meetings/22nd-session-indian-ocean-tuna-commission-s22 [Accessed on 04.12.2-18].

IOTC, 2018. Report of the 22nd Session of the Indian Ocean Tuna Commission, IOTC-2018-S22-R[E], 21-25 May 2018, Bangkok, Thailand. 144 pp. Available at http://www.iotc.org/meetings/22nd-session-indian-ocean-tuna-commission-s22 [Accessed on 04.12.2-18].

ISSF, 2018. Status of the world fisheries for tuna: October 2018. ISSF Technical Report 2018-21. International Seafood Sustainability Foundation, Washington, D.C., USA. 103 pp. Available at: https://iss-foundation.org/about-tuna/status-of-the-stocks/ [Accessed on 06.12.2018].