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Tuna, bigeye

Thunnus obesus

Method of production - Caught at sea
Capture method - Purse seine (FAD associated)
Capture area - Atlantic (FAO 34,47)
Stock area - Atlantic
Stock detail - All Areas
Certification -
Fish type - Oily fish

Sustainability rating Click for explaination of rating

This fish, caught by the methods and in the area listed above, is the least sustainable fish to eat and should be avoided. Click on the rating icon above to read more and on the alternatives tab below to find sustainable fish to eat.


Sustainability overview

The latest stock assessment undertaken in 2015 revealed a somewhat changed perception of the bigeye stock after adjustments were made to account for previous misreported catches, indicating that that the stock is overfished and subject to overfishing. Reported catches have been lower than the TAC of 85,000t, yet it is suggested that the TAC should have been revised to better account for the high proportion of juvenile bigeye being caught in the FAD purse seine fisheries. High catch rates of juvenile bigeye tuna, primarily from FAD purse seine fisheries, have substantially reduced the maximum sustainable yield for the stock. There is also concern that current measures may not be sufficient to prevent increases in effort.

Bycatch of vulnerable species continues to be of concern, particularly in the pelagic longline fishery, FAD associated purse seine, and to a lesser extent in free schooling purse seine fisheries. Monitoring and reporting of interactions with vulnerable bycatch species needs improvement across all fleets. Increasing FAD catches are of concern due to the higher bycatch rates and unknown ecosystem impacts. Pole and line is more selective and the only gear type that is not red rated for this fishery though a stock assessment of the bait fish used by this gear type is needed.

MCS advises against buying any products from red rated tuna fisheries.

Biology

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 250m 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. Bigeyes are considered moderately resilient to exploitation.

Stock information

Stock area
Atlantic

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Stock information
Atlantic bigeye and other Atlantic tuna stocks are assessed by the International Commission for the Conservation of Atlantic Tunas (ICCAT). The latest stock assessment undertaken in 2015 revealed a somewhat changed perception of the stock after adjustments were made to account for previous misreported catches, indicating that that the stock is overfished and subject to overfishing (Biomass, B at 0.67Bmsy and Fishing mortality, F, at 1.23Fmsy). The ICCAT scientific committee note that (like previous assessments) there is considerable uncertainty in the assessment and that many assumptions have had to be made. Large gaps in historical data from poor reporting and IUU fisheries in addition to ongoing gaps in fine scale data reporting for many fleets is still a problem.

Reported catches have been lower than the 2012 to 2015 annual TAC of 85,000t (provisional 2014 reported catch is 72,585t), yet it is suggested that the TAC should have been revised to better account for the high proportion of juvenile bigeye being caught in the FAD purse seine fisheries. High catch rates of juvenile bigeye tuna, primarily from FAD purse seine fisheries, have substantially reduced the maximum sustainable yield for the stock.

The bigeye tuna species is assessed as Vulnerable by the IUCN (International Union for the Conservation of Nature).

Management

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. There are five main tuna RFMOs worldwide and it is their responsibility to carry out data collection, scientific monitoring and management of these fisheries. Eastern Pacific Ocean bigeye tuna is managed and assessed by the Inter-American Tropical Tuna Commission (IATTC). 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.

Stock management measures implemented in 2004 have been largely attributed to the recovery trend for bigeye tuna in the EPO until 2010. However, there remain concerns that current management measures are insufficient to prevent increases in effort and reductions in the biomass. There is no Total Allowable Catch (TAC) for the fishery. The main stock conservation measures established include: a 62-day closure for purse seiners greater than 182 tons capacity; a seasonal closure of the purse seine fishery in an area known as "El Corralito", west of the Galapagos Islands, where catch rates of small bigeye are high; a full retention requirement for bigeye, skipjack and yellowfin tunas for all purse seine vessels (yet the degree of enforcement regime may vary depending on the country or authority); and country specific catch limits for the main longline nations including: China, Japan, Korea, and Chinese Taipei. Despite these measures, there remains some concern that adequate measures are not in place to prevent overfishing from occurring.

There is 100% observer coverage on large purse seiners in the EPO and 5% observer coverage on longliners greater than 20m.

To help address IUU, the IATTC maintains an IUU Vessel List; maintains a register of authorised fishing vessels; and prohibits transhipments at sea for most vessels (Some exemptions apply) and requires most other transhipments to be documented and observed as part of the regional observer programme.

Capture information

Approximately 30% of the bigeye catch from the Atlantic is taken in purse seine fisheries that set gear on floating objects including Fish Aggregation Devices (FADs). Purse seiners generally target far younger yet many more fish than longlining. When a high proportion of young fish are removed from a population, the potential maximum catch from a fishery is reduced. The longline fishery catches medium to large (60kg) bigeye tuna compared with the purse seine fishery which generally catches fish of about 4kg. The increasing use of FADs is of concern due to the unknown impacts such gear might have on natural species composition of schools, natural migratory patterns, growth rates and predation rates of affected pelagic species. FAD fisheries also catch a higher proportion of small fish and bycatch species compared with non FAD associated purse seiners. There are measures employed to reduce bycatch such as sorting grids, in addition to spatial and temporal closures yet continued monitoring is required to assess their effectiveness. Purse seines are associated with lower mortality rates of sharks, turtles and certainly birds, compared with longlining, however their widespread use means they still have a significant impact on shark and turtle populations, and certainly on some cetacean species.

To reduce catches of juvenile tuna, there is a two month prohibition of fishing on floating objects in an area off the West African coast, with 100% observer coverage required during this time/area closure. There is also a requirement to develop and report annually on FAD management plans, including construction design and materials yet monitoring and reporting is deficient for many coastal states and effectiveness of measures has not been assessed.

There is also a prohibition to retain at risk shark species including bigeye thresher, oceanic whitetip, hammerhead, silky and porbeagle sharks. This has been in place for over four years (except for porbeagle), yet ICCAT has not received records of compliance from many member states.

Read more about capture methods


References
Froese, R. and Pauly, D. Editors, 2013. FishBase. World Wide Web electronic publication. Available at www.fishbase.org [Accessed Dec 2015].

ICCAT, 2015. Report of the Standing Committee on Research and Statistics (SCRS). Madrid, Spain, 28 Sept to 2 Oct 2014. Available at https://www.iccat.int/com2015/DocENG/PLE_104_ENG.pdf [Accessed Dec 2015].

ICCAT, 2015. Report of the Standing Committee on Research and Statistics (SCRS). Madrid, Spain, 28 Sept to 2 Oct 2014. Available at https://www.iccat.int/com2015/DocENG/PLE_104_ENG.pdf [Accessed Dec 2015].

ICCAT, 2015. Resolutions, recommendations and other decisions. Available at https://www.iccat.int/en/RecsRegs.asp [Accessed Dec 2015]. ICCAT, 2014. Inter-sessional meeting of the sub-committee on ecosystems. Olhao, Portugal, 1 to 5 September 2014. Available at https://www.iccat.int/Documents/Meetings/Docs/2014_SCECO_Rep_ENG.pdf [Accessed Nov 2014].

ISSF, 2015. ISSF Tuna Stock Status Update, 2015: Status of the world fisheries for tuna. ISSF Technical Report 2015-03A. International Seafood Sustainability Foundation, Washington, D.C., USA. Available at http://iss-foundation.org/wp-content/uploads/downloads/2015/11/ISSF-2015-03A-Status-of-the-Stocks-Nov-2015.pdf [Accessed Dec 2015].

IUCN 2015. IUCN Red List of Threatened Species. Version 2015.4. . [Accessed December 2015].

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