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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. In the Atlantic tuna stocks are assessed by the International Commission for the Conservation of Atlantic Tunas (ICCAT). 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. There remain large data deficiencies in most tuna fisheries, particularly with regards to fine scale spatial and temporal data for both target and especially vulnerable bycatch species. For this reason, it is important to choose tuna that has been caught by vessels that are well regulated by their flag state. Commercial buyers in particular should establish what measures their source country and fleet are taking to improve these deficiencies. Fleets on the West African coast need improved monitoring and enforcement as non-compliance remains a problem. The committee also stress the importance of reducing catches of juvenile tuna in the Gulf of Guinea.

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.

The Scientific Committee note that the continued increase in harvests on Fish Aggregation Devices (FADs) could reduce the maximum sustainable yields from yellowfin, bigeye and bycatch stocks. Should ICCAT wish to improve the long-term yields, effective measures to reduce FAD-related and other fishing mortality of small bigeye should be found.

A TAC of 85,000t is in place for 2012 to 2015 and provisional catch reports indicate that this is being adhered to, however the total permissible available to member states exceeds this TAC. Significant longline and purse seine effort has moved to this fishery as a result of piracy in the Indian Ocean in previous years and there is still some risk that that the TAC could be exceeded despite their being a vessel register and limit for vessels over 20m in length.

ICCAT requires 5% observer coverage on all purse seine, longline and pole and line vessels greater than 15m in length.

To reduce IUU, ICCAT have developed an IUU vessel register and a register of vessels authorised to undertake transhipments at sea. Additionally, transhipments at sea can only take place if an ICCAT Observer is onboard the receiving vessel.

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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