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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
Accreditation -
Fish type - Oily fish

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This fish, caught by the methods and in the area listed above, is not the most sustainable choice of fish to eat. Click on the rating icon above to read more and on the alternatives tab below to find more sustainable fish to eat.


Sustainability overview

Bigeye in the Atlantic is at about the level which can produce the maximum sustainable yield and fishing mortality similarly is just below Fmsy, indicating the fishery is in a healthy state. It is noted though that the fishery could not sustainably tolerate an increase in effort, so it is important for the individual Member nations to adopt catch levels in accordance with scientifically recommended TAC of 85,000t (for 2012-15). ICCAT management has only been partially effective at controlling catch and effort as some illegal and unreported fishing is believed to be occurring in some regions. Bycatch and discards for the longline and FAD associated purse seine fisheries are thought to be significant and both fleets need to adopt better measures to mitigate this and improve monitoring. The pole & line and non-FAD purse seine fisheries represent the best options.

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 bigeye stock declined rapidly in the 1990s due to high catch levels, but has recently stabilized at a healthy level (approximately Bmsy). The latest stock assessment was undertaken in 2010 and has been updated to include 2011 data. The ICCAT scientific committee note that (like previous ones) 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 current gaps in CPUE and catch-at-size data for many fleets is still a problem. Additionally, IUU fishing is still known to be occurring.

There are indications in recent years that biomass may be increasing and fishing mortality decreasing. The 2012 provisional catch of 70,536t is a little lower than the previous five year average of 76,732t and is also lower than both the estimated mean Maximum Sustainable Yield (MSY) of 92,000t and the Total Allowable Catch (TAC) of 85,000t. The 2009 assessment indicated that biomass was about equal to that which could support MSY (1.01Bmsy, range 0.72 to 1.34Bmsy) suggesting that the stock was not overfished. Similarly, fishing mortality, F, at 0.95Fmsy (Range 0.65 to 1.55Fmsy) suggested that overfishing was not occurring. Recent catches suggest this is still the case.

Bigeye 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. For this reason, it is important to choose tuna that has been caught by vessels that are well regulated by their flag state. 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.

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.

Selected ICCAT management and conservation recommendations regarding bigeye tuna fisheries include:

A TAC of 85,000t is in place 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.

Regarding Fish Aggregation Device (FAD) purse seine fisheries: there is a two month prohibition of fishing on floating objects in an area off West Africa, with 100% observer coverage during this time/area closure. There is also a requirement to develop and report annually on FAD management plans, including construction design and materials.

Regarding bycatch: a prohibition to retain at risk shark species including bigeye thresher, oceanic whitetip, hammerhead and silky sharks. This has been in place for over three years, yet ICCAT has not received records of compliance from the majority of member states. For longline fisheries, a combination of at least two bird mitigation measures are required to be used in most areas, yet monitoring and data relating to this and other vulnerable bycatch species is deficient.

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

32% of the bigeye catch from the Atlantic is taken in purse seine fisheries that set gear on floating objects including Fish Aggregation Devices (FADs). FADs are thought to have negative ecosystem effects and have been linked to changes in migratory patterns, growth rates and predation rates of affected pelagic species. Non FAD associated purse seiners still encounter bycatch, yet on a much smaller scale compared with FAD fisheries. There are measures employed to reduce bycatch such as sorting grids, in addition to spatial and temporal closures and yet continued monitoring is required to assess their effectiveness. Purse seiners target far younger yet many more fish than longlining. The longline fishery catches medium to large (60kg) bigeye tuna compared with the purse seine fishery which generally catches fish of about 4kg. In some fisheries,

Read more about capture methods


References
ICCAT, 2012. Bigeye executive summary report 2012-13. Available at http://www.iccat.int/Documents/SCRS/ExecSum/BET_EN.pdf [Accessed Dec 2012].
ICCAT, 2010. Report of the 2010 ICCAT bigeye tuna stock assessment session . Pasaia, Gipuzkoa, Spain - July 5 to 9, 2010. Available at http://www.iccat.int/Documents/Meetings/Docs/2010_BET_Assessment_REP_ENG.pdf [Accessed Dec 2012].
ISSF, 2011. Bigeye: Atlantic Ocean. Available at http://iss-foundation.org/science/status-of-the-stocks/ [Accessed Dec2012].
IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. . [Accessed December 2013].

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