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Tuna, bigeye

Thunnus obesus

Method of production - Caught at sea
Capture method - Purse seine (FAD associated)
Capture area - South East and Eastern Central Atlantic (FAO 34,47)
Stock area - Atlantic
Stock detail - All Areas
Certification - None
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 2012 to 2015 annual TAC of 85,000t but above the incoming 2016 to 2018 TAC of 65,000t. It is projected that even the 2016 TAC will have a less than 50% probability of achieving a stock that is not overfished and not undergoing overfishing by 2028. High catch rates of juvenile bigeye tuna, primarily from FAD purse seine fisheries, have substantially reduced the maximum sustainable yield for the stock.

Bycatch of vulnerable species continues to be of concern, particularly in the pelagic longline fishery and FAD associated 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.

The 5% observer coverage required on vessels over 15m in length is also considered insufficient and several countries are failing to meet this requirement.

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 1.28Fmsy). In general, data availability has improved since 2010 but there is still a lack of information regarding detailed fishing effort and catch-at-size data from certain fleets. The Committee notes, as it did in previous assessments, that there is considerable uncertainty in the assessment of stock status.

Reported catches have been lower than the 2012 to 2015 annual TAC of 85,000t (provisional 2015 reported catch is 77,942t) but above the incoming 2016 to 2018 TAC of 65,000t. It is projected that even the 2016 TAC will have a less than 50% probability of achieving a stock that is not overfished and not undergoing overfishing by 2028.

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. Outside of the closure, a maximum of 500 FADs are permitted per vessel. Increased harvests on FADs could have had negative consequences for the productivity of bigeye tuna fisheries (e.g. reduced yield at MSY and increased SSB required to produce MSY) and, therefore, to increase long term sustainable yield, the Scientific Committee recommends that effective measures be found to reduce FAD-related and other fishing mortality of small bigeye tunas.

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.

Reported catches have been lower than the 2012 to 2015 annual TAC of 85,000t but above the incoming 2016 to 2018 TAC of 65,000t. It is projected that even the 2016 TAC will have a less than 50% probability of achieving a stock that is not overfished and not undergoing overfishing by 2028. Significant longline and purse seine effort has moved to this fishery as a result of piracy in the Indian Ocean in previous years which could increase the probability of the TAC being exceeded, despite there being a vessel register and limit on the number of authorised 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, but encourages countries to increase this in line with recommendations by the scientific committee, which concluded that current level of observers seems to be inappropriate to provide reasonable estimates of total by-catch and recommended increasing the minimum level to 20%. In addition, ICCAT has noted that the current mandatory level of observer coverage of 5% has not been implemented by many of the fleets and in 2016, updated their recommendation for a minimum of 5% observer coverage of fishing effort in each of the pelagic longline, purse seine, bait boat, traps, gillnet and trawl fisheries.

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 on-board the receiving vessel.

The scientific committee has recommended taking measures to reduce the bigeye TAC to levels that would allow a recovery with a high degree of probability and within a short timeframe and to find effective measures to reduce FAD-related and other fishing mortality of small bigeye tunas. The committee has concluded that the current area/time closure has not been effective at reducing the mortality of juvenile bigeye tuna.

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. Average weight of bigeye tuna caught in free schools is more than twice the average weight of those caught around FADs. 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 do 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, yet ICCAT has not received records of compliance from the majority of member states. Porbeagle is significantly overfished, and whilst there is a zero EU TAC for porbeagle, it is still caught incidentally and discarded, and also landed by other fleets. In 2016 additional measures for blue shark were introduced, mainly focussed on improved data recording, with potential to introduce Harvest Control Rules. Monitoring of bycatch is deficient in these fisheries and the scientific committee strongly recommends improvements in data collection.

In 2016, work was begun to improve ICCAT's understanding of the trophic ecology of pelagic ecosystems that are important and unique for species managed in this area. ICCAT also introduced a number of recommendations, to improve compliance and reporting. ICCAT has noted that the current mandatory level of observer coverage of 5% has not been implemented by many of the fleets and in 2016, updated their recommendation for a minimum of 5% observer coverage of fishing effort in each of the pelagic longline, purse seine, bait boat, traps, gillnet and trawl fisheries.

Read more about capture methods


References
Collette, B., Acero, A., Amorim, A.F., Boustany, A., Canales Ramirez, C., Cardenas, G., Carpenter, K.E., Chang, S.-K., Chiang, W., de Oliveira Leite Jr., N., Di Natale, A., Die, D., Fox, W., Fredou, F.L., Graves, J., Viera Hazin, F.H., Hinton, M., Juan Jorda, M., Minte Vera, C., Miyabe, N., Montano Cruz, R., Nelson, R., Oxenford, H., Restrepo, V., Schaefer, K., Schratwieser, J., Serra, R., Sun, C., Teixeira Lessa, R.P., Pires Ferreira Travassos, P.E., Uozumi, Y. & Yanez, E., 2011. Thunnus obesus. The IUCN Red List of Threatened Species 2011: e.T21859A9329255. Available at http://dx.doi.org/10.2305/IUCN.UK.2011-2.RLTS.T21859A9329255.en [Accessed Dec 2016].

Froese, R. and D. Pauly. Editors. 2016. FishBase. Available at www.fishbase.org [Accessed Dec 2016].

ICCAT, 2016. Resolutions, recommendations and other decisions. Available at https://www.iccat.int/en/RecsRegs.asp [Accessed Dec 2016].

ICCAT, 2016. Report of the standing committee on research and statistics. Madrid, Spain 3 to 7, October 2016. Available at https://www.iccat.int/Documents/Meetings/Docs/2016_SCRS_ENG.pdf [Accessed Nov 2016].

ISSF, 2016. ISSF Tuna stock status update, 2016: Status of the world fisheries for tuna. ISSF Technical Report 2016-05B. International Seafood Sustainability Foundation, Washington, D.C., USA. Available at http://iss-foundation.org/knowledge-tools/technical-and-meeting-reports/download-info/issf-2016-05b-status-of-the-world-fisheries-for-tuna-nov-2016/ [Accessed Nov 2016].

ISSF, 2016. Status of the world fisheries for tuna: Management of tuna stocks and fisheries, 2016. ISSF Technical Report 2016-14.International Seafood Sustainability Foundation, Washington, D.C., USA. Available at http://iss-foundation.org/downloads/13305/ [Accessed Nov 2016].

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