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Tuna, bigeye

Thunnus obesus

Method of production - Caught at sea
Capture method - Longline
Capture area - North West and Central Pacific (FAO 61,71,77)
Stock area - WCPO
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

Bigeye tuna have a high susceptibility to fishing pressure. Fishing mortality on the stock in the Western and Central Pacific Ocean has generally been increasing over time. The latest assessment undertaken in 2014 indicates that the stock is being subject to considerable overfishing and recent catches are well above the estimated maximum sustainable yield (MSY). Similar to previous assessments, the assessment indicated that a 36% reduction in fishing mortality is needed in order to return fishing mortality to sustainable levels (Fmsy). Similarly, the biomass has declined over the duration of the fishery and is in an overfished state. 38% of the catch is taken in FAD associated purse seine sets, which encounter high proportions of juvenile tuna and relatively high proportions of bycatch compared with purse seine sets on free schools. The high catch of juvenile bigeye from these fleets has significantly reduced the achievable MSY of the fishery.

The WCPFC has implemented a wide range of management measures, particularly on the FAD purse seine fishery, in order to reduce fishing mortality to sustainable levels by 2017, yet it is not clear how successful these measures will be.

44% of the catch is taken in pelagic longline fisheries. Whilst these fisheries target larger, mature bigeye, they are associated with the bycatch of vulnerable species including sharks, turtles and sea birds. Whilst there are a range of mitigation measures required to be employed, the effectiveness of these measures is yet to be evaluated. Generally, the reporting of interactions with vulnerable species needs to be improved.

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. Bigeye are considered moderately resilient to exploitation.

Stock information

Stock area
WCPO

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Stock information
Bigeye tuna in the Western Central Pacific Ocean is assessed by the Western and Central Pacific Fisheries Commission (WCPFC). The total bigeye catch in 2015 was 134,084t, which was a 16% decrease over 2014 and a 13% decrease over the average for 2010-14. The last stock assessment was conducted in 2014 and included data from over 60 years. Fishing mortality, F, was estimated to have increased through time, particularly in recent years, and levels were far in excess of sustainable levels (1.57Fmsy), therefore overfishing is occurring. Similar to previous assessments, the assessment indicated that a 36% reduction in fishing mortality was needed in order to return to sustainable levels (Fmsy). The Spawning Biomass, SB, was estimated to have declined over the duration of the fishery and was below the level that would support the Maximum Sustainable Yield (Bmsy at 0.77SBmsy). The fishery is therefore in an overfished state.

The latest estimate of MSY 108,520t is higher than previous assessments though it is noted that this is considerably lower that what could have been achieved prior to the introduction and widespread use of Fish Aggregation Device (FAD) associated purse seining which takes a high proportion of juvenile bigeye.

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

44% of bigeye catches in the WCPO are taken in pelagic longline fisheries. The number of boats in the fishery fluctuates between 4,000 and 5,000 and involves large, distant, water freezer vessels and smaller (typically less than 100t) offshore vessels, usually domestically based. Pelagic longlining in the WCPO is associated with the incidental capture and mortality of vulnerable species including sharks, turtles, sea birds and billfish. Various bycatch mitigation measures are available and required.

In 2017 a new seabird measure came into force for longliners, requiring vessels to use one or more of the following, depending on vessel size and fishing location: weighted branch lines, night setting, tori lines, side setting with a bird curtain and weighted branch lines, blue-dyed bait; deep setting line shooter; management of offal discharge. The commission is also beginning a project in 2017 to estimate seabird mortality.

Scientific Committee research in 2016 into sea turtle mitigation in longline fisheries found that interaction rates are lower when large circle hooks are used, higher at hooks closest to floats and higher when squid baits are used. Current measures relating to turtles include: the use of circle hooks for shallow set gear to reduce turtle capture, along with the requirement to carry line cutters and de-hookers and to promptly release turtles or to foster to recovery any sick or comatose turtles captured.

In 2016, catches of silky sharks in the longline fishery were around three times higher than in the purse seine fishery. Shark measures include: full utilisation of permissible sharks and retention of no more than 5% of fins to total shark weight; a prohibition to land silky and oceanic whitetip sharks; a prohibition on the use of wire leaders and lines running directly off the longline floats or drop lines - known as shark lines; and for fisheries specifically targeting sharks, WCPFC countries are also required to develop management plans, demonstrating how they intend to avoid or reduce catches of highly depleted shark species. Data on the effectiveness of the WCPFC ban on shark finning is very limited. In 2016, WCPFC adopted measures to improve recording of manta and mobula rays discarded and released, and to treat these species as key shark species for assessment and research.

In general, the effectiveness of the above measures has not been evaluated. Monitoring is deficient and the reporting of interactions with vulnerable species is poor.

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].

ISC, 2016. Report of the sixteenth meeting of the international scientific committee for tuna and tuna-like species in the North Pacific Ocean: plenary session. Sapporo, Hokkaido, Japan, 13-18 July 2016. Available at http://isc.fra.go.jp/reports/isc/isc16_reports.html [Accessed Dec 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].

WCPFC, 2016. Conservation and management measures. Available at https://www.wcpfc.int/conservation-and-management-measures [Accessed Dec 2016].

WCPFC, 2016. Scientific committee meeting: summary report. Twelfth regular session. Bali, Indonesia, 3-11 August 2016. Available at https://www.wcpfc.int/meetings/sc12 [Accessed Dec 2016].

WCPFC, 2016. Thirteenth regular session of the commission. Denarau Island, Fiji 5-9 December 2016. Available at https://www.wcpfc.int/meetings/13th-regular-session-commission [Accessed Dec 2016].

WCPFC, 2014. Western and Central Pacific fisheries commission scientific committee: summary report. Tenth regular session. Majuro, Republic of the Marshall Islands, 6-14 August 2014. Available at https://www.wcpfc.int/meetings/10th-regular-session-scientific-committee [Accessed Dec 2016].

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