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Tuna, bigeye

Thunnus obesus

Method of production - Caught at sea
Capture method - Purse seine (non FAD associated)
Capture area - Western & Central Pacific - WCPO (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 it is being subject to considerable overfishing. 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 now in an overfished state. 41% 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 Maximum Sustainable Yield (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.

MCS recommends avoiding 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. 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). Bigeye catches in 2013 were about 145,900t, an 8% decrease from 2012 yet approximately equal to the last five year average of 145,000t. The last stock assessment was conducted in 2014 and included data from over the last 60years. Fishing mortality, F, is estimated to have increased through time, particularly in recent years, and current levels are 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 is needed in order to return to sustainable levels (Fmsy). The Spawning Biomass, SB, is estimated to have declined over the duration of the fishery and is now 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 total bigeye catch in 2014 was 161,229t, which was a 5% increase over 2013 and a 5% increase over the average between 2010 and 2013. Whilst not directly correlated, this is 48% above the estimated MSY.

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

Only 5% of the total bigeye catch in the WCPO is taken purse seining on free schooling fish (as opposed to using Fish Aggregation Devices, FADs). These fisheries target smaller bigeye that aggregate closer to the surface whereas the longliners target larger fish that inhabit deeper waters. Many juvenile fish are often caught in purse seine fisheries and the method is also associated with bycatch of sharks, turtles and cetaceans. Whilst the proportion of this bycatch compared with the total catch is relatively low, the high volume of tuna that is caught means that even a low proportion of this can still be relatively high in terms of total volume or individuals. FAD associated purse seine fisheries generally catch higher proportions of juvenile tunas and vulnerable bycatch species. Poorly designed FADs can also entangle such species. In this fishery, the high catch of juveniles has significantly reduced the Maximum Sustainable Yield (MSY) achievable from the stock. The widespread use of FADs is also of concern due to the unknown impacts such gear might have on natural species composition of tuna schools, migratory patterns, growth rates and predation rates of affected pelagic species.

Bycatch mitigation measures such as sorting grids are in place to reduce mortality of turtles, sharks and non-target species in general yet the effectiveness of these measures have yet to be demonstrated. Vessels are prohibited to knowingly set a purse seine around tuna schools associated with a live whale shark, marine mammal or turtle, and all reasonable steps are required to be taken to ensure the safe release of these animals and details of the interaction are to be recorded if accidentally encircled. Permissible sharks are to be fully utilized and no more than 5% of fins to total shark weight can be retained; and there is a prohibition to land silky and oceanic whitetip sharks. The WCPFC have implemented a raft of management measures in recent years to limit and reduce the impacts of the purse seine fisheries on the tuna stocks, specifically skipjack, bigeye and yellowfin. Click on the Management TAB for more information about these measures.

Read more about capture methods


References
Froese, R. and Pauly, D. Editors, 2015. FishBase. Available at www.fishbase.org [Accessed Nov 2015].

ISSF, 2015. Status of the world fisheries for tuna: management of tuna stocks and fisheries. Nov 2015 Update. Available at http://iss-foundation.org/knowledge-tools/reports/technical-reports/download-info/issf-technical-report-2015-03-a-status-of-the-world-fisheries-for-tuna-november-2015/ [Accessed Dec 2015].

WCPFC, 2015. Conservation and management measures. Available at http://www.wcpfc.int/system/files/booklets/31/CMM%20and%20Resolutions.pdf [Accessed Dec 2015].

WCPFC, 2015. Scientific committee eleventh regular session summary report. Pohnpei, Federated States of Micronesia. 5 to 13 August 2015. Available at https://www.wcpfc.int/system/files/SC11%20Summar%20Report%20-%2019Oct2015-with%20ES.docx [Accessed Dec 2015].

WCPFC, 2015. Western and Central Pacific Fisheries Commission tuna fishery yearbook 2014. Available at https://www.wcpfc.int/system/files/YB_2014.pdf [Accessed Dec 2015].

WCPFC, 2014. Stock assessment of bigeye tuna in the Western and Central Pacific Ocean. Available at https://www.wcpfc.int/system/files/SC10-SA-WP-01%20%5BBET%20Assessment%5D_rev1_25July.pdf [Accessed Dec 2014].

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