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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
Accreditation -
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.

Unless pole and line caught, bigeye from the WCPO is best avoided at this stage until the stock recovers to healthier levels.

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). Provisional 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 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) with SB estimated 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 in 1970s, prior to the introduction widespread use of Fish Aggregation Device (FAD) associated purse seining which takes a high proportion of juvenile bigeye.

Management

Most tuna stocks range across and are accessed by numerous coastal states, making harmonised and effective management of these individual stocks very difficult. As a result, 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. The bigeye tuna fishery in the Western and Central Pacific Ocean (WCPO) is managed by the Western and Central Pacific Fisheries Commission (WCPFC). Whilst the RFMOs are responsible for the development of management and conservation measures, the degree to which they are implemented, monitored and enforced varies significantly between coastal states.

There is no Total Allowable Catch (TAC) set for this stock although a range of measure have recently been implemented to try and reduce effort with the aim of reducing fishing mortality to sustainable levels (below Fmsy) by the end of 2017. As the FAD purse seine fishery takes a considerable proportion of juvenile bigeye and has a disproportionately large impact on the stock, these primarily apply to this gear type, including: a 3-month closure of fishing on FADs in EEZ waters and on the High Seas between 20degrees N and 20degrees S; plus each member shall choose between extending this FAD closure through October, or limit the number of FAD sets to less than the number of sets made by its vessels in a prescribed reference period; in 2015 and 2016, additional measures will be extended to either a 5 month FAD closure or stronger limits in the number of FAD sets per vessel. In 2017, a prohibition on FAD sets on the high seas will apply, except for vessels purse seine flagged to Kiribati. Additionally there is a requirement to submit FAD management plans, including information on strategies used to implement the closure and other measures for reducing mortality of juvenile bigeye. It isn't clear how successfully these measures have been implemented yet.

Beyond these measures for FAD fisheries, other measures include: limits for days at sea and a requirement by member states to prevent days at sea increasing; country specific catch limits for bigeye caught by longliners with monthly reporting requirements; a purse seine landing obligation for bigeye, skipjack and yellowfin between 20N and 20S; and a limit between 20N and 20S in the number of purse seine and longline vessels with freezing capacity.

100% regional observer coverage is required for all purse seine vessels fishing on the high seas and in waters under the jurisdiction of one or more coastal States, or vessels fishing in waters under the jurisdiction of two or more coastal States during the same trip; and on all purse seiners fishing between 20N and 20S. 5% coverage is required on longliners greater than 20m in length.

To help address IUU, the WCPFC maintains an IUU Vessel List; prohibits transhipments at sea between purse seiners (Some exemptions apply) and requires all other transhipments to be documented and 100% observed as part of the regional observer programme.

Capture information

Only 6% 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, 2013. FishBase. World Wide Web electronic publication. Available at www.fishbase.org [Accessed Nov 2013].

ISSF, 2014. ISSF Tuna Stock Status Update, 2014: Status of the world fisheries for tuna. ISSF Technical Report 2014-09. International Seafood Sustainability Foundation, Washington, D.C., USA. Available at http://iss-foundation.org/wp-content/uploads/downloads/2014/09/ISSF-2014-09-Status-of-the-Stocks-Sept-11-BYCATCH-APPENDIX.pdf [Accessed Nov 2014].

IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. . [Accessed December 2013].

WCPFC, 2015. Conservation and management measures. Available at https://www.wcpfc.int/conservation-and-management-measures [Accessed Jan 2015].

WCPFC, 2014. Scientific committee tenth regular session: summary report. 6 to 14 August 2014. Majuro, Republic of the Marshall Islands. Available at https://www.wcpfc.int/system/files/SC10%20-%20final_posted_1.docx [Accessed Jan 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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