Tuna, bigeye

Thunnus obesus

Method of production — Caught at sea
Capture method — Pole & line
Capture area — Western & Central Pacific - WCPO (FAO 61,71,77)
Stock area — Western and Central Pacific
Stock detail

All Areas


Picture of Tuna, bigeye

Sustainability rating two info

Sustainability overview

A new stock assessment was conducted in 2017 and indicated that the stock is likely not in an overfished state and not undergoing overfishing. There are large uncertainties in these results though. In 2018, the Western and Central Pacific Fisheries Commission (WCPFC) adopted a bridging measure to manage bigeye, yellowfin and skipjack until harvest strategies are in place or February 2021. As part of this, spawning biomass depletion ratios for bigeye and yellowfin are to be maintained at or above recent average levels. Whiles there is no Total Allowable Catch (TAC) set for this stock, a range of measures have been implemented to reduce effort. Measures mainly focus on the Fish Aggregation Devices (FAD) purse seine fishery which takes a considerable proportion of juvenile bigeye and has a disproportionately large impact on the stock. Only 3% of the catch is taken in pole & line fisheries. These are very selective but do rely on a substantial amount of small live fish for bait. The total catch of bait fish in pole & line fisheries is generally considered low compared to targeted fisheries for these species and is unlikely to overexploit these stocks, but could have implications for local availability.

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

Criterion score: 0.25 info

Stock Area

Western and Central Pacific

Stock information

Bigeye tuna in the Western Central Pacific Ocean is assessed by the Western and Central Pacific Fisheries Commission (WCPFC).

A new stock assessment was conducted in 2017 and is considered to be the most advanced and comprehensive assessment yet conducted for this species, with improvements to both the data and the model used. It shows that the bigeye stock may be in better condition than previously thought. There is an 84% probability that the stock is not in an overfished state, with recent Spawning Biomass (SB) at 32% of unfished levels as opposed to 20% in 2014. It is also thought that the stock is not undergoing overfishing (77% probability), with the ratio of recent fishing mortality (F) to FMSY at 0.83 (in 2014 it was 1.57). These positive changes are primarily due to three factors: the inclusion of new growth curve information, the inclusion of a new regional assessment structure, and estimated increases in recruitment in recent years. It should be noted though there is much greater uncertainty in this stock assessment, primarily owing to the first two factors. The significance of the recent high recruitment and the progression of these fish to the spawning potential component of the stock are encouraging, but the cause is unknown: it could be one or a combination of environmental conditions or management measures, or other factors. Recent recruitment improvements for Pacific yellowfin, skipjack and bigeye tunas, as well as observations that strong recruitment levels in the East Pacific Ocean had been recorded in 2015 and 2016, associated with a major El Nino event, lend weight to the environmental hypothesis. In 2017 there were no projections of potential changes in spawning biomass in the future under current levels of fishing mortality, but it is hoped these will be developed in future years.

Consistent with previous assessments, the latest one shows that the stock has been continuously declining for about 60 years, since the late 1950’s, except for a recent small increase. The bigeye catch for 2016 (152,806 t) was an increase on 2015 catch and around average for the past ten years. Levels of fishing mortality and depletion differ between regions, and fishery impact was higher in the tropical region, with particularly high fishing mortality on juvenile bigeye tuna in these regions. The scientific committee therefore continues to recommend a precautionary approach to management, including reducing fishing mortality of juveniles.

The bigeye tuna species is assessed as Vulnerable by the IUCN (International Union for the Conservation of Nature).

Management

Criterion score: 0.5 info

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 tuna fisheries in the Western and Central Pacific Ocean (WCPO) are 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 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.

WCPFC has put in place a lower limit for the spawning biomass of bigeye, skipjack, and yellowfin tuna of 20 percent of unfished levels (SB/SBF=0, aka spawning biomass depletion ratio), below which the stock should not fall. For skipjack there is also an interim target of 50 percent of unfished levels, which was adopted in 2015 and has been reached. The scientific committee recommends reducing fishing mortality on juvenile bigeye and yellowfin in the tropics, through preventing increases in overall fishing mortality, until targets for these two stocks can be agreed. The commission is looking to establish harvest strategies for key fisheries and stocks but has not yet completed this work and so, as of January 2018, a bridging measure is in force to manage bigeye, yellowfin and skipjack - the measure lasts until harvest strategies are in place or until February 2021. As part of this, spawning biomass depletion ratios for bigeye and yellowfin are to be maintained at recent levels (the average from 2012-2015) and skipjack at 50 percent. The bridging measure includes a number of new measures and consolidates some pre-existing ones, and is outlined below.

In the tropical region, between 20 degrees North and 20 degrees South, the following applies:
The use and deployment of FADs is prohibited for 5 months of the year (3 months for Kiribati and the Philippines). VMS polling frequency increases to every 30 minutes during the FAD closure.
Effort limits (in vessel days) apply to purse seining on the high seas (excluding Small Island Developing States, SIDS): limits vary by country. In order not to undermine the effectiveness of this, countries cannot transfer effort into areas outside of this region.
To create an incentive to reduce the non-intentional capture of juvenile fish, to discourage waste and to encourage an efficient utilization of fishery resources, purse seiners must retain on board and then land or transship at port all bigeye, skipjack, and yellowfin tuna (also applies to national waters).
All purse seine vessels operating on the high seas or within national waters in this region must carry an observer. This also applies to purse seiners anywhere in the convention area fishing in waters under multiple countries’ jurisdiction or moving between the high seas and national waters.
The number and capacity of large (over 24m) purse seiners and longliners (with freezing capacity and ice-chilled) operating in this area is frozen to 2016 levels (excluding SIDs and Indonesia).

Other measures that apply to the wider convention area:
The number of drifting FADs with activated instrumented buoys deployed at any one time is limited to 350 per purse seine vessel.
Catch and/or effort limits apply for purse seining within national waters (both the type of limit and the amount vary by country).
There are country-specific limits on longline bigeye catch, and by 2020 hard limits for bigeye and a framework to allocate them amongst countries shall be developed.
Catches by other commercial tuna fisheries for bigeye, yellowfin or skipjack tuna (excluding those taking less than 2,000 tonnes) shall not exceed either the average level for the period 2001-2004 or the level of 2004.

More generally:
There is a requirement to submit FAD management plans, including information on strategies used to implement closures and other measures for reducing mortality of juvenile bigeye. A number of aspects in the bridging measure have been brought forward from previous management measures, and it isn’t clear how well they have been implemented, especially given the ongoing increases in total catch of skipjack. The scientific committee recommends more comprehensive data collection relating to FADs.
Observer coverage on purse seiners is poor in areas not specified in the bridging measure and only 5% coverage is required on longliners greater than 20m in length. 20% is considered to be the minimum to be effective.
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.
In 2017 a Compliance Monitoring Scheme was introduced to assess and improve compliance with obligations, and penalise non-compliance.

Capture Information

Criterion score: 0 info

3% of the bigeye tuna catch is made in pole and line fisheries. Pole and line fishing is a very selective method of fishing with virtually no impact on non-target species, yet considerable amounts of bait fish are used in these fisheries to attract the bigeye. Whilst bait fish are usually small, resilient species, it is important that these stocks are monitored and some basic management applied.

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 on 09.11.2017].

IPNLF, 2012. Ensuring sustainability of live bait fish, International Pole and Line Foundation, London, 57 pp.

WCPFC, 2017. Provisional outcomes document, WCPFC14-2017-outcomes, from the Western & Central Pacific Fisheries Commission Fourteenth Regular Session (As at 18 December 2017), 3-7 December 2017, Manila, Philippines. 15 pp. Available at https://www.wcpfc.int/meetings/wcpfc14 {Accessed on 18.12.2017]

WCPFC, 2017. Summary report of the thirteenth regular session of the Northern Committee of the Commission for the Conservation and Management of Highly Migratory Fish Stocks in the Western and Central Pacific Ocean, 28 August - 1 September 2017, Busan, Republic of Korea. 56 pp. Available at https://www.wcpfc.int/meetings/nc13 [Accessed on 17.11.17].

WCPFC, 2017. Summary Report of the Thirteenth Regular Session of the Scientific Committee of the Commission for the Conservation and Management of Highly Migratory Fish Stocks in the Western and Central Pacific Ocean, 9 - 17 August 2017, Rarotonga, Cook Islands. 281 pp. Available at https://www.wcpfc.int/meetings/sc13 [Accessed on 27.11.2017]