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Bigeye tuna - Atlantic
Marine Resource  Fact Sheet
Stock status report 2018
Bigeye tuna - Atlantic
Fact Sheet Citation  
Bigeye tuna in the Atlantic
Owned byInternational Commission for the Conservation of Atlantic Tunas (ICCAT) – More
Related observationsLocate in inventorydisplay tree map
 
Species:
FAO Names: en - Bigeye tuna, fr - Thon obèse(=Patudo), es - Patudo, ru - Тунец большеглазый
Geographic extent of Bigeye tuna - Atlantic
Main Descriptors
Considered a single stock: Yes        Spatial Scale: Regional
Management unit: Yes        Reference year: 2017
 
 
Biological State and Trend
State & Trend Descriptors
PartnerFIRMS
Exploitation rateF2017/FMSY = 1.63 range (1.14-2.12)High fishing mortalityRed
Abundance levelB2017/BMSY = 0.59 range (0.42-0.80)Low abundance
History
 

An assessment for bigeye tuna was conducted in July 2018 following a data preparation meeting in April 2018. The previous stock assessment was conducted for bigeye tuna in 2015. The last year fishery data used was 2017. Information including biology, fisheries, tagging, genetic studies and stock modeling can be found in the ICCAT special editions of the Bigeye Tuna Year Program (Anon. 2005a), the Second World Meeting on Bigeye Tuna (Anon. 2005b) and Chapter 2 of the ICCAT Manual.
Habitat and Biology
Climatic zone: Tropical; Temperate.   Horizontal distribution: Oceanic.   Vertical distribution: Pelagic.  


Bigeye tuna are distributed throughout the Atlantic Ocean between 50ºN and 45ºS, but not in the Mediterranean Sea. This species swims at deeper depths than other tropical tuna species and exhibits extensive vertical movements. Similar to the results obtained in other oceans, pop-up tagging, and sonic tracking studies conducted on adult fish in the Atlantic have revealed that they exhibit clear diurnal patterns: they are found much deeper during the daytime than at night. Spawning takes place in tropical waters when the environment is favorable. From nursery areas in tropical waters, juvenile fish tend to diffuse into temperate waters as they grow larger. Catch information from surface gears indicate that the Gulf of Guinea is a major nursery ground for this species. Dietary habits of bigeye tuna are varied and prey organisms like fish, mollusks, and crustaceans are found in their stomach contents. Bigeye tuna exhibit relatively fast growth: about 105 cm fork length at age three, 140 cm at age five and 163 cm at age seven. Bigeye tuna over 200 cm are relatively rare. Bigeye tuna become mature at about 3.5 years old. Young fish form schools mostly mixed with other tunas such as yellowfin tuna and skipjack. These schools are often associated with drifting objects, whale sharks and sea mounts. This association appears to weaken as bigeye tuna grow larger. Indian and Pacific Oceans tagging data showed that bigeye longevity is over 10 years, which may also imply lower natural mortality rates for the Atlantic Ocean.  An Atlantic wide research Tagging Program (AOTTP) was initiated in 2015 that will contribute to improve the biological knowledge on bigeye tuna.
Geographical Distribution
Jurisdictional distribution: Highly migratory


The geographical distribution of bigeye tuna is very wide and covers almost the entire Atlantic Ocean between 50ºN and 45ºS, but not in the Mediterranean Sea.
Water Area Overview
Spatial Scale: Regional

Geo References
Resource Structure
Considered a single stock: Yes


Various pieces of evidence, such as a lack of identified genetic heterogeneity, the time-area distribution of fish and movements of tagged fish, suggest an Atlantic-wide single stock for this species, which is currently accepted by the Committee. However, the possibility of other scenarios, such as north and south stocks, should not be disregarded.
Exploitation
 

The stock has been exploited by three major gears (longline, baitboat and purse seine fisheries) and by many countries throughout its range of distribution and ICCAT has detailed data on the fishery for this stock since the 1950s. Scientific sampling at landing ports for purse seine vessels of the EU and associated fleets have been conducted since 1980 to estimate bigeye tuna catches (Table 1, Figure 1). The size of fish caught varies among fisheries: medium to large for the longline fishery, small to large for the directed baitboat fishery, and small for other baitboat and for purse seine fisheries.

The major baitboat fisheries are located in Ghana, Senegal, the Canary Islands, Madeira and the Azores. The tropical purse seine fleets operate in the Gulf of Guinea and off Senegal in the East Atlantic and off Venezuela in the West Atlantic. In the eastern Atlantic, these fleets are comprised of vessels flying flags of Ghana, EU-France, EU-Spain and others which are mostly managed by EU companies. In the western Atlantic the Venezuelan fleet dominates the purse-seine catch of bigeye tuna. While bigeye tuna is now a primary target species for most of the longline and some baitboat fisheries, this species has always been of secondary importance for the other surface fisheries. In the surface fishery, unlike yellowfin tuna, bigeye tuna are mostly caught while fishing on floating objects such as logs or man-made fish aggregating devices (FADs). In 2017, landings of bigeye in weight caught by longline represent 45%, while purse seiner and baitboat plus other surface fleets represent 36% and 20%, respectively. 

The total annual Task I catch (Table 1, Figure 2) increased up to the mid-1970s reaching 60,000 t and fluctuated over the next 15 years. In 1991, catch surpassed 100,000 t and continued to increase, reaching a historic high of about 135,000 t in 1994. Reported and estimated catch has been declining since then and fell to 59,141 t 2006. Since 2007 catches has increased, although with some fluctuations from year to year, until the most recent year of data 2017. The preliminary estimate for 2017 is 78,482 t. 

After the historic high catch in 1994, all major fisheries exhibited a decline in catch while the relative share by each fishery in total catch remained relatively constant until 2008. These reductions in catch were related to declines in fishing fleet size (longline) as well as decline in CPUE (longline and baitboat). Although the general trend of decreasing catches continued for longline and baitboat, the purse seiner catches increased, as did the relative contribution of purse seine in the total catches in the period 2010-2017. The number of estimated active purse seiners declined by more than half from 1994 until 2006, but then increased as some vessels have returned from the Indian Ocean to the Atlantic and since 2014, the number of these purse seine vessels has remained stable. Other surface fisheries also have increased the catches in recent years from around 1,000 t in 2011 to around 7,000 t. in 2017, mainly due to the development of the new Brazilian handline vessel associated-school fishery.

Significant catches of small bigeye tuna continue to be channeled to local West African markets and sold as “faux poissons” in ways that make their monitoring and official reporting challenging. Monitoring of such catches has recently progressed through a coordinated approach that allows ICCAT to properly account for these catches and thus increase the quality of the basic catch and size data available for assessments. Currently those catches are included for the main purse seine fleet in the ICCAT Task I data used for the assessment up to 2017.

Mean average weight of bigeye tuna decreased prior to 2004 but has remained relatively stable at around 10 kg for the last decade (BET-Figure 3). This mean weight, however, is quite different for the different fishing gears in recent years, around 55 kg for longliners, around an average of 10 kg for baitboats, and 6 kg for purse seiners. Since 2000, several longline fleets have shown increases in the mean weight of bigeye tuna caught, with the average longline-caught fish increasing from 40 kg to 60 kg between 2000 and 2008. During the same period, purse seine-caught bigeye tuna had average weights between 5 and 6 kg. Average weight of bigeye tuna caught in free schools is more than double the average weight of those caught around FADs. Since 1991, when bigeye catches were identified separately for FADs for EU and other CPCs purse seine fleets, the majority of bigeye tuna are caught in sets associated with FADs; particularly since the mid-2000s (60%-80%). Similarly, baitboat-caught bigeye tuna weighted between 6 and 10 kg up to 2011, but with greater inter-annual variability in average weight compared to longline or purse seine caught fish, while it increased to around 18 kg in 2014 to decrease to 10 kg again since then.


Figure 1: Geographical distribution of the bigeye tuna catch by major gears and decade. The maps are scaled to the maximum catch observed during 1960-2016.




Figure 2: Bigeye estimated and reported catches for all the Atlantic stock (t). The value for 2017 represents preliminary estimates.




Figure 3: Trend of mean weight for bigeye based on the catch-at-size data for 1975-2017 by major fisheries (BB=Baitboats, LL=Longlines, PS=Purse seine). The mean weight of the baitboat fishery (BB) reflects various baitboat fleets operating in different areas of the Atlantic Ocean


Assessment
 
Assessment Model

The 2018 stock assessment was conducted using similar assessment models to those used in 2015 with updating data and new relative abundance indices up to 2017. Stock status evaluations for Atlantic bigeye tuna used in 2018 several modeling approaches, ranging from non-equilibrium (MPD) and Bayesian state-space (JABBA) production models to integrated statistical assessment models (Stock Synthesis). The results of different model formulations considered to be plausible representations of the stock dynamics were used to characterize stock status and the uncertainties in the status evaluations.

The main change from the previous assessment was the development and use of a single Joint Longline standardized abundance index (Document SCRS/2018/058) instead of each individual CPCs standardized CPUE indices used in the 2015 assessment; some of them showing conflicting trends. The joint longline standardized index was constructed using operational detailed data of longline major fleets (Japan, Korea, United States and Chinese Taipei) from 1959-2017 (BET-Figure 4).

The development of this joint standardized CPUE index was motivated to reduce data conflicts that arise when CPUE trends differ for different fleets in the same period. It was concluded that the joint index was an improvement over fleet-specific indices because of the integrated temporal and spatial coverage it afforded to index stock biomass, and because it minimizes data conflicts in the stock assessment models. The joint index uses the vessel effect that accounts for different fishing efficiency of each vessel to produce the standardized index. The selectivity used to model the index should reflect the selectivity of the combined fleets used to produce the index. The use of the index in the stock assessment model requires an assumption of its selectivity (size composition), which should reflect the selectivity of the combined fleets used to produce the index. However, given the modelled shift in the selectivity of Chinese Taipei since 2003, size composition data from Chinese Taipei was not used to estimate selectivity of the joint index in the stock assessment to maintain continuity of the time series.

In addition, a number of standardized indices of abundance were developed by national scientists for selected fleets for which data were available at finer spatial and/or temporal resolution for the assessment. These indices represented data from six different fleets: five longline fleets (Japan, Uruguay, Brazil, Chinese Taipei, USA) and one baitboat fleet (EU-Spain operating off Dakar) which were used in different stock assessment methods as sensitivity runs (BET-Figure 5).

The Stock Synthesis integrated statistical assessment model allows the incorporation of more detailed information, both for the biology of the species as well as fishery data, including the size data and selectivity by different fleet and gear components. As Stock Synthesis allows modelling of the changes in selectivity of different fleets as well as to investigate the effect of the length/age structure of the catches of different fisheries in the population dynamic, productivity and fishing mortality, it was the agreed model to be used for the management advice. The Stock Synthesis uncertainty grid includes 18 model configurations that were investigated to ensure that major sources of structural uncertainty were incorporated and represented in the assessment results. Although the results of two production models, non-equilibrium and Bayesian state-space, are not used for management advice they supported the Stock Synthesis stock assessment results.

Results of the uncertainty grid of Stock Synthesis runs show a long-term decline in SSB with the current estimate being at the lowest level in the time series (BET-Figure 6) and increasing trend of fishing mortality (average F on ages 1-7) starting in the early 1990s, with the highest fishing mortality at 1994 and has remained high since then (BET-Figure 6).

SS3 uncertainty grid, despite a broad range of assumptions regarding stock productivity (steepness) and model parameterization, shows trajectories of increasing F decreasing B towards the red area of the Kobe plot (F> FMSY and SSB<SSBMSY), overfishing starting in around 1994 and an overfished stock at around 1996-1997, and being in the red quadrant of the Kobe plot since then (BET-Figure 7). According to the results of the SS3 uncertainty grid, Atlantic bigeye stock is currently overfished (SSB/SSBMSY =0.59, ranging from 0.42 to 0.80) and undergoing overfishing (F/FMSY = 1.6, ranging from 1.14 to 2.12) with very high probability (99%) (BET-Figure 8).

The current MSY may be below what was achieved in past decades because overall selectivity has shifted to smaller fish. Calculations of the time-varying benchmarks from SS3 uncertainty grid show a long-term increase in SSBMSY and a general long term decrease in MSY (BET-Figure 9).

The Committee is confident that uncertainty of the stock assessment results has decreased from previous stock assessments. This is likely the result of the use of the improved joint LL index, the confirmation that catches continue to exceed TACs, and the use of a single model platform for the provision of the management advice.


Figure 4: CPUE Series joint longline abundance index constructed with operational detailed data from the major longline fleets from 1959 to 2017.




Figure 5: Annual relative indices of abundances for bigeye tuna from different fleets used in the stock assessment as sensitivity runs.


Figure 6: Trajectories of Spawning Stock Biomass (SSB), Fishing mortality (average F on ages 1-7) and recruitment (age 0) for the 18 Stock Synthesis uncertainty grid runs for Atlantic bigeye tuna.




Figure 7: Trajectories of SSB/SSBMSY and F/FMSY estimated from the 18 Stock Synthesis uncertainty grid runs for Atlantic bigeye tuna. For each run the benchmarks are calculated from the year-specific selectivity and fleet allocations.




Figure 8: Stock Synthesis: (a) Kobe phase plot for the deterministic runs of the 18 Stock Synthesis uncertainty grid runs for Atlantic bigeye tuna. For each run the benchmarks are calculated from the year-specific selectivity and fleet allocations. (b) Kobe plot of SSB/SSBmsy and F/Fmsy for stock status of Atlantic bigeye tuna in 2017 based on the log multivariate normal approximation across the 18 uncertainty grid model runs of Stock Synthesis with an insert pie chart showing the probability of being in the red quadrant (99.5 %), green quadrant (0.2 %), and in yellow (0.3 %). Blue square is the median and marginal histograms represent distribution of either SSB/SSBmsy or F/Fmsy.


Overall Assessment Results
Projection

Projections were conducted for the uncertainty grid Stock Synthesis for a range of fixed catches from 35,000 to 90,000 t for 15 years (which corresponds to 2 generation times of bigeye) from 2019-2033. For some of the projections, the modelled stock could not sustain higher constant catches over several years in the long term (BET-Table 3). In such cases, projections were adjusted to prevent this undesirable projection behavior and made it possible to produce Kobe 2 Strategic Matrices. The results of projections of the Stock Synthesis are provided in the form of K2SM with probabilities that overfishing is not occurring (F<=FMSY), stock is not overfished (SSB>=SSBMSY) and the joint probability of being in the green quadrant of the Kobe plot (i.e. F<= FMSY and SSB>= SSBMSY) (BET-Table 4).

It was noted in 2018 that the modeled probabilities of the stock achieving levels consistent with the Convention objective of the projected time period in 2028 and 2033 was 28% and 44%, respectively, for a future constant of 65,000 t, which is the TAC established in Rec. 16-01. Projections with current TAC level is not expected to end overfishing (F<FMSY) with 50% probability until 2032. Higher probabilities of rebuilding require longer timeframes and/or larger reduction of current catches (BET-Table 4). It was also noted that the modeled probabilities of the stock being in the green quadrant at the end of the projected period in 2033, as well as the probability to end overfishing by 2033, was 1% for a future constant catch at current levels of around 78,482 t. Moreover, when projecting at current catch level 56% of the model runs resulted in SSB levels below 10% of SSBMSY by 2032 (BET-Table 3).

It needs to be noted that projections made by the Committee assume that future constant catches represent the total removals from the stock, and not just the reported catches and the current selectivity pattern is maintained. Any future changes in selectivity due to changes in the ratios of relative mortality exerted by the different fleets – such as an increase in the relative mortality of small fish – will change and add to the uncertainty of these projections.
Table 2: Estimated probabilities of the Atlantic bigeye tuna stock being below FMSY (overfishing not occurring), above BMSY (not overfished) and above BMSY and below FMSY (green zone) in a given year for catch level ('000 t), based upon the 2015 assessment outcomes.


Management
Management unit: Yes


Effects of current regulations

During the period 2005-2008 an overall TAC was set at 90,000 t. The TAC was later lowered (Rec. 09-01 and later modified by Rec. 14-01) to 85,000 t. Estimates of reported catch for 2009-2015 (BET-Table 1) have been always lower than 85,000 t. The TAC was again reduced to 65,000 t in Recommendation 15-01 which entered into force in 2016 and the catches in 2016 and 2017 have exceeded the TAC by 20% (i.e. catches around 78,000 t.), which contributed to a further decline in stock size since the 2015 assessment. Note that because this TAC does not affect all countries that can catch bigeye tuna, in theory the total catch removed from the stock could exceed the TAC.

Concern over the catch of small bigeye tuna partially led to the establishment of spatial closures to surface fishing gear in the Gulf of Guinea (Recs. 04-01, 08-01, 11-01, 14-01, 15-01). The Committee examined trends on average bigeye tuna catches by areas as a broad indicator of the effects of such closures as well as changes in juvenile bigeye and yellowfin catches due to the moratorium. The efficacy of the area-time closure agreed in Rec. 15-01 was evaluated by examining fine-scale (1x1 degree) skipjack, yellowfin, and bigeye catch by month distributions. After reviewing this information, the Committee concluded that the moratorium has not been effective at reducing the mortality of juvenile bigeye tuna, and any reduction in yellowfin tuna mortality was minimal, largely due to the redistribution of effort into areas adjacent to the moratorium area and increase in number of fishing vessels.
Management Advice

The Atlantic bigeye tuna stock was estimated to be overfished and that overfishing was occurring in 2017. Maintaining the catches at 2016 and 2017 levels (around 78,000 t.) in the future, which exceeded the TAC of 65,000 t by 20%, the probability of achieving Convention objectives by 2033 (B>BMSY, F<FMSY) is expected to reduce to around 1% (BET-Table 4).

The Commission should urgently ensure that catches are appropriately reduced to end overfishing and allow the stock to recover following the Decision Framework adopted in paragraph 3 of Rec. 11-13. Furthermore, the Committee notes that the necessary reduction of fishing mortality could be not achieved with current and previous FAD time area closures and/or changes to fleet allocation alone.

The Commission should be aware that increased harvests on small fishes by FADs and other fisheries as well as the development of new fisheries 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, should the Commission wish to increase long-term sustainable yield, the Committee continues to recommend that effective measures be found to reduce fishing mortality of small bigeye tunas.


ATLANTIC BIGEYE TUNA SUMMARY
Maximum Sustainable Yield 76,232 t (72,664-79,700 t)1
Current (2017) Yield 78,482 t2
Relative Biomass (B2017/BMSY) 0.59 (0.42-0.80)1
Relative Fishing Mortality 
F2017/FMSY
1.63 (1.14-2.12)1
Stock Status

Overfished: Yes

Overfishing: Yes

Conservation & management measures in effect:

[Rec. 16-01]

Total allowable catch for 2016-2018 is set at 65,000 t for Contracting Parties and Cooperating non-Contracting Parties, Entities or Fishing Entities.

Be restricted to the number of their vessels notified to ICCAT in 2005 as fishing for bigeye tuna.

Specific limits of number of longline boats; China (65), Chinese Taipei (75), Philippines (5), Korea (14), EU (269) and Japan (231).

Specific limits of number of purse seine boats; EU (34) and Ghana (17).

No fishing with natural or artificial floating objects during January and February in the area encompassed by the African coast, 20º W, 5ºN and 4ºS.

No more than 500 FADs active at any time by vessel.

Use of non-entangling FADs..


1 - Combined result of SS3 18 uncertainty grid. Median and 10 and 90% percentile in brackets.

2 - Reports for 2017 reflect most recent data but should be considered provisional .
Source of information
 
Report of the Standing Committee on Research and Statistics (SCRS). “Bigeye, Executive Summary.” Madrid, Spain - October 2018. Click to openhttp://www.iccat.int/Documents/SCRS/ExecSum/BET_ENG.pdf
Report of the 2018 ICCAT Bigeye Tuna Stock Assessment Meeting. Pasaia, Spain 16 – 20 July 2018. Click to openhttps://www.iccat.int/Documents/SCRS/DetRep/BET_SA_ENG.pdf
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