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Yellowfin tuna - Atlantic
Marine Resource  Fact Sheet
Stock status report 2019
Yellowfin tuna - Atlantic
Fact Sheet Citation  
Yellowfin 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 - Yellowfin tuna, fr - Albacore, es - Rabil, ru - Тунец желтоперый
Geographic extent of Yellowfin tuna - Atlantic
Main Descriptors
Considered a single stock: Yes        Spatial Scale: Regional
Management unit: Yes        Reference year: 2018
 
 
Biological State and Trend
State & Trend Descriptors
PartnerFIRMS
Exploitation rateFcur/FMSY = 0.96 range (0.56 - 1.50)Moderate fishing mortalityGreen
Abundance levelBcur/BMSY = 1.17 range (0.75 - 1.62)Intermediate abundance
History
 

A stock assessment for yellowfin tuna was conducted in 2019 using catch and effort data through 2018. Readers interested in a more complete summary of the state of knowledge on yellowfin tuna stock status should consult the detailed 2019 Report of ICCAT Yellowfin Tuna Stock Assessment Session (ICCAT YFT 2019 Stock Assessment Report).
Habitat and Biology
Climatic zone: Tropical.   Horizontal distribution: Oceanic.   Vertical distribution: Pelagic.  


Yellowfin tuna is a cosmopolitan species distributed mainly in the tropical and subtropical oceanic waters of the three oceans. The exploited sizes typically range from 30 cm to 170 cm FL. Juvenile yellowfin tuna form mixed schools with skipjack and juvenile bigeye, and are mainly limited to surface waters, while larger fish form schools in surface and sub-surface waters. Spawning on the equatorial zone of the Gulf of Guinea, occurs primarily from December to April. Spawning also takes place in the Gulf of Mexico, the southeastern Caribbean Sea and off Cabo Verde, although the peak spawning can occur in different months in these regions. The relative importance of the various spawning grounds is unknown.

The length at 50% maturity was estimated at 115 cm FL using vitellogenesis as a maturity threshold. Lacking additional information about the relationship between fecundity and age/length, the Committee agreed to retain a fecundity schedule based upon length - or weight-at-age at the peak of the spawning season.

New information from yellowfin otolith aging sampled in the US Gulf of Mexico and the western Atlantic, indicated up to 18 year old fish using annual otolith increment counts and validated using 14C bomb radiocarbon techniques. A second study of yellowfin tuna captured in the Ascension Islands also observed ages up to 18 years and confirmed that individuals as old as 18 years occur outside of the US, and closer to the areas where fishing pressure is higher (e.g. Gulf of Guinea). In addition, preliminary results of the AOTTP (the Atlantic Ocean Tropical Tuna Tagging Programme) OTC validation have also supported the annual deposition of otolith increments. This information supported a change in maximum age from 11 to 18 years (Figure 1).

New information concerning growth was also available from AOTTP. The data suggest that the growth of yellowfin tuna is better estimated using a Richards function than a von Bertalanffy function. Therefore, the age-structured models used that functional shape (Figure 2). The AOTTP data also support the previous conclusion that growth rates are relatively slow initially, increasing at the time the fish leave the nursery grounds.

Tagging studies of yellowfin in the Pacific and Indian Oceans suggest that natural mortality is age-specific, and higher for juveniles than for adults. As was done in the previous assessments of yellowfin and bigeye tuna, an age-specific natural mortality function (e.g. Lorenzen) was developed and applied to the 2019 assessment of yellowfin tuna. The implied natural mortality based on the tmax of 18 is 0.35 yr-1, which is lower than the 2016 assessment assumption of 0.54 yr-1 based on a tmax of 11 years (Figure 3). The most recent stock assessment does not consider sex-specific natural mortality or growth, yet there are disparities in average size by gender. Males are predominant in the catches of larger sized fish (over 145 cm), which could result if large females experience a higher natural mortality rate, perhaps as a consequence of spawning. In contrast, females are predominant in the catches of intermediate sizes (120 to 135 cm), which could result from differential growth (e.g. females having a lower asymptotic size than males). Recent results from studies in the Indian Ocean suggest a combination of the two hypotheses.

Younger age classes of yellowfin tuna (40-80 cm) exhibit a strong association with floating objects (FOBs: any type of object that can affect fish aggregation). The Committee noted that this association with FOBs, which increases the vulnerability of these smaller fish to surface fishing gears, may also have an impact on the biology and on the ecology of yellowfin due to changes in feeding and migratory behaviors. These uncertainties in stock structure, natural mortality, and growth could have important implications for the stock assessment.
Figure 1. The size at age of YFT fish sampled off Ascension Island, the USA and South Africa (AOTTP), by gender. Ages of USA and AOTTP samples were assigned based on assumed birthday. No adjustment was made to annulus count for Ascension data.


Figure 2. Vector plot of the growth increments of AOTTP fish measured upon recovery. The relative age of each fish at the time of tagging is estimated from the length at tagging by inverting the von Bertalanffy (left panel) and Richards (right panel) growth equations using parameters estimated by SS. The age at recapture is then taken to be the age at tagging plus the time at liberty. Each growth trajectory (shown in grey) starts on the fitted curve (shown in red).






Geographical Distribution
Jurisdictional distribution: Highly migratory


Yellowfin tuna is a cosmopolitan species distributed mainly in the tropical and subtropical oceanic waters (Figure 4) of the three oceans.
Figure 4. Geographical distribution of yellowfin tuna total catches by decade. The maps are scaled to the maximum catch observed during 1960-2017. Note: the last panel (f) shows only 8 years of information. Thus, apparent changes in the size of the pie charts (in f) should not be interpreted as a reduction in catch during 2010-2017.
Water Area Overview
Spatial Scale: Regional

Geo References
Resource Structure
Considered a single stock: Yes


Although the distinct spawning areas might imply separate stocks, or substantial heterogeneity in the distribution of yellowfin tuna, a single stock for the entire Atlantic is currently assumed. This assumption is based upon information such as observed transatlantic movements indicated by conventional tagging and longline catch data that indicates yellowfin are distributed continuously throughout the tropical Atlantic Ocean. Movement rates and timing, migratory routes, and local residence times remain uncertain, but recent tagging activities (e.g. AOTTP) offer insights (Figure 5). In addition, some electronic tagging studies in the Atlantic as well as in other oceans suggest that there may be some degree of extended local residence times and/or site fidelity.
Figure 5. Apparent movements (straight line distance between the tagging location and that of recovery) calculated from conventional tagging from the historical ICCAT tagging database (top panel) and the current AOTTP activities (bottom panel).
Exploitation
 

Fishery indicators

Yellowfin tuna have been exploited by three major gears (longline, baitboat and purse seine fisheries) and by many countries throughout its range. Detailed data are available since the 1950s (Table 1). Overall Atlantic catches have declined by nearly half from the peak in 1990 (193,584 t) to 106,288 t estimated for 2013 but increased to an average of 140,143 t during 2016-2018. The most recent catch distribution is given in Figure 4.

In the eastern Atlantic, purse seine catches declined between 1990 and 2007 (129,144 t to 47,961 t) but have subsequently increased to 90,250 t in 2018 (Table 1; Figure 6). Baitboat catches declined between 1990 (19,717 t) and 2018 (7,255 t). Longline catches, which were 10,253 t in 1990, declined to 5,031 t in 2018. In the western Atlantic, purse seine catches were as high as 25,749 t during the mid-1980s but have since declined to 3,008 t in 2018. Baitboat catches also declined since a peak in 1994 (7,094 t), and for 2018 were estimated to be 943 t. Since 1990, longline catches have generally fluctuated between 10,000 t and 20,000 t.

It is difficult to discriminate fishing effort between free schools (composed of large yellowfin tunas) and FOB fishing (targeting skipjack) in the East Atlantic because the fishing strategies can change from one year to the next. In addition, the sea time devoted to activities on FOBs and the assistance provided by supply vessels are difficult to quantify. Nominal purse seine effort, expressed in terms of carrying capacity, decreased regularly from the mid-1990s until 2006. Since that time, several European Union purse seiners have transferred their fishing effort to the East Atlantic due to piracy in the Indian Ocean, and a fleet of new purse seiners has started operating from Tema (Ghana), whose catches are probably underestimated. These factors have contributed to the growth in carrying capacity of the purse seiners, which is approaching the level observed in the early 1990s. The nominal effort of bait boats has remained stable for over 20 years. By 2010, overall carrying capacity of the purse seine fleet had increased significantly, to about the same level as in the 1990s, and it has increased by nearly 50% since. These estimates do not include all purse seine vessels currently fishing for tropical tunas in the Atlantic. The total number of purse seine vessels (estimated by the Committee) targeting tropical tunas in the eastern Atlantic has increased in the last five years by 18%, from 49 in 2014 to 58 in 2018. FOB based fishing has accelerated even more rapidly than free school fishing.

Numerous changes have occurred in the yellowfin fishery since the early 1990s (e.g. the progressive use of FOBs and the latitudinal expansion and the westward extension of the fishing area). Since 2011, significant catches of yellowfin tuna have been obtained by EU purse seiners south of 15°S off the coast of West Africa (in association with skipjack and bigeye on FOBs). There has also been a significant increase in catches of yellowfin and bigeye by a new Brazilian “vessel associated-school” handline fishery, where the vessel is used to aggregate fish, operating in the western Atlantic. These catches have tripled from 5,200 t in 2013 to nearly 17,000 t in 2017, with a slight decrease to 15,000 t in 2018. Finally, a new strategy of fishing on floating objects off Mauritania (north of 15°N) began in 2012. Catches on floating objects in this area tended to consist almost entirely of skipjack, therefore, effort directed in this manner may have a minimal impact on yellowfin tuna.

Four indices of abundance were used in various stock assessment model runs used to develop management advice (Figure 7). A major advancement in this assessment was the development of a joint longline index using high resolution catch and effort information from the main longline fleets operating in the Atlantic (Japan, US, Brazil, Korea and Chinese Taipei). The indices were developed for 3 regions, but only two were used in the assessment: The North Atlantic (Region 1), and the tropical area (Region 2). A new echosounder-based buoy associated index (BAI) index was developed and was assumed to represent the abundance of juvenile yellowfin tuna. An index of larger yellowfin tuna (>80 cm, 10 kg) in free schools for the EU purse seine fleet (EUPSFS index) was also used.

The recent average weight in European purse seine catches, which represent the majority of the landings, had declined to about half of the average weight of 1990. This decline is at least in part due to changes in selectivity associated with fishing on floating objects beginning in the 1990s, which was observed in the increased catches of small yellowfin. A declining trend in average weight and a corresponding increase in the catch of small yellowfin is also evident in eastern tropical baitboat catches. Longline mean weights and catch at size have been more variable.


Figure 6. Yellowfin tuna total catch 1950 – 2018 by main fishing gear group.
Figure 7. Annual abundance indices used for the Atlantic yellowfin tuna stock assessment reference cases. Regions 1 and 2 for joint longline mean the area of index that are northern and tropical areas, respectively. Buoy-derived abundance index was used only in Stock Synthesis and joint longline index in region 1 only for JABBA.
Assessment
 
Assessment Model

A full stock assessment was conducted for yellowfin tuna in 2019, applying two production models (JABBA, MPB) and one age-structured model (Stock Synthesis) to the available catch data through 2018. The four Stock Synthesis model runs, were regarded as representing alternative recruitment, and steepness hypotheses. Likewise, the JABBA runs addressed different hypotheses about initial priors for r, and about which indices of abundance were representing the population. Finally, the base case selected for MPB estimated biomass and fishing mortality trends that varied somewhat from JABBA. The Committee decided that, in order to capture this uncertainty in the population dynamics for developing the management advice, it was best to incorporate results from all of the accepted model runs.

The trend in the estimated biomass (relative to BMSY) for all models shows a general continuous decline through time. Stock Synthesis runs suggest a few periods of large increases in spawning biomass associated with episodes of high recruitment. The model estimates that such very high recruitments have happened three times in the period 1960 to 2017. Production models show much less pronounced increases in total biomass at the equivalent times. Note, however, that for all models there are large uncertainties in the value of biomass at any point in the history, including 2018. Most model runs lead to biomasses at the end of 2018 above the level that produces MSY (Figure 8).

Estimates of historical fishing mortality (relative to FMSY) show similar trends for all models. For most model runs, fishing mortality increased progressively until the early 1980s, it varied in level until the mid-1990s, after which it declined gradually until the mid-2000s. Since the mid-2000s, the fishing mortality has had a generally increasing trend with fluctuations until 2018. Overall the models estimate that the fishing mortality in 2018 was near the fishing mortality that would produce MSY. Again, for all models there are large uncertainties in the value of fishing mortality at any point in the history, including 2018 (Figure 9).

It is important to note that the Stock Synthesis model is the only one that can provide estimates of recent recruitment (Figure 10). Recruitments were not estimated to vary from the stock-recruit relationship for 2018, due to the large uncertainty in terminal year recruitment estimates. The estimate of recruitment in 2017 is also more uncertain than for previous years, in part because there is no 2018 size frequency data to corroborate or contrast with it. Stock Synthesis models which use the buoy index suggest very high recruitment in 2017, whereas models that do not use the buoy index suggest that recruitment in 2017 was above average but not particularly high.

The Committee gave equal weight to surplus production model and integrated assessment model results. Within surplus production models, JABBA and MPB were also given equal weight. Each run within a modeling platform (JABBA, and Stock Synthesis) were also given equal weight. For the combined results used to develop management advice, the median estimate of B2018/BMSY was 1.17 and the median estimate of F2018/FMSY was 0.96. The median MSY estimate was 121,298 t. Combining the results of all models provides a way to estimate the probability of the stock being in each quadrant of the Kobe plot in 2018 (Figure 11). The corresponding probabilities are 54% in the green (not overfished not subject to overfishing), 21% in the orange (subject to overfishing but not overfished), 2% in the yellow (overfished but not subject to overfishing) and 22% in the red (overfished and subject to overfishing). In summary, the results point to a stock status of not overfished (24% probability of overfished status), with no overfishing (43% probability of overfishing taking place).

The Committee cautioned that the differences between the 2016 and 2019 assessment results are not due to stock recovery. In fact, the 2019 models indicate that the stock biomass declined between 2014 and 2018. Instead, the perceived improvement is more likely due to changes in key data inputs (M, growth, indices) and the suite of models applied (JABBA, MPB, and Stock Synthesis).

The Committee noted that catch reports for 2018 were incomplete, at the time when the assessment was conducted with 42% of the total catch being estimated using the average from the previous three years by CPC and gear type. Furthermore, no size data for 2018 were available at the time of the assessment. The 2018 estimated catch assumed for the stock assessment was 131,042 t. This was revised upwards to 135,689 t after additional reporting. It was not possible to re-run the stock assessment results with the new 2018 catch estimates, however a change of this magnitude is not expected to have substantial implications.


Figure 8. Estimates of relative Biomass (B/BMSY) obtained for all model runs used to develop the management advice.


Figure 9. Estimates of relative fishing mortality (F/FMSY) obtained for all model runs used to develop the management advice.




Figure 10. Annual estimates of Age-0 recruits (left panels) and recruitment deviations with 95% confidence intervals (right panels) for Stock Synthesis model runs. Models which used the buoy index suggest very high recruitment in 2017, whereas models that do not use the buoy index suggest that recruitment in 2017 was not particularly high. Note: Production models (JABBA, MPB) do not produce estimates of recruitment.


Figure 11. Kobe plot estimated from the combination of Stock Synthesis, JABBA and MPB model runs chosen to develop the management advice. The trajectory of individual runs are shown in the detailed report, and in Figures 8 and 9 above.
Overall Assessment Results
Projection

Outlook

Combined catch projections from 9 runs (JABBA (Base Case, S2, S3, and S5), MPB, Stock Synthesis (runs 1, 2, 3 and 4) were provided at constant catches ranging 0 t and from 60,000 to 150,000 t. The method used to combine the projection results is described in section 4.4 of the detailed report of the Assessment Session (SCRS/2019/011). In the projection results from the Stock Synthesis and JABBA models, some iterations were predicted with exceptionally small biomass ratios and extremely high F ratios indicating the potential for stock collapse. A probability of biomass being less than 20% of the biomass that supports MSY was calculated for each projection year and catch scenario (Table 2). The probability increased with higher catch levels and in later projected years. The probabilities more than 1% or 10% were observed with the constant catch more than 110,000 t or 140,000 t, respectively. The highest probability was 23.3% with 150,000 t constant catch in 2033. It should be noted that the reference chosen, 20% of biomass that supports MSY, was selected for informational purposes and has not been adopted formally by the SCRS for tropical tunas.

The combined projections show that 120,000 t constant catch will maintain more than 50% probability of being in green quadrant through 2033 (Figure 12 and Table 3).


Figure 12. Trends of projected relative biomass (left panel, B/BMSY) and fishing mortality (right panel, F/FMSY) of Atlantic yellowfin stock under different TAC scenarios (0, 60000 – 150000 t) from JABBA, MPB, and SS3 using 9 runs (JABBA (Base Case, S2, S3, and S5), MPB, Stock Synthesis (runs 1-4)). Each line represents the median of 20000 iterations by projected year. In 2019, the catch was assumed to be 131,042 t, equal to the 2018 estimated landings.


Table 2. Estimated probabilities of biomass the Atlantic YFT stock levels < 20% of BMSY in the combined projections of JABBA (Base Case, S2, S3, and S5), MPB, Stock Synthesis (runs 1-4) in a given year for a given catch level (0, 60,000 – 150,000 t). This result was used to develop the management advice of Atlantic YFT stock.


Table 3. Estimated probabilities of the Atlantic YFT stock (a) being below FMSY (overfishing not occurring), (b) above BMSY (not overfished) and (c) above BMSY and below FMSY (green zone) in a given year for a given catch level (0, 60,000 – 150,000 t), based upon the combined projections of JABBA (Base Case, S2, S3, and S5), MPB, Stock Synthesis (runs 1-4). This result was used to develop the management advice of Atlantic YFT stock.


Management
Management unit: Yes


Effects of current regulations

Concern over the catch of small yellowfin 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). In previous years, 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 (1ox1o) skipjack, yellowfin, and bigeye catch by month distributions. After reviewing this information, the Committee concluded that the moratorium had 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.

Rec. 11-01 (reiterated in Rec. 16-01) also implemented a TAC of 110,000 t for 2012 and subsequent years. During 2012 and 2014, overall catches exceeded the TAC by 3-5%. Since then, overages have increased substantially, to 17% (128,298 t) in 2015, 35% (148,874 t) in 2016, 24% (135,865 t) in 2017 and 23% (135,689 t) in 2018.


Management Advice

The Committee expressed strong concern that catches above 120,000 t are expected to further degrade the condition of the yellowfin stock if they continue. Furthermore, given that significant overages are frequent, existing conservation and management measures appear to be insufficient, and the Committee recommends that the Commission strengthen such measures.

The Commission should also be aware that increased harvests on small yellowfin tuna has had negative consequences to both long-term sustainable yield and stock status (Figure 13), and that continued increases in the harvest of small yellowfin tuna will continue to reduce the long-term sustainable yield the stock can produce. Should the Commission wish to increase long-term sustainable yield, the Committee continues to recommend that effective measures be found to reduce fishing mortality on small yellowfin tuna (e.g. FOB-related and other fishing mortality of small yellowfin tuna).


ATLANTIC YELLOWFIN TUNA SUMMARY
Maximum Sustainable Yield 121,298 t (90,428 - 267,350 t)1
Current (2018) Yield 135,689 t
Relative Biomass (B2018/BMSY) 1.17 (0.75 - 1.62)1
Relative Fishing Mortality 
F2018/FMSY
0.96 (0.56 - 1.50)1
2018 Total Biomass3 729,436 t
Stock Status

Overfished: No4

Overfishing: No5

Conservation & management measures in effect:

[Rec. 16-01]

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.

TAC of 110,000 t (since Rec. 11-01).

Specific authorization to fish for tropical tunas for vessels 20 meters or greater

Specific limits of number of longline and/or purse seine boats for a number of fleets

Specific limits on FADs, non-entangling FADs required.


1. Minimum and maximum values of 90%LCI and 90%UCI among all runs by the Stock Synthesis, JABBA, and MPB

2. SSB (Stock Synthesis) or exploited biomass (production models)

3. Mean of the central estimates of the SS, JABBA and MPB models

4. 24% probability of overfished status

5. 43% probability of overfishing taking place


Figure 13. Effect of changes in overall fisheries selectivity on estimate of MSY and reference points used for the determination of stock status (Dynamic SSBMSY, FMSY and MSY for the Stock Synthesis runs). For each year, reference points are calculated with the selectivity of each gear for that year, and relative yearly catch of each fleet.
Source of information
 
Report of the Standing Committee on Research and Statistics (SCRS). “Yellowfin, Executive Summary.” Madrid, Spain - October 2019. Click to openhttps://www.iccat.int/Documents/SCRS/ExecSum/YFT_ENG.pdf
Report of the 2019 ICCAT Yellowfin Tuna Stock Assessment Meeting. Grand Bassam, Cote d’Ivoire 8 – 16 July 2019. Click to openhttps://www.iccat.int/Documents/Meetings/Docs/2019/REPORTS/2019_YFT_SA_ENG.pdf
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