Fishery Resources Monitoring System

Français
Northern Bluefin tuna - Western Atlantic, 2011
Marine Resource  Fact Sheet
Stock status report 2012
Northern Bluefin tuna - Western Atlantic, 2011
Fact Sheet Citation  
Northern bluefin tuna in the Western Atlantic
Owned byInternational Commission for the Conservation of Atlantic Tunas (ICCAT) – More
Related observationsLocate in inventorydisplay tree map
 
Species:
FAO Names: en - Atlantic bluefin tuna, fr - Thon rouge de l'Atlantique, es - Atún rojo del Atlántico, ar - تونة حمراء أطلسيَّة, zh - 大西洋蓝鳍金枪鱼
Geographic extent of Northern Bluefin tuna - Western Atlantic
Main Descriptors
Considered a single stock: Yes        Spatial Scale: Regional
Management unit: Yes
 
 
History
 

In 2012, the SCRS conducted an update of the 2010 assessment of Atlantic bluefin tuna (Anon. 2011f). In this update, the available data included catch, effort and size statistics through 2011. As previously discussed, there are considerable data limitations for the eastern stock up to 2007. While catch data reporting for the eastern and Mediterranean fisheries has substantially improved since 2008 and some historical statistical data have been recovered, none-the-less, most of the data limitations that have plagued previous assessments remain and will require new approaches in order to improve the scientific advice the Committee can offer. The SCRS strongly recommends the continuation of enhanced data collection program and the replacement of current assessment methods with appropriate approaches that take unquantified uncertainties into account.

During the last decade, there has been an overall shift in targeting towards large bluefin tuna, mostly in the Mediterranean. As the majority of these fish are destined for fattening and/or farming operations, it is crucial to get precise information about the total catch, the size composition, the area and flag of capture. Progress has been made over the last years, but current information that consists in individual weight after fattening remain too uncertain to be used within stock assessment models. Therefore, real size samples at time of the catch are still required. Pilot studies using dual camera system or acoustic coupled with video system have been presented at the SCRS since 2010. The results are encouraging and could already provide catch composition of greater precision than this of current Mediterranean fleets Task II data. The SCRS strongly encourages the CPCs to make the stereoscopic camera systems or any alternative technique that would provide equivalent precision to recover size information from farms operational for the coming year.

The Atlantic-wide Research Program for Bluefin Tuna (GBYP) research plan outlined the research necessary for improving the scientific advice that the Committee provides to the Commission. This plan was presented to and approved by the Commission and the GBYP was started in 2010. The Committee continues to strongly and unanimously support the GBYP, particularly with respect to obtain fisheries-independent indices of stock size, and welcomes the Commission’s continued commitment to the Program. In the absence of such a significant and sustained effort, it remains highly unlikely that the Committee will improve its scientific diagnosis and management advice in the foreseeable future.

In 2012, the SCRS also reviewed new information on the biology, spatial dynamics, catch statistics and fisheries catch rates. The SCRS also discussed progress made by the GBYP and other research program about the aerial survey, tagging, data mining, biological sampling, stock mixing and new modeling approaches. The new information is summarized in SCRS/2012/139.


Habitat and Biology
Climatic zone: Temperate.   Vertical distribution: Pelagic.  


Atlantic bluefin tuna (BFT) mainly live in the pelagic ecosystem of the entire North Atlantic and its adjacent seas, primarily the Mediterranean Sea. Bluefin tuna have a wide geographical distribution living mostly in temperate Atlantic waters and adjacent seas (Figure 1). Archival tagging and tracking information confirmed that bluefin tuna can sustain cold as well as warm temperatures while maintaining stable internal body temperature. Bluefin tuna preferentially occupy the surface and subsurface waters of the coastal and open-sea areas, but archival tagging and ultrasonic telemetry data indicate that bluefin tuna frequently dive to depths of 500m to 1,000m. Bluefin tuna is also a highly migratory species that seems to display a homing behavior and spawning site fidelity in both the Mediterranean Sea and Gulf of Mexico, which constitute the two main spawning areas being clearly identified today. Less is known about feeding migrations within the Mediterranean and the North Atlantic, but results from electronic tagging indicated that bluefin tuna movement patterns vary considerably between individuals, years and areas. The appearance and disappearance of important past fisheries further suggest that important changes in the spatial dynamics of bluefin tuna may also have resulted from interactions between biological factors, environmental variations and fishing. Although the Atlantic bluefin tuna population is managed as two stocks, conventionally separated by the 45°W meridian, its population structure remains poorly understood and needs to be further investigated. Recent genetic and microchemistry studies as well as work based on historical fisheries tend to indicate that the bluefin tuna population structure is complex.

Currently, the SCRS assumes that eastern Atlantic and Mediterranean bluefin tuna mature at approximately 25 kg (age 4) and western Atlantic bluefin tuna at approximately 145 kg (age 9). Recent information received by the SCRS indicated that some individuals caught in the West Atlantic as small as 47 kg (age 5) were mature. Juvenile and adult bluefin tuna are opportunistic feeders (as are most predators). However, in general, juveniles feed on crustaceans, fish and cephalopods, while adults primarily feed on fish such as herring, anchovy, sand lance, sardine, sprat, bluefish and mackerel. Juvenile growth is rapid for a teleost fish (about 30cm/year), but slower than other tuna and billfish species. Fish born in June attain a length of about 30-40 cm long and a weight of about 1 kg by October. After one year, fish reach about 4 kg and 60 cm long. Growth in length tends to be lower for adults than juveniles, but growth in weight increases. At 10 years old, a bluefin tuna is about 200 cm and 170 kg and reaches about 270 cm and 400 kg at 20 years. Bluefin tuna is a long-lived species, with a lifespan of about 40 years, as indicated by recent studies from radiocarbon deposition.

The Committee recognized that there have been important recent contributions to the understanding of bluefin tuna biology and ecology that should have significant impacts on the assessment (and possibly on the management) of the resource. The new information includes conversion factors, maturity, growth, migrations and stock structure. This new information needs to be evaluated in detail, prior to incorporation in the current stock assessment models. To accomplish this objective, the Committee suggested convening an intersessional meeting on bluefin tuna biology and ecology in 2013.


Geographical Distribution
Jurisdictional distribution: Highly migratory


Bluefin tuna have a wide geographical distribution living mostly in temperate Atlantic waters and adjacent seas (Figure 1).
Water Area Overview
Spatial Scale: Regional

Geo References
Resource Structure
Considered a single stock: Yes


Bluefin tuna is also a highly migratory species that seems to display a homing behavior and spawning site fidelity in both the Mediterranean Sea and Gulf of Mexico, which constitute the two main spawning areas being clearly identified today. Less is known about feeding migrations within the Mediterranean and the North Atlantic, but results from electronic tagging indicated that bluefin tuna movement patterns vary considerably between individuals, years and areas. The appearance and disappearance of important past fisheries further suggest that important changes in the spatial dynamics of bluefin tuna may also have resulted from interactions between biological factors, environmental variations and fishing. Although the Atlantic bluefin tuna population is managed as two stocks, conventionally separated by the 45°W meridian, its population structure remains poorly understood and needs to be further investigated. Recent genetic and microchemistry studies as well as work based on historical fisheries tend to indicate that the bluefin tuna population structure is complex.

TThe Committee received new information on stock structure derived from otolith microchemistry, but the potential conclusions were still limited by sample size considerations. It is anticipated that with the expanded biological sample collections now being undertaken by CPCs and through the GBYP, more information on stock structure will be forthcoming in coming years. One current study that had more complete information examined natal origin of bluefin tuna caught in Bay of Biscay in 2009 to 2011, and concluded that a large fraction (95-100%) of the catch originated in the Mediterranean. The Committee also considered the evidence of the recent strong 2003 year class in both the eastern and western fisheries. In the west, otolith microchemistry results suggest that the natal origin of the 2003 year-class in recent U.S. catches is about equal proportions of eastern and western individuals. It is unclear, however, how strong the western 2003 year-class is, because of recent changes in location of Japanese fishing and stock mixing.

Substantial progress has been achieved by the GBYP “Biological Sampling and Analysis” program in relation to stock structure (genetics and microconstituents) and direct age estimations.

Important electronic and conventional tagging contributions from national programs, NGOs, industry and the GBYP were presented, and these ongoing efforts are expected to provide significant insight into bluefin tuna stock structure, mixing and migrations in the Convention area.

The Committee recognized that there have been important recent contributions to the understanding of bluefin tuna biology and ecology that should have significant impacts on the assessment (and possibly on the management) of the resource. The new information includes conversion factors, maturity, growth, migrations and stock structure. This new information needs to be evaluated in detail, prior to incorporation in the current stock assessment models. To accomplish this objective, the Committee suggested convening an intersessional meeting on bluefin tuna biology and ecology in 2013.


Figure1 Distribution of Atlantic bluefin catches by gear for the period 1950-2007.
Exploitation
 

Fishery indicators

The total catch for the West Atlantic peaked at 18,671 t in 1964, mostly due to the Japanese longline fishery for large fish off Brazil (that started in 1962) and the U.S. purse seine fishery for juvenile fish (Table1and Figure 2). Catches dropped sharply thereafter with the collapse of the bluefin tuna by-catch longline fishery off Brazil in 1967 and decline in purse seine catches, but increased again to average over 5,000 t in the 1970s due to the expansion of the Japanese longline fleet into the northwest Atlantic and Gulf of Mexico and an increase in purse seine effort targeting larger fish for the sashimi market. The total catch for the West Atlantic including discards has generally been relatively stable since 1982 due to the imposition of quotas. However, since a total catch level of 3,319 t in 2002 (the highest since 1981, with all three major fishing nations indicating higher catches), total catch in the West Atlantic declined steadily to a low of 1,638 t in 2007 and then increased in 2008 and 2009 to 2,000 t and 1,980 t, respectively. The catch in 2011 was 1,986 t (Figure 2). The decline through 2007 was primarily due to considerable reductions in catch levels for U.S. fisheries. Since 2002, the Canadian annual catches have been relatively stable at about 500-600 t (733 t in 2006); the 2006 catch was the highest recorded since 1977. The 2011 Canadian catch (including dead discards) was 510 t. Japanese catches have generally fluctuated between 300-500 t, with the exception of 2003 (57 t), which was low for regulatory reasons, and 2009 (162 t). Japanese landings for 2011 were 578 t.

The average weight of bluefin tuna taken by the combined fisheries in the West Atlantic were historically low during the 1960s and 1970s (Figure 3), for instance showing an average weight of only 33 kg during the 1965-1975 period. However, since 1980 they have been showing a quite stable trend and at a quite high average weight of 93 kg.

The overall number of Japanese vessels engaged in bluefin fishing has declined from more than 100 vessels to currently less than 10 vessels in the West Atlantic. After reaching 2,014 t in 2002 (the highest level since 1979), the catches (landings and discards) of U.S. vessels fishing in the northwest Atlantic (including the Gulf of Mexico) declined precipitously during 2003-2007. The United States did not catch its quota in 2004-2008 with catches of 1,066, 848, 615, 858 and 922 t, respectively. However, in 2009 the United States fully realized its base quota with total catches (landings including dead discards) of 1,272 t and in 2011 the U.S. catches totaled 884t and were below the quota partly owing to a reduction in dead discards and fishing effort in the Gulf of Mexico.

The indices of abundance used in the 2010 assessment were updated through 2011 (Figure 4). The catch rates of juvenile bluefin tuna in the U.S. rod and reel fishery fluctuate with little apparent long-term trend, but exhibit a pattern that is consistent with the strong year-class estimated for 2003 and show small increases in 2010 and 2011. The catch rates of adults in the U.S. rod and reel fishery remain low, but increased in 2010 to the highest level since 2002 and showed a small decrease in 2011. The catch rates of the Japanese longline fishery north of 30oN fluctuated significantly since 2007, showing considerably high values for 2007, 2009, and 2011 fishing years. These high indices might be related to the abundance of relatively small-sized bluefin (135-150cm, 50-60kg). The catch rates from the U.S. Gulf of Mexico longline fishery showed a gradual increasing trend from 1996 to 2008 and a slight decrease afterwards. The catch rates in the Gulf of St. Lawrence have increased rapidly since 2004 and the catch rates in 2011 were the highest in the time series considered in the assessment. The catch rates in southwest Nova Scotia have continued to follow an increasing trend since 2000. The Gulf of Mexico larval survey (the only fishery independent indicator) continues to fluctuate around the low levels observed since the 1980s.


Figure2 Historical catches of western bluefin tuna: (a) by gear type and (b) in comparison to TAC levels agreed by the Commission.
Figure3 Mean weight of western bluefin tuna catches by purse seine, longline, rod and reel, and all gears combined (estimated from the catch-at-size compiled information).
Figure4 Updated indices of abundance for western bluefin tuna. The dashed portions of the larval survey and Gulf of St. Lawrence CPUE series bridge the gaps between years where data were missing or otherwise considered unreliable by the 2012 SCRS (and not used in the base assessment).
Assessment
 




Assessment Model

The SCRS cautions that the conclusions of this assessment do not capture the full degree of uncertainty in the assessments and projections. An important factor contributing to uncertainty is mixing between fish of eastern and western origin. Based on earlier work, the estimates of stock status can be expected to vary considerably depending on the type of data used to estimate mixing (conventional tagging or isotope signature samples) and modeling assumptions made. Mixing models will be further investigated prior to the next assessment. Another important source of uncertainty is recruitment, both in terms of recent levels (which are estimated with low precision in the assessment), and potential future levels (the "low" vs. "high" recruitment hypotheses which affect management benchmarks). Improved knowledge of maturity at age will also affect the perception of changes in stock size. Finally, the lack of representative samples of otoliths requires determining the catch at age from length samples, which is imprecise for larger bluefin tuna. Many of these deficiencies are being addressed by current research programs.

The assessment used to provide management advice in 2010 was updated this year with data collected through 2011. The estimated trends are consistent with the previous analysis in that spawning stock biomass (SSB) declined steadily from 1970 to 1992 and has since fluctuated between 25% and 36% of the 1970 level (Figure 5). In recent years, however, there appears to have been a gradual increase in SSB from 27% in 2003 to an estimated 36% in 2011. Since 1998, when the rebuilding plan was adopted, the SSB has increased by 19%. The stock has experienced different levels of fishing mortality (F) over time, depending on the size of fish targeted by various fleets (Figure 5). Fishing mortality on spawners (ages 9 and older) declined markedly after 2003.

Estimates of recruitment were very high in the early 1970s (Figure 5), and previous analyses involving longer catch and index series suggest that recruitment was also high during the 1960s. Since 1977, recruitment has varied from year to year without trend with the exception of a strong year-class in 2003. The previous assessment estimated that the 2003 year-class was the largest since 1974, but the current assessment estimates two somewhat smaller year classes (2002 and 2003) instead. The Committee continues to believe the 2003 year class was large based on the progression of size classes through various fisheries; and the estimate of two adjacent but smaller year classes is likely an artifact of the lack of direct observations of the age of fish in the catch and recent regulations in the United States that limited the take of fish in that size range. In 2012, the 2003 year class has started to contribute to the spawning biomass.

A key factor in estimating MSY-related benchmarks is the highest level of recruitment that can be achieved in the long term. Assuming that average recruitment cannot reach the high levels from the early 1970s, recent F (2008-2010) is 61% of FMSY and B2011 is about 140% of BMSY (Figure 6, Figure 7). Estimates of stock status are more pessimistic if a high recruitment scenario is considered (F =160% of FMSY, B =19% of BMSY).


Figure 5 Median estimates of spawning biomass (age 9+), fishing mortality on spawners, apical fishing mortality (F on the most vulnerable age class) and recruitment for the base VPA model. The 80% confidence intervals are indicated with dotted lines. The recruitment estimates for the last three years of the VPA are considered unreliable and have been replaced by the median levels corresponding to the low recruitment scenario.
Figure 6 Estimated status of stock relative to the Convention objectives (MSY) by year (1973 to 2011) and recruitment scenario (black=high recruitment potential, blue=low recruitment potential). The light blue dots represent the status estimated for 2011 and the clouds of symbols depict the corresponding bootstrap estimates of uncertainty..The lines give the historical point estimates. The marginal density plots shown above and to the right of the main graph reflect the frequency distribution of the bootstrap estimates of each model with respect to relative biomass (top) and relative fishing mortality (right). The frequency distributions of the combined model bootstraps are shown in light blue. The red lines represent the benchmark levels (ratios equal to 1.0).
Figure 7 Pie chart summarizing stock status, showing the proportion of model outputs that are not overfished and not undergoing overfishing (green), either overfished or undergoing overfishing (yellow) and both overfished and undergoing overfishing (red).




Overall Assessment Results


Outlook

A medium-term outlook evaluation of changes in spawning stock size and yield over the remaining rebuilding period under various management options was conducted. Future recruitment was assumed to fluctuate under two scenarios: (i) average levels observed for 1976-2008 (87,000 fish, the low recruitment potential scenario) and (ii) levels that increase as the stock rebuilds (MSY level of 280,000 fish, the high recruitment potential scenario). The Committee has no strong evidence to favor either scenario over the other and notes that both are plausible (but not extreme) lower and upper bounds on rebuilding potential.

The outlook for bluefin tuna in the West Atlantic is similar to that from the 2010 assessment (Figures 8-11). The low recruitment scenario suggests the stock is above the MSY level with greater than 60% probability and catches of 2,500 t or lower will maintain it above the MSY level. Constant catches of 2,000 t would result in 2019 SSB nearly equal to that in 2012. If the high recruitment scenario is correct, then the western stock will not rebuild by 2019 even with no catch, although catches of 1,200 t or less are predicted to have a 60% chance to immediately end overfishing and initiate rebuilding.

The Committee notes that considerable uncertainties remain for the outlook of the western stock, including the effects of mixing and management measures on the eastern stock.


Figure 8 Projections of spawning stock biomass (SSB) for the Base Case assessment under low recruitment potential (top panels) and high recruitment potential (bottom panels) and various levels of constant catch. The labels “50%” and “60%” refer to the probability that the SSB will be greater than or equal to the values indicated by each curve. The curves corresponding to each catch level are arranged sequentially in the same order as the legends. A given catch level is projected to have a 50% or 60% probability of meeting the convention objective (SSB greater than or equal to the level that will produce the MSY) in the year that the corresponding curve meets the dashed horizontal line.
Figure 9 Kobe II matrices (updated during the 2012 stock assessment) giving the probability that the spawning stock biomass will exceed the level that will produce MSY (B>BMSY, not overfished) in any given year for various constant catch levels under the low recruitment, high recruitment, and combined scenarios. The current TAC of 1,750 t [Rec. 10-03] is indicated in bold.
Figure 10 Kobe II matrices (updated during the 2012 stock assessment) giving the probability that the fishing mortality rate (F) will be less than the level that will produce MSY (F<FMSY, no overfishing) in any given year for various constant catch levels under the low recruitment, high recruitment, and combined scenarios. The current TAC of 1,750 t [Rec. 10-03] is indicated in bold.
Figure 11 Kobe II matrices (updated during the 2012 stock assessment) giving the joint probability that the fishing mortality rate will be less than the level that will produce MSY (F<FMSY) and the spawning stock biomass (SSB) will exceed the level that will produce MSY (B>BMSY) in any given year for various constant catch levels under the low recruitment, high recruitment, and combined scenarios. The current TAC of 1,750 t [Rec. 10-03] is indicated in bold.




Management
Management unit: Yes


Effects of current regulations

The Committee previously noted that Recommendation 08-04, which was implemented in 2009, was expected to result in a rebuilding of the stock towards the convention objective, but also noted that there has not yet been enough time to detect with confidence the population response to the measure. This statement is also true for Recommendation 10-03, which was implemented in 2011. Nevertheless, the available fishery indicators (Figure 4) as well as the current assessment suggest the spawning biomass of western bluefin tuna continues to increase.


Management Advice


In 1998, the Commission initiated a 20-year rebuilding plan designed to achieve BMSY with at least 50% probability. In response to recent assessments, the Commission recommended a total allowable catch (TAC) of 1,900 t in 2009, 1,800 t in 2010 [Rec. 08-04] and 1,750 t in 2011 [Rec. 10-03].

The current (2012) assessment indicates similar historical trends in abundance as in previous assessments. The strong 2003 year class has contributed to stock productivity such that total biomass has been increasing in recent years.

Future stock productivity, as with prior assessments, is based upon two hypotheses about future recruitment: a ‘high recruitment scenario” in which future recruitment has the potential to achieve levels that occurred in the early 1970s and a “low recruitment scenario” in which future recruitment is expected to remain near present levels. The results of this assessment have shown that long term implications of future biomass are different between the two hypotheses and the issue of distinguishing between them remains unresolved.

Probabilities of achieving BMSY within the Commission rebuilding period were projected for alternative catch levels (Figure 9). The "low recruitment scenario" suggests that biomass is currently sufficient to produce MSY, whereas the "high recruitment scenario" suggests that BMSY has a very low probability of being achieved within the rebuilding period. Despite this large uncertainty about the long term future productivity of the stock, under either recruitment scenario current catches (1,750 t) should allow the biomass to continue to increase. Larger catches in excess of 2,000 t will prevent the possibility of the 2003 year class elevating the productivity potential of the stock in the future. The Commission may wish to protect the 2003 year class to enhance its contribution to the spawning biomass. Maintaining catch at current levels (1,750 t) is expected to allow the spawning biomass to increase, which may help resolve the issue of low and high recruitment potential. For example, should the high recruitment hypothesis be correct, allowing substantial increases in spawning biomass should lead to higher recruitment.

As noted previously by the Committee, both the productivity of western Atlantic bluefin and western Atlantic bluefin fisheries are linked to the eastern Atlantic and Mediterranean stock. Therefore, management actions taken in the eastern Atlantic and Mediterranean are likely to influence the recovery in the western Atlantic, because even small rates of mixing from East to West can have considerable effects on the West due to the fact that eastern plus Mediterranean resource is much larger than that of the West.


WEST ATLANTIC BLUEFIN TUNA SUMMARY

(Catches and Biomass in t)

Current (2011) Catch (including discards)

1,986 t

Assuming Low Potential Recruitment

Maximum Sustainable Yield (MSY)

2,634 (2,452-2,834)1

    BMSY

12,943 (12,717-13,268) 1

    B2011/BMSY

1.4 (1.14-1.72)1

    FMSY

0.17 (0.14-0.19) 1

    F0.1

0.11 (0.10-0.12)1

    F2008-2010 /FMSY2

0.61 (0.49-0.74)1

    F2008-2010 /F012

0.92 (0.77-1.12)1

   Stock status

Overfished: NO

Overfishing: NO

Assuming High Potential Recruitment

Maximum Sustainable Yield (MSY)

6,472 (5,736-7,500)1

    BMSY

93,621 (77,288-116,679) 1

    B2011/BMSY

0.19 (0.13-0.29)1

    FMSY

0.064 (0.056-0.074) 1

    F0.1

0.11 (0.10-0.12)1

    F2008-2010 /FMSY2

1.57(1.24-1.95)1

    F2008-2010 /F012

0.92 (0.77-1.12)1

   Stock status

Overfished: YES

Overfishing: YES

Management Measures: 

 

[Rec. 08-04] TAC of 1,900 t in 2009 and 1,800 t in 2010, including dead discards

[Rec. 10-03] TAC of 1,750 t in 2011 and 2012, including dead discards.

1 Median and approximate 80% confidence interval from bootstrapping from the assessment.
2 F 2008-2010 refers to the geometric mean of the estimates for 2008-2010 (a proxy for recent F levels).

Biological State and Trend
Exploitation rate: F2008-2010 /F0.1 =0.92 (0.77-1.12); F2008-2010/FMSY: Low Recruitment=0.61 (0.49-0.74), High Recruitment=1.57(1.24-1.95)
Abundance level: B2011/BMSY: Low Recruitment=1.4 (1.14-1.72), High Recruitment=0.19 (0.13-0.29)
Source of information
 
“Report of the 2012 Meeting of the Standing Committee on Research and Statistics.” International Commission for the Conservation of Atlantic Tunas (ICCAT). 2013 ICCAT Report for biennial period, 2012-13 PART I (2012) - Vol. 2 ICCAT Click to openhttp://www.iccat.int/Documents/BienRep/REP_EN_12-13_I_2.pdf
powered by FIGIS  © FAO, 2014
Powered by FIGIS
crawl