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Bigeye tuna - Eastern Pacific (EPO)
Fact Sheet Title  Fact Sheet
Stock status report 2016
Bigeye tuna - Eastern Pacific (EPO)
Fact Sheet Citation  
Owned byInter-American Tropical Tuna Commission (IATTC) – ownership
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Species List:
Species Ref: en - Bigeye tuna, fr - Thon obèse(=Patudo), es - Patudo, ru - Тунец большеглазый
ident Block Bigeye tuna - Eastern Pacific (EPO)
Aq Res
Biological Stock:  Biological Stock         Value: Regional
Management Unit:  Management Unit         Reference Year: 2015
 
 
Aq Res State Trend
Aq Res State Trend
Aq Res State Trend Aq Res State Trend
Aq Res State TrendFishing mortality rate close to that for MSYModerate fishing mortalityGreen
[?]
Aq Res State TrendSpawning biomass close to that for MSYIntermediate abundance
Aq Res State Trend
Aq Res State TrendFully exploited
Habitat Bio
Bottom Type: Unspecified.   Depth Zone: Abyssal >1000m.   Horizontal Dist: Oceanic.   Vertical Dist: Pelagic.  

Geo Dist
Geo Dist: Highly migratory

Water Area Overview
Spatial Scale:  Spatial Scale

Water Area Overview
Aq Res Struct
Biological Stock:  Biological Stock
Exploit
 

The annual catches of bigeye during 1985-2015 are shown in (Table A-1). Overall, the catches in both the EPO and WCPO have increased, but with considerable fluctuations. In the EPO, the average catch for the period was 104 thousand t, with a low of 73 thousand t in 1989 and a high of 149 thousand t in 2000. In the WCPO the catches of bigeye increased to more than 77 thousand t during the late 1970s, decreased during the early 1980s, and then increased steadily to 111 thousand t in 1996. In 1997 the total jumped to 153 thousand t, and reached a high of 178 thousand t in 2004. Since 2004 the catch has fluctuated between 130 and 155 thousand t.

The annual retained catches of bigeye in the EPO by purse-seine and pole-and-line vessels during 1986-2015 are shown in Table A-2a. During 1993-1994 the use of fish-aggregating devices (FADs), placed in the water by fishermen to aggregate tunas, nearly doubled, and continued to increase in the following years. This resulted in greater catches of bigeye by purse-seine vessels. Before this increase, the annual retained catch of bigeye taken by purse-seine vessels in the EPO was about 5 thousand t (Table A-2a), (Table A-2a (cont.)), (Table A-2a (cont.)). As a result of the development of the FAD fishery, bigeye catches increased from 10 thousand t in 1993 to 35 thousand t in 1994, and further increased to between 44 and 95 thousand t during 1995-2014. The preliminary estimate of the retained catch in the EPO in 2015 is 63 thousand t.

During 2000-2014 the purse-seine catch of the species discarded at sea has steadily decreased, from 5% in 2000 to less than 1% in 2014, for an average discard rate of about 2.1%. No bigeye catch has been reported by pole-and-line vessels in recent years.

From 1986 to 1993, before the increase in the use of FADs, longliners caught an average of 95% of the bigeye in the EPO (average 88 thousand t; range; 71 to 104 thousand t). During 2000-2014 this average dropped to 38%, with a low of 25% in 2008 (average: 42 thousand t; range: 26 to 74 thousand t) (Table A-2a). The preliminary estimate of the longline catch in the EPO in 2015 is 38 thousand t (Table A-2a).

Small amounts of bigeye are caught in the EPO by other gears, as shown in Table A-2a.

See also fishery fact sheet:EPO Tunas and billfishes fishery
Total catches (retained catches plus discards) of bigeye tuna by the purse-seine fisheries, and retained catches for the longline fisheries, in the eastern Pacific Ocean, 1975-2015. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2015 catch data are preliminary.
Bio Assess
 
Assess Models
Type:  Age-structured
An integrated statistical age-structured stock assessment model, Stock Synthesis

This report presents the most current stock assessment of bigeye tuna (Thunnus obesus) in the eastern Pacific Ocean (EPO). An integrated statistical age-structured stock assessment model (Stock Synthesis 3.23b) was used in the assessment.
Assumption

The assessment is conducted as if there were a single stock of bigeye in the EPO, with minimal net movement of fish between the EPO and the western and central Pacific Ocean (WCPO). Its results are consistent with the results of other analyses of bigeye tuna on a Pacific-wide basis. However, the distribution of the bigeye catches extends across the equatorial Pacific Ocean. In addition, a large amount of conventional and electronic tagging data has recently accumulated from the Pacific Tuna Tagging Program, which has focused its bigeye tagging efforts between 180° and 140°W since 2008. The tag recoveries clearly show that there is extensive longitudinal movement of bigeye across the IATTC’s management boundary at 150°W, in particular from west to east. The IATTC staff is collaborating with Secretariat of the Pacific Community (SPC) on an updated Pacific-wide bigeye stock assessment. This research will incorporate the new tagging data in a spatially-structured population dynamics model, which will help to evaluate potential biases resulting from the current approach of conducting separate assessments for the EPO and WCPO.
Data

There have been substantial changes in the bigeye tuna fishery in the EPO over recent decades (Figure D-1)
Figure D-1: Total catches (retained catches plus discards) of bigeye tuna by the purse-seine fisheries, and retained catches for the longline fisheries, in the eastern Pacific Ocean, 1975-2015. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2015 catch data are preliminary.

Initially, the majority of the bigeye catch was taken by longline vessels. With the expansion of the fishery on fish-aggregating devices (FADs) since 1993, the purse-seine fishery has taken an increasing component of the bigeye catch. In recent years, purse-seine catches of bigeye were taken primarily between 5°N and 5°S across the equatorial Pacific as far west as the western boundary (150°W) of the EPO (Figure A-3a-b).
Figure A-3a: Average annual distributions of the purse-seine catches of bigeye, by set type, 2010-2014. The sizes of the circles are proportional to the amounts of bigeye caught in those 5° by 5° areas.
Figure A-3b: Annual distributions of the purse-seine catches of bigeye, by set type, 2015. The sizes of the circles are proportional to the amounts of bigeye caught in those 5° by 5° areas.

The longline catches of bigeye in the EPO are predominantly taken below 5°S, but a substantial portion is also taken north of 10°N (Figure A-4)
Figure A-4: Distributions of the average annual catches of bigeye and yellowfin tunas in the Pacific Ocean, in metric tons, by Chinese, Japanese, Korean, and Chinese Taipei longline vessels, 2010-2014. The sizes of the circles are proportional to the amounts of bigeye and yellowfin caught in those 5° by 5° areas.

The assessment of bigeye tuna in the eastern Pacific Ocean in 2015 is similar to the previous assessment, except that separate series of length-frequency data for Japanese longline commercial and training vessels are now available, and both were used in the assessment.
Results
Assess Indicator
Assess Indicator: Recruitment
Assess Indicator
Assess Indicator: Fishing mortality

There have been important changes in the amount of fishing mortality caused by the fisheries that catch bigeye tuna in the EPO. On average, since 1993 the fishing mortality of bigeye less than about 15 quarters old has increased substantially, and that of fish more than about 15 quarters old has also increased, but to a lesser extent) (Figure D-3)
Figure D-3: Average annual fishing mortality, by all gears, of bigeye tuna recruited to the fisheries of the EPO. Each panel illustrates the average fishing mortality rates that affected the fish within the range of ages indicated in the title of each panel. For example, the trend illustrated in the top panel is an average of the fishing mortalities that affected the fish that were 1-4 quarters old.

The increase in the fishing mortality of the younger fish was caused by the expansion of the purse-seine fisheries that catch tuna in association with floating objects. It is clear that the longline fishery had the greatest impact on the stock prior to 1995, but with the decrease in longline effort and the expansion of the floating-object fishery, at present the impact of the purse-seine fishery on the bigeye stock is far greater than that of the longline fishery (Figure D-4).
Figure D-4: Trajectory of the spawning biomass of a simulated population of bigeye tuna that was not exploited (top line) and that predicted by the stock assessment model (bottom line). The shaded areas between the two lines show the portions of the impact attributed to each fishing method. t = metric tons.

The discarding of small bigeye has a small, but detectable, impact on the depletion of the stock.
Assess Indicator
Assess Indicator: SSB

At current levels of fishing mortality (F), and if recent levels of effort and catchability continue and recruitment remains average, the spawning biomass (S) is predicted to continue rebuilding and stabilize at about 0.22, above the level corresponding to the maximum sustainable yield (MSY) (0.21) (Figure D-5).
Figure D-5: Estimated spawning biomass ratios (SBRs) of bigeye tuna in the EPO, including projections for 2016-2026 based on average fishing mortality rates during 2013-2015. The dashed horizontal line (at 0.21) identifies the SBR at MSY. The solid line illustrates the maximum likelihood estimates, and the estimates after 2016 (the large dot) indicate the SBR predicted to occur if fishing mortality rates continue at the average of that observed during 2013-2015. The dashed lines are the 95-percent confidence intervals around these estimates.

The recent fishing mortality rates are estimated to be below the level corresponding to MSY, whereas recent spawning biomasses are estimated to be slightly below that level (Table D-1).

and,
Figure D-6: Kobe (phase) plot of the time series of estimates of spawning stock size (top panel: spawning biomass; bottom panel: total biomass aged 3+ quarters) and fishing mortality relative to their MSY reference points. The colored panels represent interim target reference points (SMSY and FMSY; solid lines) and limit reference points (dashed lines) of 0.38 SMSY and 1.6 FMSY, which correspond to a 50% reduction in recruitment from its average unexploited level based on a conservative steepness value (h = 0.75) for the Beverton-Holt stock-recruitment relationship. Each dot is based on the average fishing mortality rate over three years; the large dot indicates the most recent estimate. The squares around the most recent estimate represent its approximate 95% confidence interval. The triangle represents the first estimate (1975).



These interpretations are uncertain and highly sensitive to the assumptions made about the steepness parameter (h) of the stock-recruitment relationship, the weighting assigned to the size-composition data (in particular to the longline size-composition data), the growth curve, and the assumed rates of natural mortality (M) for bigeye.


Projection

The results of this assessment indicate a recovering trend for bigeye in the EPO during 2005-2009, subsequent to IATTC tuna conservation resolutions initiated in 2004 (Figure D-5).
Figure D-5: Estimated spawning biomass ratios (SBRs) of bigeye tuna in the EPO, including projections for 2016-2026 based on average fishing mortality rates during 2013-2015. The dashed horizontal line (at 0.21) identifies the SBR at MSY. The solid line illustrates the maximum likelihood estimates, and the estimates after 2016 (the large dot) indicate the SBR predicted to occur if fishing mortality rates continue at the average of that observed during 2013-2015. The dashed lines are the 95-percent confidence intervals around these estimates.

However, although the resolutions have continued since 2009, the rebuilding trend was not sustained during 2010-2012, and the spawning biomass ratio (SBR) gradually declined to a historically low level of 0.16 at the start of 2013. This decline may be related to a series of recent below-average recruitments which coincided with a series of strong La Niña events (Figure D-2).
Figure D-2: Estimated annual recruitment of bigeye tuna to the fisheries of the EPO. The estimates are scaled so that the estimate of virgin recruitment is equal to 1.0 (dashed horizontal line). The solid line shows the maximum likelihood estimates of recruitment, and the shaded area indicates the approximate 95% intervals around those estimates.

More recently, the SBR is estimated to have increased slightly, from 0.16 in 2013 to 0.20 at the start of 2016; in the model, this increase is driven mainly by the recent increase in the catch per unit of effort (CPUE) of the longline fisheries that catch adult bigeye. There is uncertainty about recent and future levels of recruitment and biomass.


Management
Management:  Management
Sources
 
Inter-American Tropical Tuna Commission (IATTC).  “"Tunas and billfishes in the eastern Pacific Ocean in 2015. Inter-American Tropical Tuna Commission." Fishery Status Report. IATTC 2015.” Click to openhttp://www.iattc.org/PDFFiles2/FisheryStatusReports/FisheryStatusReport14.pdf
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