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Bigeye tuna - Eastern Pacific (EPO)
Marine Resource  Fact Sheet
Stock status report 2015
Bigeye tuna - Eastern Pacific (EPO)
Fact Sheet Citation  
Owned byInter-American Tropical Tuna Commission (IATTC) – More
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Species:
FAO Names: en - Bigeye tuna, fr - Thon obèse(=Patudo), es - Patudo, ru - Тунец большеглазый
Geographic extent of Bigeye tuna - Eastern Pacific (EPO)
Main Descriptors
Considered a single stock: Yes        Spatial Scale: Regional
Management unit: Yes        Reference year: 2014
 
 
Biological State and Trend
State & Trend Descriptors
PartnerFIRMS
Exploitation rateFishing mortality rate close to that for MSYModerate fishing mortality
Abundance levelSpawning biomass close to that for MSYIntermediate abundance
FAO
Exploitation stateFully exploited
Habitat and Biology
Bottom type: Unspecified.   Depth zone: Abyssal ( >1000m).   Horizontal distribution: Oceanic.   Vertical distribution: Pelagic.  

Geographical Distribution
Jurisdictional distribution: Highly migratory

Water Area Overview
Spatial Scale: Regional

Geo References
Resource Structure
Considered a single stock: Yes
Exploitation
 

The annual catches of bigeye during 1985-2014 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 103 thousand t, with a low of 72 thousand t in 1985 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 154 thousand t, and reached a high of 180 thousand t in 2004. Since 2004 the catch has fluctuated between 128 and 153 thousand t.

The annual retained catches of bigeye in the EPO by purse-seine and pole-and-line vessels during 1985-2014 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, it increased from 35 thousand t in 1994 to between 44 and 95 thousand t during 1995-2013. The preliminary estimate of the retained catch in the EPO in 2014 is 60 thousand t.

During 1999-2013 the purse-seine catch of the species discarded at sea has steadily decreased, from 9% in 1999 to less than 1% in 2013, for an average discard rate of about 2.5%. No bigeye catch has been reported by pole-and-line vessels in recent years.

From 1985 to 1993, before the increase in the use of FADs, longliners caught an average of 95% of the bigeye in the EPO (average 86 thousand t; range; 66 to 104 thousand t). During 1999-2013 this average dropped to 39%, 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 2014 is 35 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, 1975-2014. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2014 catch data are preliminary.
Assessment
 
Assessment Model
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

Several assumptions regarding processes such as growth, recruitment, movement, natural mortality, and fishing mortality, have also been made. 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.

This model is the same as that used in the previous full assessment conducted in 2013 (IATTC Stock Assessment Report 14) which included several improvements. First of all, a new Richards growth curve estimated externally from an integrated analysis of otolith age-readings and tag-recapture observations was introduced. This curve reduced the uncertainty about the average size of the oldest fish (L2 parameter). In addition, the parameters which determine the variance of the length-at-age were also taken from the new externally-derived growth estimates. Diagnostic analyses with the previous base case model configuration indicated a dominant influence of the size-composition data in determining the productivity (the R0 parameter) of the bigeye stock, and conflicts among datasets were also found. As a result, improvements were made in the previous full assessment on the weighting assigned to the different datasets. Specifically, the size-composition data of all fisheries were down-weighted. In addition, the number of catch per unit of effort (CPUE) data series used as indices of abundance was reduced in order to minimize conflict trends among data sets. Rather than fitting to a total of ten CPUE series (two purse-seine indices and eight longline indices), a reduced set of indices of abundance was chosen to best represent the bigeye stock trends (the early and late periods of the Central and Southern longline fisheries).
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, 1975-2014. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2014 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, 2009-2013. 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, 2014. 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, 2009-2013. The sizes of the circles are proportional to the amounts of bigeye and yellowfin caught in those 5° by 5° areas.

The stock assessment requires a substantial amount of information. Data on retained catch, discards, CPUE, and size compositions of the catches from several different fisheries have been analyzed. Catch and CPUE data for the surface fisheries have been updated, and include new data for 2014. New or updated longline catch data are available for China (2013), Japan (2008-2013), Korea (2013), Chinese Taipei (2011-2013), the United States (2012-2013), French Polynesia (2013) and Vanuatu (2013-2014). Longline catch data for 2014 are available for China, Japan, Chinese Taipei, and Korea from the monthly report statistics. For longline fisheries with no new catch data for 2014, catches were assumed to be the same as in 2013. New or updated CPUE data are available for the Japanese longline fleet (2008-2013). New purse-seine length-frequency data are available for 2014 and updates are available for 2013. New or updated length-frequency data are available for the Japanese longline fleet (2011-2013).
Results
Assessment Indicator
Type: Recruitment

A prominent feature in the time series of estimated bigeye recruitment is that the highest recruitment peaks of 1983 and 1998 coincide with the strongest El Niño events during the historic period of the assessment (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.

There was a period of above-average annual recruitment during 1994-1998, followed by a period of below-average recruitment in 1999-2000. The recruitments were above average from 2001 to 2006, and were particularly strong in 2005. More recently, the recruitments were below average during 2007-2009, and have fluctuated around average during 2010-2013. The most recent annual recruitment estimate (2014) is estimated to be slightly above average levels. However, this estimate is highly uncertain, and should be regarded with caution, due to the fact that recently-recruited bigeye are represented in only a few length-frequency data sets.

Over the range of spawning biomasses estimated by the base case assessment, the abundance of bigeye recruits appears to be unrelated to the spawning potential of adult females at the time of hatching.
Assessment Indicator
Type: 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.
Assessment Indicator
Type: SSB

Since the start of 2005, the spawning biomass ratio (SBR; the ratio of the spawning biomass at that time to that of the unfished stock) gradually increased, to a level of 0.30 at the start of 2010. This may be attributed to a combined effect of a series of above-average recruitments since 2001, the IATTC tuna conservation resolutions and decreased longline fishing effort in the EPO during 2004-2009. However, although the resolutions have continued since 2009, the rebuilding trend was not sustained during 2010-2013, and the SBR gradually declined to a low historic level of 0.19 at the start of 2013 (Figure D-5).
Figure D-5: Estimated spawning biomass ratios (SBRs) of bigeye tuna in the EPO, including projections for 2015-2024 based on average fishing mortality rates during 2012-2014. The dashed horizontal line (at about 0.21) identifies the SBR at MSY. The solid line illustrates the maximum likelihood estimates, and the estimates after 2015 (the large dot) indicate the SBR predicted to occur if fishing mortality rates continue at the average of that observed during 2012-2014. The dashed lines are the 95-percent confidence intervals around these estimates.

This decline could be related to a period dominated by below-average recruitments that began in late 2007 and coincides with a series of particularly strong La Niña events. More recently, the SBR is estimated to have increased slightly, from 0.19 in 2013 to 0.22 at the start of 2015; 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.

At the beginning of 2015, the spawning biomass of bigeye tuna in the EPO appears to have been about 6% above SMSY, and the recent catches are estimated to have been about 13% lower than the maximum sustainable yield (MSY). If fishing mortality is proportional to fishing effort, and the current patterns of age-specific selectivity are maintained, FMSY is about 14% higher than the current level of effort (Table D-1).
Projection

According to the base case results, the most recent estimate indicates that the bigeye stock in the EPO is not overfished (S>SMSY) and that overfishing is not taking place (F<FMSY) (Figure D-6).
Figure D-6: Kobe (phase) plot of the time series of estimates of spawning stock size and fishing mortality relative to their MSY reference points. The panels represent interim target reference points (S(MSY) and F(MSY); solid lines) and limit reference points (dashed lines) of 0.38 S(MSY) and 1.6 F(MSY), 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 is the first estimate (1975).

Likewise, the current base case model indicates that the interim limit reference points 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, have not been exceeded (Figure D-6). These interpretations, however, are subject to uncertainty, as indicated by the approximate confidence intervals around the most recent estimate in the phase plots, which allows F>FMSY). Also, they are strongly dependent on the assumptions made about the steepness parameter of the stock-recruitment relationship, the assumed levels of adult natural mortality, the growth curve, and the weighting assigned to the size-composition data.

The MSY of bigeye in the EPO could be maximized if the age-specific selectivity pattern were similar to that of the longline fisheries, because they catch larger individuals that are close to the critical weight. Before the expansion of the floating-object fishery that began in 1993, the MSY was greater than the current MSY and the fishing mortality was much less than FMSY (Figure D-7).
Figure D-7: Estimates of MSY-related quantities calculated using the average age-specific fishing mortality for each year. (S(recent) is the spawning biomass at the beginning of 2015.)

At current levels of fishing mortality, and if recent levels of effort and catchability continue and average recruitment levels persist, the spawning biomass is predicted to continue rebuilding and stabilize at an SBR of 0.25 around 2022, above the level corresponding to MSY (0.21) (Figure D-5). If a stock-recruitment relationship is assumed, it is estimated that catches will be lower in the future at current levels of fishing effort, particularly for the surface fisheries (Figure D-8).
Figure D-8: Historic and projected annual catches of bigeye tuna during 2015-2024 for the surface (top panel) and longline (bottom panel) fisheries, based on fishing mortality rates during 2012-2014. Predicted catches are compared between the base case, the analysis assuming F(MSY) and the analysis in which a stock-recruitment relationship (h=0.75) was used. t=metric tons.

These simulations are based on the assumption that selectivity and catchability patterns will not change in the future. Changes in targeting practices or increased catchability of bigeye as abundance declines (e.g. density-dependent catchability) could result in differences from the outcomes predicted here.
Overall Assessment Results

Summary

  1. The results of this assessment indicate a recovery trend for bigeye tuna in the EPO during 2005-2009, subsequent to IATTC tuna conservation resolutions initiated in 2004. However, the decline of the spawning biomass that began at the start of 2010 reduced both summary and spawning biomasses to their lowest historic levels at the start of 2013, and persisted through 2013. This decline may be related to a series of recent below-average recruitments which coincide with a series of strong la Niña events. More recently, the SBR is estimated to have increased slightly, from 0.19 in 2013 to 0.22 at the start of 2015; in the model, this increase is driven mainly by the recent increase in the CPUE of the longline fisheries which catch adult bigeye. At current levels of fishing mortality, and if recent levels of effort and catchability continue and average recruitment levels persist, the spawning biomass is predicted to continue rebuilding, and stabilize at about 0.25, above the level corresponding to MSY (0.21).
  2. There is uncertainty about recent and future recruitment and biomass levels.
  3. The recent fishing mortality rates are estimated to be below the level corresponding to MSY whereas recent levels of spawning biomass are estimated to be slightly above that level. These interpretations are uncertain and highly sensitive to the assumptions made about the steepness parameter of the stock-recruitment relationship, the assumed rates of natural mortality for adult bigeye, the growth curve, and the weighting assigned to the size-composition data, in particular to the longline size-composition data. The results are more pessimistic if a stock-recruitment relationship is assumed, if lower rates of natural mortality are assumed for adult bigeye, if the length of the oldest fish is assumed to be greater, and if a greater weight is assigned to the size-composition data, in particular for the longline fisheries.
  4. The IATTC staff is collaborating with the Secretariat of the Pacific Community (SPC) on an updated Pacific-wide wide bigeye stock assessment. This research will incorporate the new bigeye 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

Management
Management unit: Yes
Source of information
 
Inter-American Tropical Tuna Commission (IATTC).  “"Tunas and billfishes in the eastern Pacific Ocean in 2014. Inter-American Tropical Tuna Commission." Fishery Status Report. IATTC 2015.” Click to openhttp://www.iattc.org/PDFFiles2/FisheryStatusReports/FisheryStatusReport13.pdf
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