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Bigeye tuna - Eastern Pacific (EPO)
Marine Resource  Fact Sheet
Stock status report 2020
Bigeye tuna - Eastern Pacific (EPO)
Fact Sheet Citation  
Owned byInter-American Tropical Tuna Commission (IATTC) – 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 - Eastern Pacific (EPO)
Main Descriptors
Considered a single stock: Yes        Spatial Scale: Regional
Management unit: Yes        Reference year: 2019
 
 
Biological State and Trend
State & Trend Descriptors
PartnerFIRMS
Exploitation rateFishing mortality rate close to that for MSYModerate fishing mortalityGreen
Abundance levelSpawning biomass close to that for MSYIntermediate abundance
FAO Categories
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 total annual catches of bigeye in the Pacific Ocean during 1990-2019 are shown in (Table A-1). Overall, the catches in both the EPO and WCPO have increased, but with considerable fluctuations. In the WCPO they averaged more than 77 thousand t during the late 1970s, decreased during the early 1980s, and then increased steadily to 119 thousand t in 1992; they jumped to 168 thousand t in 1998, and reached a high of 180 thousand t in 2004, since when they have fluctuated between 123 and 156 thousand t. In the EPO, the average catch during 1990-2019 was 105 thousand t, with a low of 83 thousand t in 1993 and a high of 149 thousand t in 2000.

The annual retained catches of bigeye in the EPO by purse-seine and pole-and-line vessels during 1990-2019 are shown in Table A-2a. The introduction of fish-aggregating devices (FADs), placed in the water by fishers to attract tunas, in 1993 led to a sudden and dramatic increase in the purse-seine catches. Prior to 1993, the annual retained purse-seine catch of bigeye in the EPO was about 5 thousand t by 1994 it was 35 thousand t, and in 1996 was over 60 thousand t. During 1997-2018 it has fluctuated between 44 and 95 thousand t; the preliminary estimate for 2019 is 71 thousand t. (Table A-2a).

During 2000-2019 the percentage of the purse-seine catch of bigeye discarded at sea has steadily decreased, from 5% in 2000 to less than 1% in 2014, averaging about 1.7%.

Before the expansion of the FAD fishery, longliners caught almost all the bigeye in the EPO, averaging 88 thousand t annually during 1985-1992. Since 1993, the annual average catch has declined by 50%, to 44 thousand t, and the preliminary estimate for 2019 is less than 25 thousand t.

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) by the purse-seine (PS) fisheries, and retained catches by the longline (LL) fisheries, of bigeye tuna in the eastern Pacific Ocean, 1975-2019. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. 2019 data are preliminary.
Assessment
 
Assessment Model
Type:  Age-structured
An integrated statistical age-structured stock assessment model, Stock Synthesis

For the full version of this analysis, see documents SAC-11-05, SAC-11-06, SAC-11-INF-F, and SAC-11-08.

Bigeye tuna are distributed in tropical and temperate waters across the Pacific Ocean. In the eastern Pacific Ocean (EPO), the majority of catch before 1993 was taken by longline fisheries that target large bigeye as shown in Figure D-1. Due to the expansion of purse-seine fisheries associated with floating objects, purse-seine fisheries that target small bigeye have replaced longline fisheries as the dominant fishery type for EPO bigeye since 1996.

The reference models for the benchmark assessment of bigeye were built based on three overarching hypotheses (SAC-11-INF-J).

The first overarching hypothesis is about the cause of the apparent recruitment shift which coincides with the expansion of the floating-object fishery, assuming that shift is either real or an artifact of model misspecification. The second overarching hypothesis consists of two levels. The first level represents the cause of the apparent recruitment shift given it is an artifact of model misspecification. It is assumed that mis-specified process is either known (movement, mortality, selectivity, or growth) or unknown (other than those four processes). The second level represents the cause of the misfit to the length composition data from the longline fishery that has asymptotic selectivity. It is assumed that the misfit is due to random observation error or an artifact of model misspecification (in growth, selectivity, or natural mortality).

The third overarching hypothesis is about the steepness of the Beverton-Holt stock-recruitment relationship, which was assumed in the reference models to be 0.7, 0.8, 0.9, or 1.0.
Data

In 2020, stock status indicators (SSIs) were developed for bigeye using the data collected in the EPO as a whole (SAC-11-05). For the floating-object fishery, the main fishery for bigeye since 1993, fishing effort has been continuously increasing. The fishing effort associated with the longline fishery, in comparison, does not show a noticeable long-term trend and remained around the median level since 2013 (Figure B-2).
Figure B-2: Indicators of total effort in the EPO, based on purse-seine data closure-adjusted capacity, 2000-2019; annual total number of sets, by type, 1987-2019) and based on longline data for 2000-2018 (effort reported by all fleets, in total numbers of hooks; proportion of the effort corresponding to Japan). The dashed horizontal lines are the 5th and 95th percentiles, the solid horizontal line is the median.
Data

This increase in fishing effort corresponds to increased catch, decreased catch per set, and decreased average length for the floating-object fishery during the same time (Figure D-2).
Figure D-2: Indicators of catch, catch per set, and average length for bigeye tuna in the EPO based on purse-seine data.
Results
Assessment Indicator
Type: Recruitment

In total, 44 reference models were retained in the benchmark assessment for bigeye tuna (SAC-11-06). These reference models on which the management advice is based were combined using relative weights determined by several criteria, including performance on model diagnostics (SAC-11-INF-J).

The results from the 44 reference models for bigeye show that (1) the recruitment shift is apparent in some but not all models (Figure D-4);
Figure D-4: Comparison of annual relative recruitment estimates for bigeye tuna in the eastern Pacific Ocean from the twelve reference models (only the estimates that correspond to steepness = 1.0 are shown). The shaded areas represent the 95% confidence intervals and the black line represents the combined estimates across the twelve models.

(2) all models show a decreasing trend in spawning biomass while the scale of the spawning biomass varies dramatically among models (Figure D-5);
Figure D-5: Comparison of spawning biomass estimates for bigeye tuna in the eastern Pacific Ocean from the twelve reference models (only the estimates that correspond to steepness = 1.0 are shown). The shaded areas represent the 95% confidence intervals and the two black lines represent the combined estimates across the two groups of reference models. Black triangles mark the combined estimates across all reference models.

(3) since 2000, the fishing mortality on juvenile bigeye (age 1-8 quarters) has increased while that on adult bigeye (age 13-39 quarters) has decreased (Figure D-6).


Figure D-6: Comparison of average annual fishing mortality, by age groups, of bigeye tuna in the eastern Pacific Ocean (only the estimates that correspond to steepness = 1.0 are shown). The black dots show the combined values across all models with a steepness of 1.0.
Assessment Indicator
Type: Fishing mortality

Regarding management quantities, the 44 reference models estimate that (1) at the beginning of 2020, the spawning biomass of bigeye ranged from 14% to 212% of the level at dynamic MSY; 26 models suggest that it was below that level; (2) at the beginning of 2020, the spawning biomass of bigeye ranged from 51% to 532% of the limit reference level; five models suggest that it was below that limit; (3) during 2017-2019, the fishing mortality of bigeye ranged from 51% to 223% of the level at MSY; 26 runs suggest that it was above that level; (4) during 2017-2019, the fishing mortality of bigeye ranged from 32% to 114% of the limit reference level; three models suggest that it was above that limit (Figure D-8).
Figure D-8: Kobe plot of the most recent estimates of spawning biomass (S) and fishing mortality (F) relative to their MSY reference points (SMSY_d and FMSY) estimated by the 44 converged reference model runs (see Table 4). Each dot is based on the average F over the most recent three years. The dashed lines represent the limit reference points averaged for the 44 converged reference model runs. The error bars represent the 95% confidence interval of the estimates. The black, purple, and green dots are the combined estimates across all models, all pessimistic models, and all optimistic models, respectively.

The results from the 44 reference models are combined in a risk analysis framework to provide management advice (SAC-11-08). The combined risk curves show that (1) the probabilities of fishing mortality during 2017-2019 (F(cur)) being higher than the target and limit reference levels are 50% and 5%, respectively; (2) the probabilities of spawning biomass at the beginning of 2020 (S(cur)) being lower than the target and limit reference levels are 53% and 6%, respectively as shown in (Figure D-9).
Figure D-9: Risk curves showing the probability of exceeding the target (blue) and limit (red) reference points for different durations of the temporal closure.
Assessment Indicator
Type: Abundance

The longline index of abundance represents adult population trend and is one of the key inputs to the stock assessment model for bigeye. It suggests a long-term decreasing trend in adult population abundance since 2000. However, the average length for the longline fishery remained relatively stable since 2000 (Figure D-3).
Figure D-3: Index of abundance and average length of bigeye in the EPO, based on Japanese longline data for 1975-2019. The dashed horizontal lines are the 5th and 95th percentiles, the solid horizontal line is the median.


Assessment Indicator
Type: Average length


Projection

Although the combined distribution suggests that the probability of (F(cur)) being higher than the limit reference level is much lower than 10%, the combined probability distribution is bimodal (Figure D-10).
Figure D-10: Combined bigeye probability density function for Fcur/FMSY, Fcur/FLIMIT, Scur/SMSY, and Scur/SLIMIT.

This bimodal pattern for bigeye is due to the substantial differences in estimates between two groups of models, one more pessimistic and one more optimistic (Figure D-5) and (Figure D-11)
Figure D-10: Combined bigeye probability density function for Fcur/FMSY, Fcur/FLIMIT, Scur/SMSY, and Scur/SLIMIT


Figure D-11:

Description: Risk curves showing the probability of exceeding the target (top) and limit (bottom) reference points for bigeye with different durations of the temporal closure, combined by pessimistic and optimistic models resulting from the bimodal combined distribution.

and, ( Table D-2).



It should be noted that the combined risk curve based only on pessimistic models shows that the probability of (F(cur)) being higher than the limit reference level reaches 10% the level beyond which additional management measures shall be established (Resolution C-16-02). The bimodality complicates the evaluation of the status of the bigeye stock and of the potential outcomes of management actions. This issue needs to be addressed in the future to improve management advice as shown in (Figure D-11)
Figure D-11:

Description: Risk curves showing the probability of exceeding the target (top) and limit (bottom) reference points for bigeye with different durations of the temporal closure, combined by pessimistic and optimistic models resulting from the bimodal combined distribution.

and, (Table D-3).
Management
Management unit: Yes
Source of information
 
Inter-American Tropical Tuna Commission (IATTC).  “"Report on tuna fishery, stocks, and ecosystem in the Eastern Pacific Ocean in 2019. Inter-American Tropical Tuna Commission." Fishery Status Report. IATTC 2020.”.
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