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Bigeye tuna - Eastern Pacific (EPO)
Marine Resource  Fact Sheet
Stock status report 2019
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: 2018
 
 
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 1989-2018 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 113 thousand t in 1996; they jumped to 158 thousand t in 1997, and reached a high of 180 thousand t in 2004, since when they have fluctuated between 132 and 156 thousand t. 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.

The annual retained catches of bigeye in the EPO by purse-seine and pole-and-line vessels during 1989-2018 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 1997-2017 it has fluctuated between 44 and 95 thousand t; the preliminary estimate for 2018 is 65 thousand t. (Table A-2a).

During 2000-2017 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 1.8%.

Before the expansion of the FAD fishery, longliners caught almost all the bigeye in the EPO, averaging 86 thousand t annually during 1985-1992. Since then this has dropped to 36%, with a low of 25% in 2008 (average: 37 thousand t; range: 26 to 60 thousand t). The preliminary estimate of the longline catch in the EPO in 2018 is 21 thousand t as shown in 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, 1975-2017 The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2017 catch data are preliminary.
Assessment
 
Assessment Model
Type:  Age-structured
An integrated statistical age-structured stock assessment model, Stock Synthesis

Various uncertainties were identified in the update assessment of bigeye tuna conducted in 2018 (SAC-09-05, SAC-09 INF-B), and its usefulness for management has been questioned. Therefore, the staff developed stock status indicators (SSIs) for bigeye, similar to those used for skipjack tuna (SAC-09-07), as an alternative basis for management advice and to monitor the stock and the fishery in the future until the uncertainties in the stock assessment are resolved (see work plan to improve the assessments of tropical tunas). The indicators are based on relative quantities; i.e., instead of comparing a value with a reference point based on the maximum sustainable yield (MSY) of a species, it is compared with the distribution of its historical values. For the full version of this analysis, see document SAC-10-06.

The staff also developed two indicators for bigeye in the EPO based on longline data: (a) abundance indices, standardized with a generalized linear model (SAR-7-07), and (b) average length of the fish in the catch.

Some analyses suggested that the method used to calculate the number of days fished on floating objects is biased towards an increasing trend in days fished, which would also bias the CPDF. The number of days fished by set type is not easy to determine, since a vessel can make several set types in the same day, and has to be estimated using the multiple regression method. This method indicates a close correlation between the number of days fished assigned to the floating-object fishery and the number of floating-object sets as shown in Figure D-2.
Figure D-2: Stock status indicators for bigeye tuna in the EPO, based on purse-seine data, 2000-2018. The dashed horizontal lines are the 5th and 95th percentiles, the solid horizontal line is the median. CPDF: catch per day fishing; OBJ: sets on floating objects.


Data

The six purse-seine indicators are based on data for all purse-seine vessels that fished during 2000-2018, in order to exclude the period prior to the mid-1990s when purse-seine catches of bigeye were negligible (Figure D-1).
Figure D-1: 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, 1975-2018. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. 2018 data are preliminary.

The distributions of the historical values of these indicators are somewhat skewed; therefore, in order to estimate the current value of each indicator relative to its historical values, the 5th and 95th percentiles are used as reference levels.


Data

Initially, the total purse-seine catch of bigeye fell from its high level in 2000, resulting from favorable environmental conditions, increased during 2002-2006, and has fallen since then, except for an increase to its average level in 2018. The catch per day fished (CPDF) of bigeye in floating-object sets generally fell during 2000-2018, reaching the lower reference level in 2018.

The capacity of the purse-seine fleet, adjusted for the closures, has fluctuated since 2000, but has increased in recent years, and is now at its upper reference level. Both the number of floating-object sets and the number of days fished in such sets generally increased during the whole period, and in 2018 were above the upper reference level, while the average weight of the bigeye in the catch has been generally decreasing, and has been at the lower reference level since 2015.

The increasing number of floating-object sets, particularly on fish-aggregating devices (FADs), and the decreasing average weight of the bigeye in the catch continue to indicate that the bigeye stock in the EPO is under increasing fishing pressure, and that measures additional to the current seasonal closures, such as limits on the number of floating-object sets, are necessary.

To investigate this relationship, data for vessels that made more than 50% of their sets on floating objects during 2000-2018 were analyzed. In this data set, the total number of sets and the number of floating-object sets showed a similar increasing trend over time as shown in Figure D-5.


Figure D-5: Quantities used to investigate the relationship between days fished and the number of floating-object (OBJ) sets, 2000-2018, based on data from purse-seine vessels that made more than 50% of their sets on floating objects.







The number of days fished and the number of vessels also increased over time, but less rapidly than the number of sets. Since the early 2000s, bigeye catch-per-set has decreased, but the CPDF has remained fairly stable. The number of days fished per vessel has declined, while the number of floating-object sets per vessel has increased. This is manifested mainly in an increase in the number of days fished with one or more sets rather than in the number of sets conducted in a single day, as shown in Figure D-6.
Figure D-6: Various quantities used to investigate the reason for the increasing number of sets. The proportion of days with a set was calculated as the annual average, for all vessels, of the proportion of days fished with one or more floating-object sets.

The reasons for the increase in the number of floating-object sets, per day and per vessel, are not clear, but it is probably due to the vessels’ increased efficiency in finding FADs with tuna, thanks to the greater number of FADs and the increased use of satellite-linked fish-detecting sonar buoys, and to the greater number of floating-object by vessels with DMLs (SAC-10 INF-D). This should be further investigated. The reason for the reduction in the catch per set is also not clear; it may be due to several factors, among them reduced abundance, more FADs that distribute the stock into smaller schools, or a change in fishing strategy (e.g. vessels choosing to fish on smaller schools).


Results
Assessment Indicator
Type: Recruitment
Assessment Indicator
Type: Fishing mortality
Assessment Indicator
Type: Abundance

All purse-seine SSIs, except catch, show strong trends over time, and in 2018 were at, or near, the respective reference levels, indicating high rates of exploitation, increased fishing mortality and reduced abundance of juveniles as shown in Figure D-2.
Figure D-2 Stock status indicators for bigeye tuna in the EPO, based on purse-seine data, 2000-2018. The dashed horizontal lines are the 5th and 95th percentiles, the solid horizontal line is the median. CPDF: catch per day fishing; OBJ: sets on floating objects.

The abundance indices for the LL-C and LL-S fisheries from the last update assessment of bigeye (SAC-09-05) were updated with data for the last quarter of 2017 and the first three quarters of 2018 (Figure D-3).


Figure D-3 Indices of abundance for bigeye tuna in the central (LL-C) and southern (LL-S) longline fisheries, 1975-2018. The red dots represent updated values for the first three quarters of 2018 and, for the LL-S fishery, also the last quarter of 2017. The solid horizontal line is the median, and the two dashed horizontal lines are the 5th and 95th percentiles.

The resulting indices for both fisheries, but especially the LL-S fishery, are much lower than in 2017, suggesting that the abundance of adults in the EPO continues to decline. In particular, one of the three updated indices for the LL-C fishery is below that lower reference level, and all four updated indices for the LL-S fishery LL-S are near, or below, the lower reference level.

It is important to note that longline indices of abundance for recent years are highly uncertain, due mainly to the decrease in both the fishing effort and spatial coverage of the Japanese longline fleet.


Assessment Indicator
Type: Average length

For the second longline indicator, in all four longline fisheries, the time series of average length of fish in the catch do not show an apparent long-term trend, and the most recent values are within the reference limits as shown in Figure D-4.
Figure D-4 Mean length of bigeye tuna caught in the four EPO longline fisheries, 1975-2018. The solid horizontal line is the median and the two dashed horizontal lines are the 5th and 95th percentiles.


Projection




Management
Management unit: Yes
Source of information
 
Inter-American Tropical Tuna Commission (IATTC).  “"Tuna fishery, stocks, and ecosystem in the eastern Pacific Ocean in 2018. Inter-American Tropical Tuna Commission." Fishery Status Report. IATTC 2019.”.
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