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Yellowfin tuna - Eastern Pacific
Fact Sheet Title  Fact Sheet
Stock status report 2022
Yellowfin tuna - Eastern Pacific
Fact Sheet Citation  
Yellow fin, Eastern Pacific Ocean (EPO)
Owned byInter-American Tropical Tuna Commission (IATTC) – ownership
ident Blockident Blockdisplay tree map
 
Species List:
Species Ref: en - Yellowfin tuna, fr - Albacore, es - Rabil, ru - Тунец желтоперый
ident Block Yellowfin tuna - Eastern Pacific
Aq Res
Biological Stock: Yes         Value: Regional
Reference year: 2021
 
 
Aq Res State Trend
Aq Res State Trend
Aq Res State Trend Aq Res State Trend
Aq Res State TrendModerate fishing mortality, close to that corresponding to MSYModerate fishing mortalityGreen
Aq Res State TrendIntermediate abundanceIntermediate abundance
Aq Res State Trend
Aq Res State TrendModerately 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: Regional

Water Area Overview
Aq Res Struct
Biological Stock: Yes
Exploit
 

The total annual catches of yellowfin in the Pacific Ocean during 1992-2021 are shown in Table A-1.

The 2021 EPO catch of 254 thousand t is 4% higher than the average of 244 thousand t for the previous 5-year period (2016-2020). In the WCPO, the catches of yellowfin reached a record high of 722 thousand t in 2020.

The annual retained catches of yellowfin in the EPO, by gear, during 1992-2021 are shown in Table A-2a. Over the most recent 15-year period (2006-2020), the annual retained purse-seine and pole-and-line catches have fluctuated around an average of 217 thousand t (range: 167 to 251 thousand t). The preliminary estimate of the retained catch in 2021, 253 thousand t, is 16% higher than that of 2020, and 11% higher than the 2006-2020 average. On average, about 0.3% (range: 0.1 to 1.0%) of the total purse-seine catch of yellowfin was discarded at sea during 2006-2020 (Table A-2a). During 1991-2005, annual longline catches in the EPO averaged about 22 thousand t (range: 12 to 31 thousand t), or about 7% of the total retained catches of yellowfin. They then declined sharply, to an annual average of 10 thousand t (range: 8 to 13 thousand t), or about 4% of the total retained catches, during 2006-2020. Catches by other fisheries (recreational, gillnet, troll, artisanal, etc.), whether incidental or targeted, are shown in Table A-2a, under “Other gears” (OTR); during 2006-2020 they averaged about 3 thousand t.

See also fishery fact sheet: EPO Tunas and billfishes fishery
Figure B-1: Total catches (retained catches plus discards) for the purse-seine fisheries, by set type (DEL, NOA, OBJ), and retained catches for the longline (LL) and other (OTR) fisheries, of yellowfin tuna in the eastern Pacific Ocean, 1975-2021. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2020 and 2021 data are preliminary.
Bio Assess
 
Assess Models
Type:  Age-structured
Stock Synthesis

A workplan to improve the stock assessments for tropical tunas was completed taking into consideration the results of the external review of yellowfin. The yellowfin review panel did not single out a particular model configuration as a replacement for the current base case model but suggested a variety of alternatives for the staff to consider. To encompass as many scenarios as possible, the staff developed a pragmatic risk assessment framework to apply for both species, which included the development of hypothesis, the implementation and weighting of models, and the construction of risk tables based on the combined result (SAC-11-08, SAC-11-INF-F,SAC-11-INF-J).

Three overarching hypotheses related to the degree of spatial mixing of the yellowfin tuna stock in the EPO where developed (SAC-11-INF-J). Of those, the high-mixing hypothesis was assumed for the benchmark assessment, with the purse-seine index assumed the most representative of the core of the exploited population (SAC-11-07). A series of lower hierarchical level hypotheses were developed regarding other major uncertainties in the previous assessment. From those, 12 reference models were developed, which combine components that address changes in selectivity and catchability, growth, asymptotic selectivity, and density-dependence in the index catchability (Table B-1).
Table B-1: Model configurations (hypotheses) used for yellowfin tuna in the EPO (from SAC-11-08 Table A)

Each reference model was run with four steepness of the stock-recruitment relationship assumptions (0.7, 0.8, 0.9, and 1.0). A total of 48 models composed the benchmark assessment for yellowfin tuna (SAC-11-07). In addition, new fishery definitions were implemented, and spline selectivity functions were adopted for most fisheries. All EPO catches were added to the models, which were fit to a standardized purse-seine index of abundance for the EPO north of 5°N and to the length-composition data from the purse-seine fisheries that operate north of 5°N, in order to avoid contamination of the signal with that of a possible southern population. The models were diagnosed for model misspecification, lack of fit, retrospective bias, among others (SAC-11-07). Rather than choosing a base-case model, all models where used to produce management advice by combining them using relative weights determined based on several criteria, including performance on model diagnostics (SAC-11-INF-J).
Assumption

Yellowfin are distributed across the Pacific Ocean, but the bulk of the catch is made in the eastern and western regions. Purse-seine catches in the vicinity of the western boundary of the EPO at 150oW are relatively low, but have been increasing, mainly in sets on floating objects (Table A-1), (Table A-2a); (Figure A-1a and A-1b)


Figure A-1a: Average annual distributions of the purse-seine catches of yellowfin, by set type, 2016-2020. The sizes of the circles are proportional to the amounts of yellowfin caught in those 5° by 5° areas.
Figure A-1b: Annual distributions of the purse-seine catches of yellowfin, by set type, 2021. The sizes of the circles are proportional to the amounts of yellowfin caught in those 5° by 5° areas.

Most of the catch in the eastern Pacific Ocean (EPO) is taken in purse-seine sets associated with dolphins and floating objects (Figure B-1).
Figure B-1: Total catches (retained catches plus discards) for the purse-seine fisheries, by set type (DEL, NOA, OBJ), and retained catches for the longline (LL) and other (OTR) fisheries, of yellowfin tuna in the eastern Pacific Ocean, 1975-2021. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2020 and 2021 data are preliminary.

Tagging studies of yellowfin throughout the Pacific indicate that they tend to stay within 1,800 km of their release positions. This regional fidelity, along with the geographic variation in phenotypic and genotypic characteristics of yellowfin shown in some studies, suggests that there might be multiple stocks of yellowfin in the EPO and throughout the Pacific Ocean. However, movement rates between these putative stocks, as well as across the 150°W meridian, cannot be estimated with currently-available tagging data.

In 2022, stock status indicators (SSIs) were developed for yellowfin using the data collected in the EPO as a whole (SAC-13-06). In the short term, the indicators show the effect of the COVID-19 pandemic. The total number of floating-object sets decreased 21.5% from 2019 to 2020. The closure-adjusted capacity slightly decreased as well. In 2021, both effort measures increased, and the current conditions are at the average 2017-2019 conditions (status quo). There was apparent decrease in the estimated floating object catches, followed by an increase in 2021. The port sampling for species composition was greatly affected as well, which caused a bias in the catch estimation (SAC-13-05). The 2020 yellowfin tuna estimated catches may be as much as 18% larger than what is shown in Figure B-1, while the 2021 may be 10% smaller, because some yellowfin tuna catches were most likely attributed to bigeye tuna, and vice-versa in 2020 and 2021(Figure B-3), respectively, due to lack of sampling on key ports as a result of the impact of the COVID-19 pandemic. The effort for the purse-seine fishery associated with dolphins and the unassociated fishery, has been stable since 2018. The longline effort was about the same in 2020 as 2019 (Figure B-2),


Figure B-2: Indicators of total effort in the EPO, based on purse-seine data closure-adjusted capacity, 2000-2021; annual total number of sets, by type, 1987-2021) and based on longline data for 2000-2020 (effort reported by all fleets, in total numbers of hooks; proportion of the effort corresponding to Japan). The dashed horizontal lines are the 10th and 90th percentiles, the solid horizontal line is the mean. The red dashed lines mark the status quo levels (average conditions in 2017-2019).

yet the relative CPUE and the average length increased (Figure B-3).
Figure B-3: Indicators (catch (t and numbers); CPUE (t/day fished); average length (cm)) for the yellowfin tuna stock in the eastern Pacific Ocean, from purse-seine fisheries; relative catch and relative average length, obtained from standardized length composition using spatiotemporal model, from longline fisheries. The lines represent the 10% and 90% percentiles (dashed lines) and the mean (solid line). The red dots are the bias-adjusted estimates for floating-object catches in the two COVID-19 years (see SAC-13-05). The red dashed lines mark the status quo reference levels (average conditions in 2017-2019).

The average length also increased in 2021 for the purse-seine fishery associated with dolphins, which also showed a slight increase in relative CPUE. In the long term, most floating-object fishery SSIs suggest that the yellowfin stock has potentially been subject to increased fishing mortality, mainly due to the increase in the number of sets in the floating-object fishery since 2005 (Figure B-2) and corresponding increase in catch for yellowfin (Figure B-3), associated with decline in catch-per-set (Figure B-3) and reduction in the average length of the fish in the catch (Figure B-3) for the floating-object fishery. This coincided with a declining trend in the yellowfin longline CPUE index based on spatiotemporal modelling since 2005, which was at the lowest historic levels in 2017–2018 (Figure B-4).


Figure B-4: Top: Spatial domain of the purse-seine and longline derived indices. Middle: Relative abundance indices derived from catch per unit of effort of purse-seine (1985-2020) and longline (1995- 2020 3rd quarter) fisheries standardized using spatiotemporal models. Bottom: Relative average size.


Data

Long term trends in some of the other SSIs do not support the interpretation that increased fishing mortality is occurring because of an increase in the numbers of floating-object sets, such as trends in catch-per-set for other set types (Figure B-3), mean length of yellowfin in the other set types (Figure B-3), and the longline SSIs (Figure B-3). The SSI based on spatiotemporal modelling of CPDF for the purse-seine fishery associated with dolphins shows a period of low values starting in 2015 (Figure B-4) which coincides with a period of increased yellowfin catches in floating-objects set (Figure B-3). The SSI based on spatiotemporal modelling of CPUE for the longline fishery does not coincide with the purse-seine one (Figure B-4), although both indices refer to large fish, with longline fish being the largest. The inconsistencies among SSIs for yellowfin may be due to an interaction between potential stock structure (i.e. the longline index referring to a southern population and the purse-seine index to a northern) and differences in the spatial distribution of effort in the different gear/set types. In addition, catch-per-set may not be a reliable indicator of abundance, particularly for the target species (i.e. yellowfin in the dolphin-associated fishery). Nonetheless, the fact that most SSIs based on the floating-object fishery are consistent with an increase in fishing mortality in that fishery means that precautionary management measures should be considered to prevent further increases. Due to the COVID-19 effect, 2020 and 2021 should be regarded with caution when interpreting long term trends in the indicators.


Results
Assess Indicator
Type: Recruitment

The 48 models of the benchmark assessment estimate similar relative recruitment trends, regardless of the steepness assumed (Figure B-5).
Figure B-5: Annual relative recruitment of yellowfin tuna to the fisheries of the EPO estimated by the 48 models and weighted average (black dashed line). The lines and dots indicate the maximum likelihood estimates of recruitment, and the shaded areas the approximate 95% confidence intervals around the estimates. The estimates are scaled so that the average recruitment is equal to 1.0 (dashed horizontal line). See model descriptions in Table B-1. The weighted average is computed using the weights assigned to each model in SAC-11-INF-J.

All biomass trajectories have declining trends, but they vary in the magnitude of the declines (Figure B-6).
Figure B-6: Spawning biomass ratios (SBRs) for yellowfin tuna in the EPO, 1985-2019. The solid lines represent the maximum likelihood estimates and the shaded areas the approximate 95% confidence intervals around those estimates estimated by the 48 models and weighted average (black dashed line). The red dashed horizontal line (at 0.077) identifies the SBR at SLIMIT. See model descriptions in Table B-1 The weighted average was computed using the weights assigned to each model in SAC-11-INF-J.

All models indicate the highest F for fish aged 21+ quarters (5.25+ years), followed by fish aged 11-20 quarters (2.75-5 years) (Figure B-7).
Figure B-7: Average annual fishing mortality (F) of yellowfin tuna in the EPO, by age group (in quarters), for all gears, estimated by the 48 models and weighted average. See model descriptions in Table B-1. The weighted average was computed using the weights assigned to each model in SAC-11-INF-J.

All models estimate similar impacts of the different types of fisheries (Figure B-8).
Figure B-8: Impact of fishing, 1985-2019: trajectory of the spawning biomass (a fecundity index, see text for details) of a simulated population of yellowfin tuna that was never exploited (dashed line) and that predicted by each model, with a steepness of 1.0 (solid line). The shaded areas between the two lines show the portions of the impact attributed to each fishing method. See model descriptions in Table B-1.

The longline and the sorted discard fisheries have the smallest impact, while the purse-seine fisheries associated with dolphins have the greatest impact during most of the assessment period (1984-2019). In 1990s the impact of the floating-object fisheries started to be noteworthy, and surpassed that of the unassociated fisheries around 2008 and that of the purse-seine fisheries associated with dolphins in 2018. At the beginning of 2020, the spawning biomass (S) of yellowfin ranged from 49% to 219% of the level at dynamic MSY (SMSY_d); 12 models suggested that it was below that level (Figure B-9).
Table B-2: Management quantities for yellowfin tuna in the EPO for each reference model summarized over the four steepness values. See explanation of code in Table B-1 (from SAC-11-08 Table 1)


Figure B-9: Kobe (phase) plot of the time series of estimates of spawning stock size (S) and fishing mortality (F) of yellowfin tuna relative to their MSY reference points. The colored panels are separated by the target reference points (SMSY and FMSY). Limit reference points (dashed lines), which correspond to a 50% reduction in recruitment from its average unexploited level, based on a conservative steepness (h) of 0.75 for the Beverton-Holt stock-recruitment relationship, are merely indicative, since they vary by model and are based on all models combined. The center point for each model indicates the current stock status, based on the average fishing mortality (F) over the last three years; The solid black circle represents all models combined; to be consistent with the probabilistic nature of the risk analysis and the HCR, it is based on P(Scur/SLIMIT<x) = 0.5 and P(Fcur/FMSY>x) = 0.5. The lines around each estimate represent its approximate 95% confidence interval.

At the beginning of 2020, the spawning biomass (S) of yellowfin ranged from 145% to 345% of the limit reference level (SLIMIT); no models suggest that it was below that limit. During 2017-2019 the fishing mortality (F) of yellowfin ranged from 40% to 168% of the level at MSY (FMSY); 14 models suggested that it was above that level. During 2017-2019, the fishing mortality of yellowfin ranged from 22% to 65% of the limit reference level (FLIMIT); no models suggest that it was above that limit. Every reference model suggests that lower steepness values correspond to more pessimistic estimates of stock status: lower S and higher F relative to the reference points.


Results
Assess Indicator
Type: Others

The degree of spatial mixing of the yellowfin tuna population in the EPO was considered the main uncertainty within the risk analysis (SAC-11-INF-J). This conclusion came from the detailed inspection of the contradictory indices of abundance. Previous assessments used five indices of abundance, one from the longline fishery and four from the purse-seine fisheries, and the length composition data from longline and purse-seine fisheries. The model was unable to reconcile indices from different fisheries and length-frequency data that apparently carried contradictory signals about the status of the stock (SAC-10 INF-F). To solve the inconsistencies, spatiotemporal models were used to produce new purse-seine and longline indices and associated length frequencies, but the inconsistencies were not resolved (Figure B-4). The mismatch was most apparent in 2001-2003, when a peak occurred earlier in the longline index and later in the purse-seine index (opposite to what was expected given the growth and selectivity assumptions of the model). By incorporating length classes in the standardization, it became clear that the differences were due mainly to the 1998 cohort (coinciding with an important El Niño year) being prominent in the longline index, while not showing in in purse-seine index, and the opposite occurring with the 1999 cohort (coinciding with an equally important La Niña year) (see Figure B-4 average length). The mismatch between the purse-seine and the longline indices continues to be striking in the updated series (Figure B-4). Spatial heterogeneity was considered as the most plausible explanation for the unresolved inconsistencies.
Bio Assess
 
Results

The results from the reference models are combined in a risk analysis to provide management advice (SAC-11-08). The probabilities of exceeding the reference points where computed using each model result and its associated weight, the final estimates are in Table B-3 (Table B-3) and Figures B-9 and B-10.
Figure B-10: Yellowfin probability density functions for Fcur/FMSY, Fcur/FLIMIT and Scur/SLIMIT broken down into different components for models developed to address: a) combined; b) issues with the index of abundance; c) misfit to the composition data for the fishery with asymptotic selectivity; and d) different assumptions on steepness (h).

All probability distributions are unimodal (Figure B-10). There is a low probability of Fcur being above FMSY (9%). The probability of Fcur being above FLIMIT is zero. The probability of the spawning biomass being below SMSY_d is low (12%). The probability of the spawning biomass exceeding SLIMIT is zero. The combined expected risk of F exceeding FMSY is below 50% for six closure durations (Table B-3; Figure B-11),
Figure B-11: Risk curves showing the probability of exceeding the target and limit reference points (RPs) for different durations of the temporal closure for yellowfin in the EPO.

varying from 26% (no closure) to 5% (100 days), with a low risk (9%) for the current closure (72 days). One model (Base-A) produced a pessimistic result (a risk above 50% of exceeding FMSY for all scenarios (Table B-3), but this model has a very low relative weight (0.01).

A key uncertainty not addressed in the assessment is the spatial structure of the stock of yellowfin tuna in the EPO. Future work to further improve the assessment will focus on it.


Sources
 
Inter-American Tropical Tuna Commission (IATTC).  “Report on tuna fishery, stocks, and ecosystem in the Eastern Pacific Ocean in 2021. Inter-American Tropical Tuna Commission. Fishery Status Report. IATTC 2022.” Click to openhttps://www.iattc.org/GetAttachment/6aff9a86-590c-4f24-b13b-a929eb4065df/IATTC-100-01_The-tuna-fishery,-stocks,-and-ecosystem-in-the-Eastern-Pacific-Ocean-in-2021-(1).pdf
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