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Skipjack tuna - Eastern Pacific
Marine Resource  Fact Sheet
Stock status report 2020
Skipjack tuna - Eastern Pacific
Fact Sheet Citation  
Owned byInter-American Tropical Tuna Commission (IATTC) – More
Related observationsLocate in inventorydisplay tree map
 
Species:
FAO Names: en - Skipjack tuna, fr - Listao, es - Listado, zh - 鲣, ru - Тунец полосатый (=скипджек)
Geographic extent of Skipjack tuna - Eastern Pacific
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 rateModerate fishing mortalityModerate fishing mortalityGreen
Abundance levelIntermediate abundanceIntermediate abundance
FAO Categories
Exploitation stateUncertain
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 skipjack during 1990-2019 are shown in (Table A-1). Most of the catch is taken in the WCPO. Most of the catch is taken in the WCPO. Prior to 1998, WCPO catches averaged about 900 thousand t; subsequently, they increased steadily, from 1.2 million t to an all-time high of 2 million t in 2014. In the EPO, the greatest catches occurred between 2003 and 2019, ranging from 153 to 350 thousand t, the record catch in 2019.

The annual retained catches of skipjack in the EPO by purse-seine and pole-and-line vessels during 1990-2019 are shown in (Table A-2a). During 2004-2018 the annual retained purse-seine and pole-and-line catch averaged 267 thousand t (range: 147 to 338 thousand t). The preliminary estimate of the retained catch in 2019, 347 thousand t, is 30% greater than the 15-year average for 2004-2018.

Discards of skipjack at sea decreased each year during the period, from 8% in 2004 to a low of less than 1% in 2018, averaging about 2% of the total catch of the species.

Catches of skipjack in the EPO by longlines and other gears are negligible as shown in Table A-2a.



See also fishery fact sheet:EPO Tunas and billfishes fishery
Figure C-1: Total catches (retained catches plus discards) for the purse-seine fisheries, by set type (NOA, OBJ) and retained catches for the other (OTR) fisheries, of skipjack tuna in the eastern Pacific Ocean, 1975- 2019. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2019 catch data are preliminary.
Figure A-2a: Average annual distributions of the purse-seine catches of skipjack, by set type, 2014-2018. The sizes of the circles are proportional to the amounts of skipjack caught in those 5° by 5° areas.
Figure A-2b: Annual distributions of the purse-seine catches of skipjack, by set type, 2019. The sizes of the circles are proportional to the amounts of skipjack caught in those 5° by 5° areas.
Assessment
 
Assessment Model
Type:  Others
Fisheries Indicators

A major management objective for tunas in the eastern Pacific Ocean (EPO) is to keep stocks at levels capable of producing maximum sustainable yields (MSYs). Management objectives based on MSY or related reference points (e.g. fishing mortality that produces MSY (FMSY); spawner-per-recruit proxies) are in use for many species and stocks worldwide. However, these objectives require that reference points and quantities to which they are compared be available. The various reference points require different amounts and types of information, ranging from biological information (e.g. natural mortality, growth, and stock-recruitment relationship) and fisheries characteristics (e.g. age-specific selectivity), to absolute estimates of biomass and exploitation rates. These absolute estimates generally require a formal stock assessment model. For many species, the information required to estimate these quantities is not available, and alternative approaches are needed.

Skipjack tuna is a notoriously difficult species to assess. Due to its high and variable productivity (i.e. annual recruitment is a large proportion of total biomass), it is difficult to detect the effect of fishing on the population with standard fisheries data and stock assessment methods. This is particularly true for the stock of the EPO, due to the lack of age-composition data, and especially tagging data, without which a conventional stock assessment of skipjack is not possible.

The continuous recruitment and rapid growth of skipjack mean that the temporal stratification needed to observe modes in length-frequency data make the current sample sizes inadequate.

Previous assessments have had difficulty in estimating the absolute levels of biomass and exploitation rates, due to the possibility of a dome-shaped selectivity curve, which would mean that there is a cryptic biomass of large skipjack that cannot be estimated.

The most recent assessment of skipjack in the EPO is considered preliminary because it is not known whether the catch per day fished for purse-seine fisheries is proportional to abundance.

Analysis of currently available tagging data is unlikely to improve the skipjack stock assessment and a fully length-structured model produced unrealistic estimates. In addition to the problems listed above, the levels of age-specific natural mortality are uncertain, if not unknown, and current yield-per-recruit (YPR) calculations indicate that the YPR would be maximized by catching the youngest skipjack in the model. Therefore, neither the biomass- nor fishing mortality-based reference points, nor the indicators to which they are compared, are available for skipjack in the EPO.

Further analysis of currently available tagging data is unlikely to improve the skipjack stock assessment and a fully length-structured model produced unrealistic estimates. In addition to the problems listed above, the levels of age-specific natural mortality are uncertain, if not unknown, and current yield-per-recruit (YPR) calculations indicate that the YPR would be maximized by catching the youngest skipjack in the model. Therefore, neither the biomass- nor fishing mortality-based reference points, nor the indicators to which they are compared, are available for skipjack in the EPO.

Since the stock assessments and reference points for skipjack in the EPO are so uncertain, developing alternative methods to assess and manage the species that are robust to these uncertainties would be beneficial. Full management strategy evaluation (MSE) for skipjack would be the most comprehensive method to develop and test alternative assessment methods and management strategies; however, developing MSE is time-consuming, and has not yet been conducted for skipjack. In addition, higher priority for MSE is given to yellowfin and bigeye tuna, as available data indicate that these species are more susceptible to overfishing than skipjack. Therefore, Maunder and Deriso (2007) investigated some simple indicators of stock status based on relative quantities. Rather than using reference points based on MSY, they compared current values of indicators to the distribution of indicators observed historically. They also developed a simple stock assessment model to generate indicators for biomass, recruitment, and exploitation rate. However, this year catch-per-set by set type replaces the catch-per-day-fished Stock Status Indicators (SSIs) used previously, which are considered unreliable due to possible biases in the method used to assign days fished to set types; also, the model-based indicators used for skipjack are no longer reported because they were based on the same CPDF data.

This analysis was originally presented in document SAC-10-09


Data

One of the major problems mentioned above is the uncertainty as to whether the catch per unit of effort (CPUE) of the purse-seine fisheries is an appropriate index of abundance for skipjack, particularly when the fish are associated with fish-aggregating devices (FADs). Purse-seine CPUE data are particularly problematic, because it is difficult to identify the appropriate unit of effort. In the current analysis, effort is defined as the amount of searching time required to find a school of fish on which to set the purse seine, and this is approximated by number of days fished.

Few skipjack are caught in the longline fisheries or dolphin-associated purse-seine fisheries, so these fisheries cannot be used to develop reliable indices of abundance for skipjack. (Figure C-1)
Figure C-1: Total catches (retained catches plus discards) for the purse-seine fisheries, by set type (NOA, OBJ) and retained catches for the other (OTR) fisheries, of skipjack tuna in the eastern Pacific Ocean, 1975- 2019. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2019 catch data are preliminary.

Within a single trip, purse-seine sets on unassociated schools are generally intermingled with floating-object or dolphin-associated sets, complicating the CPUE calculations. Maunder and Hoyle (2007) developed a novel method to generate an index of abundance, using data from the floating-object fisheries. This method used the ratio of skipjack to bigeye in the catch and the “known” abundance of bigeye based on stock assessment results. Unfortunately, the method was of limited usefulness, and more research is needed to improve it. Currently, there is no reliable index of relative abundance for skipjack in the EPO. Therefore, other indicators of stock status, such as the average weight of the fish in the catch, should be investigated.
Results
Assessment Indicator
Type: Others

The current SSIs begin in 2000 because the IATTC port-sampling program began the species composition sampling in that year, and it is after the major offshore expansion of the floating-object fishery which started in the mid-1990s. All SSIs are scaled (relative indicators) so that their average equals 1 during the 2000-2019 period. The reference levels were changed from the 5% and 95% percentiles to the 10% and 90% percentiles because extreme percentiles are less reliable with fewer years of data.

Many of the indicator values for recent years are near their reference levels (Figures B-2 and C-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.
Figure C-2: Indicators of stock status for skipjack tuna in the eastern Pacific Ocean. OBJ: floating-object fishery; NOA: unassociated fishery; DEL: dolphin associated fishery. All indicators are scaled so that their average equals one.



Most floating-object fishery SSIs suggest that the skipjack has potentially been subject to increased fishing mortality, mainly due to the increase in the number of sets in the floating-object fishery. Of particular concern is the constantly increasing trend in the number of floating object sets observed since 2005 as shown in Figure B-2.

This is reflected as an increase in catch for skipjack in floating-object sets and a decline in catch-per-set and in average length of the fish in the catch for the floating-object fishery. The interpretation of increased fishing mortality is supported by trends in average length of skipjack caught in the other set types. On the other hand, trends in catch-per-set for unassociated sets, are not consistent with this interpretation as shown in Figure C-2.

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.

Productivity and susceptibility analysis, PSA as shown in IATTC Fishery Status Report 12 and in Figure L-4 shows that skipjack has substantially higher productivity than bigeye (Figure L-4).
Figure L-4: IATTC Fishery Status Report 12. Productivity and susceptibility x-y plot for target and bycatch species caught by the purse-seine fishery of the EPO during 2005-2011.

Biomass (B) and the fishing mortality that corresponds to MSY (F(MSY)) are, respectively, negatively and positively correlated with productivity. Therefore, since skipjack and bigeye have about the same susceptibility, and susceptibility is related to fishing mortality, the status of skipjack can be inferred from the status of bigeye, but only if the fishing mortality of bigeye is below the MSY level (i.e., F<FMSY). Since an assessment of bigeye is available, inferences can be made about the status of skipjack. Productivity and Susceptibility Analysis (PSA; Duffy et al. 2019) for the tropical tuna fishery in the EPO indicated that skipjack and bigeye have about the same susceptibility to purse-seine fishing gear, and that skipjack is much more productive than bigeye. Taking the risk analysis results for bigeye as reference (SAC-11-08), the staff infers the following about the skipjack stock status in the EPO as shown in (Table A)
Figure Table A Estimated catches, in metric tons, of tunas and bonitos in the EPO, by fishing gear, 1990-2019. For purse-seine (PS) vessels, retained (Ret.) catches include all vessels; discard (Dis.) data are for Class-6 vessels only. ‘C’ indicates that the catch has been combined with the total in the ‘OTR’ column. The purse-seine and pole- and-line (LP) data for yellowfin, skipjack, and bigeye tunas have been adjusted to the species composition estimate, and are preliminary. The data for 2018-2019 are preliminary.



1.There is less than 50% probability that F(MSY) has been exceeded (P(F>F(MSY))<50%), and a less than 53% probability that S(cur) is below S(MSY) (P(S<S(MSY))<53%),



2. There is less than 5% probability that FLIMIT has been exceeded (P(F>F(LIMIT))<5%), and less than 6% probability that S(LIMIT) has been breached (P(S>S(LIMIT))<6%).

These inferences about skipjack stock status from the PSA analysis are interim: direct advice from a skipjack assessment is still needed. The staff is currently conducting a multi-year tagging study of tropical tunas in the EPO aimed at obtaining data that will contribute to, and reduce uncertainty in, tuna stock assessments, particularly for skipjack (Project E.4.a). In addition, an MSE process for tropical tunas, which includes skipjack, is ongoing at IATTC.
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.” Click to openIATTC-95-05_The fishery and status of the stocks 2019
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