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Skipjack tuna - Eastern Pacific
Marine Resource  Fact Sheet
Stock status report 2019
Skipjack tuna - Eastern Pacific
Fact Sheet Citation  
Owned byInter-American Tropical Tuna Commission (IATTC) – More
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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: 2018
 
 
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 1988-2017 are shown in (Table A-1). 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 2018, ranging from 153 to 343 thousand t, the record catch in 2016.

The annual retained catches of skipjack in the EPO by purse-seine and pole-and-line vessels during 1989-2018 are shown in (Table A-2a). During 2003-2017 the annual retained purse-seine and pole-and-line catch averaged 266 thousand t (range: 147 to 338 thousand t). The preliminary estimate of the retained catch in 2018, 287 thousand t, is 8% greater than the average for 2003-2017, but 15% less than the record catch of 2016.

Discards of skipjack at sea decreased each year during the period, from 8% in 2004 to a low of less than 1% in 2017, averaging about 3% 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- 2018. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2018 catch data are preliminary.
Figure A-2a: Average annual distributions of the purse-seine catches of skipjack, by set type, 2013-2017. 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, 2018. 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

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.

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.

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- 2018. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2018 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

We update their results to include data up to 2018. To evaluate the current values of the indicators in comparison to historical values, we use reference levels based on the 5th and 95th percentiles, as the distributions of the indicators are somewhat asymmetric. Indicators of number of sets and catch-per-set are also presented. Additional relevant indicators are also presented in SAC-10-06.

Eight data- and model-based indicators are shown in Figure C-2.



The standardized effort, which is a measure of exploitation rate, is calculated as the sum of the effort, in days fished, for the floating-object (OBJ) and unassociated (NOA) fisheries. The floating-object effort is standardized to be equivalent to the unassociated effort by multiplying by the ratio of the average floating-object CPUE to the average unassociated CPUE.

The purse-seine catch started increasing substantially in the mid-1990s, and has been above average since 2003; during 2015-2017 it was above the upper reference level, but fell below it in 2018. The floating-object CPUE has generally been above average since the early 1990s, and was above the upper reference level in 2016. The unassociated CPUE has been increasing since the early 2000s; it has been above average since about 2003, and was above the upper reference level in 2017, but fell below it in 2018.

The standardized effort indicator of exploitation rate increased starting in the early 1990s, and has been above the average level since about 2000. The average weight of skipjack has been declining since 2000, and in 2015 and 2016 was below the lower reference level, but increased slightly to above that level in 2017, then fell back to the reference level in 2018. Both biomass and recruitment have been increasing over the past 20 years, and were above their respective upper reference levels in 2015 and 2016.

The exploitation rate started increasing in the mid-1980s, and has fluctuated around the average since the mid-1990s.

The number of sets by both large and small purse-seine vessels in the floating-object fishery has increased consistently for at least the past 15 years, and at the same time the catch per set has fallen shown in Figure C-3.
Figure C-3: Number of floating-object sets, by vessel carrying capacity and total (top panel), and catch per set in the floating-object fishery (bottom panel).

The number of days fished has not increased at the same rate, and the increased number of sets is therefore likely the cause of the increased catch and catch per day fished (CPDF). The CPDF is used to create the model-based indicators and therefore the estimated increases in recruitment and abundance are probably an artifact caused by the increased number of sets. The data- and model-based indicators have yet to detect any adverse impacts of the fishery. However, the model-based indicators are probably biased and should not be considered reliable.

The average weight was at or below its lower reference level during 2015-2017, which can be a consequence of overexploitation, but can also be caused by recent recruitments being greater than past recruitments or expansion of the fishery into areas occupied by smaller skipjack. The average length is less in the western part of the EPO, but it has been declining in all areas as shown in Figure C-4.

The long-term pattern in reduced average weight is probably due to increasing fishing mortality resulting from the increasing number of sets. However, it is unknown if the current fishing mortality levels are appropriate because there are no reference points for skipjack tuna in the EPO; however, any continued decline in average length is a concern

Productivity and susceptibility analysis (PSA; see IATTC Fishery Status Report 12, Figure L-4) shows that skipjack has substantially higher productivity than bigeye. Biomass (B) and the fishing mortality that corresponds to MSY (FMSY) 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 not available, no inferences can be made at this stage about the status of skipjack. A conventional assessment of skipjack is necessary to ascertain the status of the stock, but, as noted above, this is not possible without much more extensive tagging data. The large-scale tagging program (Project E.4.a) that commenced in 2019 is therefore critical.


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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