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Skipjack tuna - Eastern Pacific
Marine Resource  Fact Sheet
Stock status report 2016
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: 2015
 
 
Biological State and Trend
State & Trend Descriptors
PartnerFIRMS
Exploitation rateModerate fishing mortalityModerate fishing mortalityGreen
[?]
Abundance levelIntermediate abundanceIntermediate abundance
FAO
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 1986-2015 are shown in (Table A-1). Most of the skipjack catch in the Pacific Ocean is taken in the WCPO. Prior to 1999, WCPO skipjack catches averaged about 900 thousand t. Beginning in 1999, catches increased steadily from 1.1 million t to an all-time high of 2 million t in 2014. In the EPO, the greatest yearly catches occurred between 2003 and 2015, ranging from 153 to 333 thousand t, the record catch in 2015.

The annual retained catches of skipjack in the EPO by purse-seine and pole-and-line vessels during 1986-2015 are shown in (Table A-2a), (Table A-2a (cont.)), (Table A-2a (cont.)). During 2000-2014 the annual retained catch averaged 234 thousand t (range 144 to 297 thousand t). The preliminary estimate of the retained catch in 2015, 329 thousand t, is 41% greater than the average for 2000-2014, and 11% higher than the record-high retained catch of 2008. Discards of skipjack at sea decreased each year during the period, from 11% in 2000 to a low of less than 1% in 2014. During the period about 4% of the total catch of the species was discarded at sea (Table A-2a).

Small amounts of EPO skipjack are caught with longlines and other gears (Table A-2a).

See also fishery fact sheet:EPO Tunas and billfishes fishery
Average annual distributions of the purse-seine catches of skipjack, by set type, 2010-2014. The sizes of the circles are proportional to the amounts of skipjack caught in those 5° by 5° areas.
Annual distributions of the purse-seine catches of skipjack, by set type, 2015. 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 are distributed across the Pacific Ocean, and it is likely that there is a continuous stock throughout the Pacific Ocean, with exchange of individuals at a local level, although large-scale movements are thought to be rare. The bulk of the catches of skipjack are made in the eastern and western regions; the purse-seine catches are relatively low in the vicinity of the western boundary of the EPO at 150°W. The movements of tagged skipjack generally cover hundreds, rather than thousands, of kilometers, and exchange of fish between the eastern and western Pacific Ocean appears to be limited. Movement rates between the EPO and the western Pacific cannot be estimated with currently-available tagging data. In some analyses the EPO was divided into six independent sub-regions to accommodate spatial structure of the population and fishery dynamics.
Assumption

Stock assessment requires substantial amounts of information and the information varies depending on the method used. The methods applied to skipjack require a variety of information, including data on retained catches, discards, indices of abundance, the size compositions of the catches of the various fisheries, tagging data, and oceanographic data. In addition, assumptions have to be made about processes such as growth, recruitment, movement, natural mortality, selectivity, and stock structure.
Data

Biomass, recruitment, and fishing mortality are estimated to be highly variable over time. The estimates are uncertain and differ among the alternative assessment methods. A large recruitment appears to have entered the population in 1999, and led to increased biomass in that year, but the increase was temporary, due to the short-lived nature of skipjack. Biomass appears to have been above average in recent years, but this may differ among regions. SEAPODYM estimates annual biomass of skipjack 30cm or larger cycling between 1,800,000 t and 2,350,000 t from 1998 to 2008, but the quality of these estimates has yet to be determined. The average weight of skipjack started declining in 2000, but has stabilized in recent years (Figure C-1).
Figure C-1: Indicators of stock status for skipjack tuna in the eastern Pacific Ocean. OBJ: floating-object fishery; NOA: unassociated fishery; CPDF: catch per day fished. All indicators are scaled so that their average equals one.

Previous assessments using a catch-at-length analysis (A-SCALA) to assess skipjack tuna in the EPO were considered preliminary because: 1) it was unknown if catch-per-day-fished for purse-seine fisheries is proportional to abundance; 2) it is possible that there is a population of large skipjack that is invulnerable to the fisheries; and 3) the structure of the EPO stock in relation to the western and central Pacific stocks is uncertain. These issues are also relevant to the other assessments.

Previous assessments estimated that maximum yields are achieved with infinite fishing mortality because the critical weight is less than the average weight at recruitment to the fishery. However, this is uncertain because of uncertainties in the estimates of natural mortality and growth. For this reason, no traditional reference points are available for skipjack tuna in the EPO. Consequently, indicators and reference levels have been used to evaluate the status of the stock.
Results
Assessment Indicator
Type: Others

Susceptibility and productivity analysis (PSA; see IATTC Fishery Status Report 12, p 149) shows that skipjack has substantially higher productivity than bigeye tuna. Biomass and fishing mortality corresponding to MSY are, respectively, negatively and positively related to productivity. Therefore, since skipjack and bigeye have about the same susceptibility, which is related to fishing mortality, the status of skipjack can be inferred from the status of bigeye. The current assessment of bigeye tuna estimates that the fishing mortality is less than FMSY; therefore, the fishing mortality for skipjack should also be less than FMSY. Since effort and skipjack biomass have been relatively constant over the past 10 years, this also implies that skipjack biomass is above BMSY.
Overall Assessment Results

Summary

The main concern with the skipjack stock is the constantly increasing exploitation rate. However, exploitation rate appears to have leveled off in recent years. The data- and model-based indicators have yet to detect any adverse consequence of this increase. The average weight has declined to levels seen in the early 1980s and was below its lower reference level in 2015 which can be a consequence of overexploitation, but it can also be caused by recent recruitments being greater than past recruitments or expansion of the fishery into areas occupied by smaller skipjack (Figure C-1).
Figure C-1: Indicators of stock status for skipjack tuna in the eastern Pacific Ocean. OBJ: floating-object fishery; NOA: unassociated fishery; CPDF: catch per day fished. All indicators are scaled so that their average equals one.

The low 2015 level is likely due to the large recruitment in 2015. However, average weight has stabilized in recent years. The tagging analyses, length-structured model, A-SCALA, and the SEAPODYM analyses do not provide any information that indicates a credible risk to the skipjack stock(s).


Management
Management unit: Yes
Source of information
 
Inter-American Tropical Tuna Commission (IATTC).  “"Tunas and billfishes in the eastern Pacific Ocean in 2015. Inter-American Tropical Tuna Commission." Fishery Status Report. IATTC 2016.” Click to openhttp://www.iattc.org/PDFFiles2/FisheryStatusReports/FisheryStatusReport14.pdf
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