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Skipjack tuna - Eastern Pacific
Marine Resource  Fact Sheet
Stock status report 2015
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: 2014
 
 
Biological State and Trend
State & Trend Descriptors
PartnerFIRMS
Exploitation rateModerate fishing mortalityModerate fishing mortality
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 1985-2014 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 1.8 million t in 2013. In the EPO, the greatest yearly catches occurred between 2003 and 2014, with the highest catch of 309 thousand t in 2006, and a range from 153 to 309 thousand t.

The annual retained catches of skipjack in the EPO by purse-seine and pole-and-line vessels during 1985-2014 are shown in (Table A-2a), (Table A-2a (cont.)), (Table A-2a (cont.)). During 1999-2013 the annual retained catch averaged 234 thousand t (range 144 to 297 thousand t). The preliminary estimate of the retained catch in 2014, 262 thousand t, is 12% greater than the average for 1999-2013, and 12% less 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 2013. During the period about 5% 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
Total catches (retained catches plus discards) of skipjack tuna by the purse-seine fisheries on floating objects and unassociated schools, and by other fisheries combined, in the eastern Pacific Ocean. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2014 catch data are preliminary.
Average annual distributions of the purse-seine catches of skipjack, by set type, 2009-2013. 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, 2014. 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. Even more data are required if catch quotas are to be used as the management tool.
Assumption

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 (Maunder 2007); 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. We update their results to include data up to 2013. 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.

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 the limited tagging data. 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 (Maunder 2002; Maunder and Harley 2005), 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 (Maunder and Harley 2005) is considered preliminary because it is not known whether the catch per day fished for purse-seine fisheries is proportional to abundance. The results from that assessment are more consistent among sensitivity analyses than the earlier assessments, which suggests that they may be more reliable. Analysis of currently available tagging data is unlikely to improve the skipjack stock assessment (Maunder 2012a) and a fully length-structured model produced unrealistic estimates (Maunder 2012b). 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 (Maunder and Harley 2005). 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.
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 assessment, 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. 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

Eight data- and model-based indicators are shown in (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 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 has been increasing since 1985, and has fluctuated around the upper reference level since 2003. The floating-object CPUE has generally fluctuated above the average level since 1990 and was at the upper reference level in 2011. The unassociated CPUE has been higher than average since about 2003, and was at its highest level in 2008; it declined in 2010, then increased to above the upper reference level in 2013. The standardized effort indicator of exploitation rate increased starting in about 1991, but decreased in 2009 and 2010. The average weight of skipjack has been declining since 2000, and in 2009 was below the lower reference level, but has increased slightly since then. The biomass, recruitment, and exploitation rate have been increasing over the past 20 years, and have fluctuated at high levels since 2003. The biomass and recruitment were close to the upper reference level in 2013.
Overall Assessment Results

Summary

The main concern with the skipjack stock was the constantly increasing exploitation rate. However, this 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 was below its lower reference level in 2009, 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. Any continued decline in average length is a concern and, combined with leveling off of catch and CPUE, may indicate that the exploitation rate is approaching, or above, the level associated with MSY.
Management
Management unit: Yes
Source of information
 
Inter-American Tropical Tuna Commission (IATTC).  “"Tunas and billfishes in the eastern Pacific Ocean in 2014. Inter-American Tropical Tuna Commission." Fishery Status Report. IATTC 2015.” Click to openhttp://www.iattc.org/PDFFiles2/FisheryStatusReports/FisheryStatusReport13.pdf
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