Fisheries and Resources Monitoring System

EspañolFrançais
Skipjack tuna - West Atlantic
Marine Resource  Fact Sheet
Stock status report 2014
Skipjack tuna - West Atlantic
Fact Sheet Citation  
Skipjack tuna - West Atlantic
Owned byInternational Commission for the Conservation of Atlantic Tunas (ICCAT) – 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 - West Atlantic
Main Descriptors
Considered a single stock: Yes        Spatial Scale: Regional
Management unit: Yes        Reference year: 2013
 
 
Biological State and Trend
State & Trend Descriptors
PartnerFIRMS
Exploitation rateF2013/FMSY = Likely<1Not applicableGray
Abundance levelB2013/BMSY = Likely>1Not applicable


History
 

Stock assessments for East and West Atlantic skipjack were conducted in 2014 (Anon. 2014) using catch data available to 2013. The last assessment of skipjack stocks was only conducted in 2008 (Anon. 2009a). Consequently, this report covers the most recent information on the state of the stocks on this species.


Habitat and Biology
Climatic zone: Tropical.   Vertical distribution: Pelagic.  


Skipjack tuna is a gregarious species that is found in schools in the tropical and subtropical waters of the three oceans (Figure 1). Skipjack is the predominant species aggregated to FADs where it is caught in association with juvenile yellowfin tuna, bigeye tuna and with other species of epipelagic fauna. Skipjack reproductive potential is considered to be high because it reaches sexual maturity around one year and it spawns opportunistically in warm waters above 25ºC throughout the year and in large areas of the ocean. Moreover, the analysis of East Atlantic tagging data has confirmed that the growth of skipjack was quicker in sub-tropical waters than in equatorial waters where it produces most of its spawn. These growth differences depending on latitude must be taken into account if the assessments are carried out on separate stocks between sub-tropical and tropical areas. It is also possible that the growth does not follow the conventional Von Bertalanffy model but rather a two-stanza model. Based on the relationships between life history characteristics and natural mortality, a natural mortality vector decreasing with size has been estimated (Figure 2). The natural mortality values estimated by this approach are greater than those used to date for East Atlantic skipjack. Lower values have been obtained by another approach which has been applied for the western stock, whose catches are however composed of larger sized individuals than in the eastern stock.

The increasing use of fish aggregation devices (FADs) since the early 1990s, have changed the species composition of free schools. It is noted that, in fact, the free schools of mixed species were considerably more common prior to the introduction of FADs. Furthermore, the association with FADs may also have an impact on the biology (growth rate, plumpness of the fish) and on the ecology (distances, movement orientation) of skipjack and yellowfin (“ecological trap” concept).


Figure 1. Distribution of skipjack catches in the Atlantic for baitboat (upper left panel) between 1950 and 2013 and for purse seiners (upper right panel) by fishing mode (free schools vs. FADs) between 1991 and 2013. Skipjack catches made by European and associated purse seiners (about 75% of the total catches) between 2000 and 2006 (lower left panel) and between 2007 and 2013 (lower right panel) showing the withdrawal from the Senegal fishing zone on free schools, due to non-renewal of the fishing agreements in 2006, and the appearance of a fishing area under FADs in 2012 North of 15oN latitude.
Figure 2. Estimates of natural mortality by size of Atlantic skipjack calculated by empirical relationships between mortality and some biological parameters (which show different values from those traditionally used in the East.
Geographical Distribution
Jurisdictional distribution: Highly migratory


Skipjack tuna is a gregarious species that is found in schools in the tropical and subtropical waters of the three oceans (Figure 1)
Water Area Overview
Spatial Scale: Regional

Geo References
Resource Structure
Considered a single stock: Yes


During the 2009 assessment the Committee reviewed the current stock structure hypothesis that consists of two separate management units, one in the East Atlantic and another in the West Atlantic, separated at 30ºW. The boundary of 30ºW was established when the fisheries were coastal, whereas in recent years the East Atlantic fisheries have extended towards the west, surpassing this longitude, and showing the presence of juvenile skipjack tuna along the Equator, west of 30ºW, following the drift of the FADs. This implies the potential existence of a certain degree of mixing .

Also some biological parameters, like growth, show differences depending on latitude, which could indicate separate stocks between sub-tropical and tropical areas.

Based on the large geographic distances between the fishing areas and current knowledge on small-scale migrations of skipjack in the Atlantic (Figure 1), the Committee has analysed the possibility of using smaller stock units. Nevertheless, the Committee does not currently have evidence, such as a sufficient amount of tag-recovery data covering the entire tropical ocean, in order to validate smaller stock units. Consequently, the Committee has decided to maintain the working hypothesis which favours two different units of eastern and western stocks but on an experimental basis to assess a sub-unit in each of the two stocks. The use of smaller areas has however been recommended to monitor the development over time of fishery indicators.


Exploitation
 

Fishery Indicators

Following the historic record in 2012 (258,300 t), the total catches of skipjack throughout the Altantic Ocean (including catches of "faux poisson" landed in Côte d’Ivoire) remain high at 221,600 t (Table1, Figure 3) This represents a very sharp rise compared to the average catches of the five years prior to 2010 (157,600 t). It is possible, however, that the catches of a segment of the Ghanaian purse seine fleet, transshipped at sea on carriers, have escaped the fishery statistics collection process before 2011. In addition, following the expert missions carried out in Ghana which have shown the existence of bias in the sampling protocol which aims to correct the multi-species compositions of the catches reported in the logbooks, Ghanaian Task I and II statistics have been reviewed in several stages (1973-2005). The last review for the period 2006-2012 shows that the skipjack catches reported by Ghana were underestimated by around 28%, which gives an average of 12,000 t/year. Therefore, all of these historical data have consequently been corrected.

In the West Atlantic, the major fishery is the Brazilian baitboat fishery, followed by the Venezuelan purse seine fleet. The preliminary estimates of catches in 2013 made in the West Atlantic amounted to 18,000 t (against the historic record of 40,000 t in 1984). This sharp decrease in 2013 compared to the large catches reported by Brazilian baitboats in 2012 is due to incomplete reporting by Brazil in 2013 (Figure 4). As the fishing effort of this fleet has not increased, these variations could be the result of changes in catchability at local level of this fishery.The average weight (Figure 5) is significantly higher than for the East Atlantic stock and has remained stable over the period.

With respect to the West Atlantic, the fishing effort of the Brazilian baitboats, which constitute the main skipjack fishery in this region, seems to have stabilised over the past 20 years. No marked trend regarding the structure of catches by size has been observed (Figure 6).
Figure 3. Total skipjack catches (t) in the Atlantic and by stock (East and West) between 1950 and 2013. Skipjack estimates in the faux poissons landed in Côte d’Ivoire were included in the skipjack trade catches in the eastern Atlantic. It is possible that skipjack catches taken in the eastern Atlantic in recent years were not reported or were under-estimated in the logbook correction of species composition based on multi-species sampling carried out at the ports. The 2013 figure is still preliminary, in particular for the East Atlantic.
Figure 4. Cumulative skipjack catches in the western Atlantic, by gear (1950-2013). The values for 2013 are preliminary.
Figure 5. Changes in the average weight of skipjack in the eastern (black) and western Atlantic (red).
Figure 6. Annual catch (in weight) by skipjack size class in the western Atlantic.
Assessment
 
Assessment Model

In all the oceans, the traditional stock assessment models are difficult to apply to skipjack because of their particular biological and fishery characteristics (on the one hand, continuous spawning, spatial variation in growth and on the other, discrimination of effort for free schools and FADs, transition between these two fishing methods which are difficult to quantify). In order to overcome these difficulties, several assessment methods, conventional and non-conventional (based solely on catches, or on development of average size) have been applied to the two stocks of Atlantic skipjack. Several fishery indicators have also been analysed in order to track the development of the state of the stock over time.

Based on the large geographic distances between the fishing areas and current knowledge on small-scale migrations of skipjack in the Atlantic (Figure 1 and Figure 7), the Committee has also analysed the possibility of using smaller stock units. While recognising the validity of this approach, the Committee does not currently have evidence, such as a sufficient amount of tag-recovery data covering the entire tropical ocean, in order to validate smaller stock units. Consequently, the Committee has decided to maintain the working hypothesis which favours two different units of eastern and western stocks but on an experimental basis to assess a sub-unit in each of the two stocks. The use of smaller areas has however been recommended to monitor the development over time of fishery indicators.

The CPUEs in the West were those of the Brazilian baitboat which remain relatively stable, those of the Venezuelan purse seiner, the US pelagic longline and a laval index (Figure 8). In addition, the average weight of skipjack caught in the West Atlantic is higher than in the East (3 to 4.5 kg compared to 2 to 2.5 kg), at least for the Brazilian baitboat fishery.

The model based on catches and the non-equilibrium surplus biomass production model have estimated respectively the MSY at 30,000 t - 32,000 t (which remains close to the previous estimates in the order of 34,000 t). The fishing mortality vector estimated by a method based on the development of average size of individuals captured over time (mainly from Brazilian catches) shows a profiles which is very close to that estimated by the non-equilibrium surplus biomass model (Figure 9).

It should be emphasised that all these analyses rest on the assumption of a single western stock from the US coast to Brazil and correspond to the current geographic coverage of this fishery.

For the western Atlantic stock, in light of the information provided by the trajectory of B/BMSY and F/FMSY ratios (Figure 10), it is unlikely that the current catch is larger than the replacement yield.


Figure 7. Apparent movements (straight line distance between the tagging location and that of recovery) calculated from conventional tagging.
Figure 8. Relative abundance indices for the eastern skipjack stock. Each index has been adjusted to its own average level given that to resolve problems regarding scaling, the indices for purse seiners and longliners have been adjusted to the level of the larvae index of the Gulf of Mexico.
Figure 9. Comparison of coefficient mortality estimates of skipjack fishing in the western Atlantic obtained from a surplus production model (ASPIC black line and solid circles) and by the model based on the average size of fish caught (so called Then Hoenig-Gedamke in red and empty circles).
Figure 10. Western skipjack stock status: trajectories of B/BMSY and F/FMSY from the ASPIC surplus production model (Schaefer type).




Overall Assessment Results
Projection

Outlook

The Committee do not conduct projections for the skipjack West Atlantic stock


Management
Management unit: Yes


Effects of current regulations

No specific management regulations are into force for the skipjack West Atlantic stock.


Management Advice

For the West Atlantic, the Committee has not formulated any management recommendation, and has only indicated that the catches should not be allowed to exceed the MSY.

Despite recent progress, the Committee has expressed its concern regarding uncertainties which the under-reporting of skipjack catches may have on the perception of the state of the stocks.


WEST ATLANTIC SKIPJACK TUNA SUMMARY

Maximum Sustainable Yield

Around 30,000-32,000 t

Current (2013) Yield1

18,000 t

Current Replacement Yield

Somewhat below 32,000 t

Relative Biomass  (B2013/BMSY)

Probably close to 1.3

Relative Fishing Mortality: F2013/FMSY

Probably close to 0.7

Management measures in effect

None

1 Preliminary estimate due to incomplete reporting by Brazil.

Source of information
 
“Report of the 2014 Meeting of the Standing Committee on Research and Statistics.” International Commission for the Conservation of Atlantic Tunas (ICCAT). 2015 ICCAT Report for biennial period, 2014-15 PART I (2014) - Vol. 2 ICCAT Click to openhttp://www.iccat.int/Documents/Meetings/SCRS2012/2014_SCRS_REP_EN.pdf
powered by FIGIS  © FAO, 2019
Powered by FIGIS
crawl