Habitat and Biology
Bottom type: Unspecified. Depth zone: Coastal (0 m - 50 m); Shelf (50 m - 200 m); Slope (200 m - 1000 m). Horizontal distribution: Neritic; Oceanic. Vertical distribution: Pelagic.
Jurisdictional distribution: Unspecified
Considered a single stock: No
The Working Group maintained the assumption of two stocks of chub mackerel: the northern stock between Cap Boujdor and the north of Morocco in the Atlantic, and the southern stock between Cap Boujdor and the south of Senegal. Nevertheless, owing to a lack of more information including on migration and possible exchanges between the two stocks, the Working Group since its 2003 meeting has proceeded with a joint assessment of the two stocks in its general distribution area.
In Zones A+B and the northern zone (Tangiers–Cap Boujdor), the chub mackerel is exploited by Moroccan coastal purse seiners which mainly target sardine but which also catch chub mackerel depending on its availability. A fleet of Spanish purse seiners targeting anchovy also operated in the northern zone from April 2007 to November 2011. However, its landings of chub mackerel were low.
A part of the Moroccan coastal purse seiners also operate in the zone between Cap Boujdor and Cap Blanc as well as a Moroccan fleet of refrigerated sea water (RSW) vessels. Furthermore a fleet of Russian pelagic trawlers operates under the Morocco–Russian fishing agreement. In 2011, ten Russian trawlers operated in this zone starting in the third quarter. Other vessels in this area are chartered vessels operated by Moroccans and trawlers operating under the Morocco–European Union fishing agreement which ended in November 2011. The fleet of Ukrainian vessels that used to operate in this area are no longer operating since 2010.
South of Cap Blanc, in the Mauritanian zone, pelagic trawlers from several countries (e.g. Russian Federation, Ukraine, Poland, Lithuania, etc.) fish mackerel on a seasonal basis. Chub mackerel is also taken as bycatch by EU vessels (Holland type) that normally fish for other species. The number of vessels has been stable the last two years.
In The Gambia and Senegal, chub mackerel is considered as bycatch of the Senegalese artisanal fleet. In 2010, a Russian fleet composed of three industrial fishing vessels operated in Senegal between March and May. In 2011, the number of Russian vessels reached eight units in the first quarter. These stopped fishing in May and four vessels returned in December. These vessels mainly target horse mackerel, but chub mackerel is the predominant species in about 6 percent of the tows. Catches in The Gambia are reported from both the artisanal and industrial fleet, however at a very low level (around 300 tonnes in 2010 and 2011). Total catches
The annual trend in catches of Scomber japonicus
by country for the period 1990–2011 is given in Table 5.2.1 and Figure 5.2.1.
Total catches in the northern fishery (north of Cap Boujdor) have remained around 50 000 tonnes over the period 2009–2011 declining since the peak of around 84 000 tonnes in 2007. Contrary to previous years, the highest catches in 2011 were taken in Zone North between Cap Cantin and Cap Spartel (20 000 tonnes) and only 13 000 tonnes were taken in Zone A, traditionally the zone with highest catches. In comparison the catch in Zone A in 2010 was around 30 000 tonnes. In Zone B, catches were around 15 000 tonnes in 2011, increasing from only 2 000 tonnes in 2010 (Table 5.2.1 and Figure 5.2.1).
Catches in Zone C (Cap Boujdor–Cap Blanc), increased in 2011 as compared to 2010 from 86 000 tonnes in 2010 to more than 150 000 tonnes in 2011, representing almost a doubling of the catch and the highest catches of chub mackerel of the time series. The Moroccan fleet, composed of the coastal purse seiners and refrigerated sea water vessels (RSW), comprised nearly 50 percent of the catch (increasing from 2010), followed by the Russian fleet with 29 percent and the EU fleet with 25 percent.
In Mauritania, the total catch of chub mackerel have been fluctuating over the time period 1990–2011, showing an overall increasing trend. Peaks in catches were observed in 1996 (100 000 tonnes) and in 2002–2003 (100 000 tonnes and 130 000 tonnes respectively). Since then, catches declined sharply to 38 000 tonnes in 2005 and 33 000 tonnes in 2006, and fluctuated at a lower level until 2010 when the catch increased to 75 000 tonnes and then to 100 000 tonnes in 2011. The bulk of catches mainly taken by the pelagic trawler fleet (mainly Russian Federation) (79 percent), followed by EU vessels from Eastern Europe with 16 percent (Table 5.2.1 and Figure 5.2.1).
In Senegal and The Gambia, chub mackerel is not a target species. Catches over the time period 1990–2012, range from around 1 200 tonnes to more than 15 000 tonnes. The overall trend in catches in Senegal is similar to that in Mauritania with high catches in the period 1996–1998, a peak of 14 000 tonnes in 2003, and high catches in 2010 (11 000 tonnes) increasing to 15 000 tonnes in 2011; the highest catch in the time series. Traditionally the artisanal fishery has reported the highest catches of this species in Senegal, but since its entry into the fishery in 2010, the Russian fleet also is a main contributor. In 2011 this fleet represented 61 percent of the catch, compared to 10 percent in 2010.
Catches in The Gambia in 2011 were somewhat lower as compared to 2011, decreasing from just about 310 tonnes to around 295 tonnes of which 81 percent was taken by the artisanal fleet.
Since 1991, the trend of total chub mackerel catches for the whole subregion has seen an overall increase over the time period. A period of high catches was observed in the 1995–1998 period reaching over 210 000 tonnes in 1997, after which catches fluctuated around an average value of 181 000 tonnes until 2006. Higher catches were observed in 2007 and 2008 (57 000 tonnes and 268 000 tonnes respectively), before decreasing to 225 000 tonnes in 2010. The catch in 2011 was 318 000 tonnes, the highest catch of the time series, mainly resulting from an increase in catches in Zone C, north of Cap Blanc with the Moroccan fleet being the main contributor but higher caches are also observed to the south of Cap Blanc in Mauritania and Senegal (Table 5.2.1 and Figure 5.2.1).
In 2011, around 74 percent of the total anchovy catch in the subregion was recorded in Mauritania. It has increased from 8 percent of the total catch in 1995 to more than 95 percent in 2003. In 2004 and
2005, catches decreased by 47 percent in Mauritania. In 2006 and 2007, catches increased again, before showing another decline in 2008 and 2009 followed by another increase in 2010 and 2011. Catches by the Russian and Ukrainian fleets represent 51 percent of the total declared catch in Mauritania. However, several indications exist that the bulk of the catches declared in Mauritania as anchovy could be juvenile horse mackerel or other species which have been processed into fishmeal and thus are impossible to identify.
In Morocco, catches of anchovy in 2011 increased compared with 2010 despite the fact that there was no change in the fleet operating in the zone. The largest part of this catch was taken in the northern zone with more than 10 000 tonnes and in Zone A with over 22 000 tonnes, representing an increase of 22 percent compared with 2010 in the two zones (Table 6.2.1). Fishing effort
Fishing effort of the Moroccan coastal purse seiners is expressed in number of positive trips. That of the pelagic trawlers in the different zones (i.e. RSW, Russian Federation, EU-type Lithuania, non-EU, Senegal Industrial) is expressed in fishing days. The effort of the artisanal fishery in Senegal (encircling gillnet and purse seine) is expressed in fishing days. Given the multispecific nature of the fishery, the nominal fishing effort for chub mackerel is the same as that described in the chapters of sardine, horse mackerel and sardinella, and thus the trends are not re-described here.
The data used for the two models ICA and XSA are given in Tables 5.2.1, 5.3.1 and 5.5.2a, b. The natural mortality used was 0.5 year-1
(same as previous years).
The parameters used for fitting the model were: the CPUE of the Russian fleet, two separate fishing periods (1992–2006 and 2007–2011) and a selection (S) on the last age set to 1.7. The reference age used was 3 and the number of years for the separability constraint was 5.
For the XSA model, the catchability depends on the size of the stock for ages below 2, and does not depend on ages above 4. The estimated number of survivors is “shrinked” to the average fishing mortality of the last two ages.
The ICA and XSA models were applied and the effect of different factors on the fit and stability of the model were tested. The criteria for the fit of the models were the minimization of the SSQ residuals and the similarity of the spawning stock biomass (SSB) and fishing mortality F curves using a retrospective analysis of the data.
Integrated catch-at-age analysis (ICA)
Extended survivors analysis (XSA)
Overall Assessment Results
The fishing mortalities for age groups 1 to 5 during the period 1992–2011 were recalculated on the basis of the fitting. The results of this analysis are given in Tables 5.6.3a, b and c. The results of the fit are given in Figure 5.6.3.Tableau 5.6.4:
Fishing mortality by age groups estimated by
the ICA and XSA models for 2011
|Age group ||1 ||2 ||3 ||4 ||5 |
|ICA ||0.09 ||0.32 ||0.47 ||0.45 ||0.79 |
|XSA ||0.09 ||0.27 ||0.28 ||0.25 ||0.33 |
With the ICA model, the estimated mean fishing mortality for 2011 was 0.41 year-1
, which is an increase as compared to the value obtained for 2010 (0.28 year-1
), noting that the 2010 value is the lowest value of the time series (1998–2011). With the XSA model, this fishing mortality was 0.24 year-1
in 2011, similar to the 2009 level. Both mean F values are lower than the natural mortality coefficient M=0.5 year-1
. The results of the two analyses show that although the levels of fishing mortality have shown some fluctuations over the last period, the rate of exploitation have remained at a similar level.
The estimated SSB curves from both models are almost identical. Mature fish constitutes around 57 percent of the total biomass while the medium values of the past 20 years have been 74 percent.
The comparison in trends of estimated biomass and landings are shown in Figures 5.6.4. a, b. The catches increased in 2011 compared with 2010, and the biomass estimated by the models has remained at the same level.
The results of the ICA and XSA model indicate that the stock is fully exploited.
The Multi-fleet Deterministic Projections (MFDP) Programme was applied to the values obtained through ICA to carry out the projections over a three-year period (current year and two years ahead). The results are shown in Figure 5.7.2.
The following scenario was considered:
- F in 2012 will be around the average value over the past three years.
- Recruitment for the forecasted years is equal to the geometric mean recruitment for the time period (1992–2011) as determined by ICA.
Within this context several levels of fishing intensity for 2013 were investigated.
The projections indicated that among the F´s options considered the F values between 0.22–0.25 year-1
would sustain catch levels of around 250 000–280 000 year while maintaining the SSB around the current level. This would imply a slight reduction in F as compared to 2011, while catches remain within the average of the last five years.
Scientific AdviceFuture research Follow-up on previous year’s recommendations
- Scientists from AtlantNiro continue the collection and reading of otoliths from Russian fleet, but the full objective of extending this work to other fleets has not yet been achieved.
- No new information regarding stock identity was provided to the Working Group
- Biological information related to length composition of catches has improved.
Given the fact that last year’s recommendations were only partially implemented in 2011, they have been carried over to this year.
for Management consideration
- Continue to encourage studies on stock identity in the region.
- Ensure that biological sampling is carried out for all fisheries in the subregion, covering all size ranges and quarters of the year.
- Increase the collection and reading of otoliths in order to establish the age-length keys by fishery and/or by zone.
- Ensure a complete regional coverage of the chub mackerel stock by acoustic surveys.
As a precautionary approach and considering the good recruitment estimations, the Working Group recommends that the catch levels should not exceed a level of around 250 000 tonnes in 2012.
Biological State and Trend