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Horse mackerel and other carangids - Southern Area of Eastern Central Atlantic, 2005
Marine Resource  Fact Sheet
Status of stocks and resources 2006
Horse mackerel and other carangids - Southern Area of Eastern Central Atlantic, 2005
Fact Sheet Citation  
Owned byFood and Agriculture Organization (FAO) – More
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Species:
Geographic extent of Horse mackerel and other carangids - Southern Area of Eastern Central Atlantic
Main Descriptors
Considered a single stock: No        Spatial Scale: Sub-Regional
 
 
Habitat and Biology
Climatic zone: Temperate.   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.  

Geographical Distribution
Jurisdictional distribution: Shared between nations

Water Area Overview
Spatial Scale: Sub-Regional



Geo References
Resource Structure
Considered a single stock: No


The two main species under consideration for stock assessment are the horse mackerel (Decapterus rhonchus and Trachurus trecae). For the other carangid species or groups of species only data on catch and effort will be presented.

Whilst waiting for more detailed information on stock identity of the carangid species in the southern CECAF region, the Working Group decided to split into four stocks: the northern stock (Guinea and Sierra Leone), the western stock (Ghana, Togo and Benin), the central stock (Nigeria and Cameroon and the southern stock (Congo and Angola).
Exploitation
 

Catch

For Decapterus spp., the catches are basically all from the industrial pelagic fishery in the northern stock (Guinea and Sierra Leone) with an annual average of around 4 000 tonnes.

The majority of catches in the southern CECAF region (mainly Decapterus rhonchus) are taken in Guinea. A large decrease can be seen in all catches from about 5 700 tonnes in 1996 to around 2 000 tonnes in 2004.

For the group of Trachurus spp species (mainly Trachurus trecae), most of the catches are carried out in the southern stock (notably Angola). A worrying decrease can be seen in catches over the whole southern CECAF region, going from over 65 000 tonnes in 1990 to less than 7 000 tonnes in 2004. It should however be noted that a slightly increasing trend in catches can be seen from 2003 in the western stock.

The other carangid species that are generally exploited in the region are Selene dorsalis, Chloroscombrus chrysurus and the Caranx spp. species. Total catch of these species has seen a decreasing trend, declining from 13 000 tonnes in 1993 to 4 000 tonnes in 2004.

In Guinea and Sierra Leone these species are mainly fished by encircling gillnets and driftnets in the artisanal fishery. In Ghana, Togo, Benin, Nigeria and Cameroon, small carangids are mainly exploited by the beach seine and the purse seine.

Effort

Most of the industrial fleets’ effort is concentrated in the Guinean EEZ. The large pelagic trawlers that target horse mackerel come from eastern European countries (Russian Federation and Ukraine). The nominal effort of this fleet (fishing days) has decreased overall from over 600 days in 1996 to around 400 days in 2004.


Assessment
 
Assessment Model
Type:  Biomass-aggregated

The logistical production model on an Excel spreadsheet was used.


Data

The model requires a time series of total catch and abundance indices of the stock.

The estimates of total catch were obtained by summing catches from different fleets from different countries as well as the catch series.

The global model was applied to the Decapterus rhonchus and Trachurus trecae data from the northern stock (Guinea, Sierra Leone) using the CPUEs of the industrial pelagic trawlers of Guinea.

For Trachurus trecae the model was applied to the data from the southern stock (Congo/Angola) using the biomass indices from the R/V DR. FRIDTJOF NANSEN acoustic surveys as an index of abundance.

Overall Assessment Results

For the northern Decapterus rhonchus and Trachurus trecae stocks, the fit of the model can be considered to be relatively satisfactory as it shows the trends in abundance indices of the period.

The results show that the two stocks are fully exploited with current biomass levels well below that at B0.1 (Table 1).

It can be considered that the adult fraction of these two stocks is more or less fully exploited. There are however, grave concerns with regard to the actual level of total catch and to the discards as well as to the level of change in exploitation pattern in the two countries that share the same stocks. Given these concerns, the results of the fit should be considered with prudence.

Table 1: Summary of the actual state of the stock and of the fisheries of horse mackerel D. rhonchus and T. trecae (northern stock)

Stock/abundance index Bcur/B0.1 Fcur/FSYcur Fcur/FMSY Fcur/F0.1
Decapterus rhonchus/CPUE, IF Guinea 54% 70% 109% 98%
Trachurus trecae/CPUE, IF Guinea 56% 64% 98% 89%

Bcur/B0.1: Ratio between the estimated biomass for the last year and the biomass corresponding to F0.1.
Fcur/FSYcur: Ratio between the observed fishing mortality coefficient during the last year of the series and the coefficient that would give a sustainable yield at current biomass levels.
Fcur/FMSY: Ratio between the observed fishing mortality coefficient during the last year of the series and the coefficient giving maximum long term sustainable yield.
Fcur/F0.1: Ratio between the observed fishing mortality coefficient during the last year of the series and F0.



For the Trachurus trecae stock in the southern stock (Congo and Angola), the model fit is satisfactory. It shows the main trends in the abundance indices.

The adjusted model shows that the stock underwent fluctuations during the period 1995–2005.

The results from the fit show that current biomass is above the level at B0.1 and that current fishing mortality is well below the level of that for the current biomass (Table 2).


Table 2: Summary of the actual state of the stock and of the fisheries of T. trecae (northern stock)

Stock/abundance index Bcur/B0.1 Fcur/FSYcur Fcur/FMSY Fcur/F0.1
Trachurus trecae//NANSEN indices 115% 22% 18% 16%

Bcur/B0.1: Ratio between the estimated biomass for the last year and the biomass corresponding to F0.1.
Fcur/FSYcur: Ratio between the observed fishing mortality coefficient during the last year of the series and the coefficient that would give a sustainable yield at current biomass levels.
Fcur/FMSY: Ratio between the observed fishing mortality coefficient during the last year of the series and the coefficient giving maximum long term sustainable yield.
Fcur/F0.1: Ratio between the observed fishing mortality coefficient during the last year of the series and F0.



The results of the assessments for the two northern horse mackerel stocks suggest that current fishing effort is at the level corresponding to maximum sustainable yield (MSY). There are however several reasons why these results should be considered with care. The assessment results depend on the assumption that the exploitation pattern is constant. If this has been recently modified, for example due to targeting smaller sizes, the assessment results could prove weak. The uncertainty surrounding total catch due to under-declaration of small fish catches invites prudence when considering the results.

For the horse mackerel stock of the southern stock, the results should be considered with care due to the absence of an important series of data on catches carried out by pelagic trawlers which catch most of this species. The results of the model using the data available to the Working Group indicate that the stock is not overexploited, which is confirmed by the abundance indices obtained during the acoustic surveys. These have shown an increasing trend, probably tied to the withdrawal of all pelagic trawlers from the relevant area.

Scientific Advice

Future research
In order to reduce uncertainties surrounding the assessment, the Working Group recommends carrying out the following research:
  • Support sampling programmes in order that they cover total catch of all the main carangid species, including horse mackerel, for all fleets in all countries of the southern CECAF region.
  • Continue biological data collection for biological studies (growth, reproduction, feeding) on the main carangid species and make these data available to the Working Group by the next meeting.
  • Begin sampling catches, bycatch and discards onboard all vessels fishing horse mackerel due to the problems arising from under-declaration of catch, especially of juveniles.


for Management considerations

Due to limitations in the model and uncertainties surrounding the data, the Working Group recommends adopting a precautionary approach and that catches not be increased above the average level of the last five years (Northern stock for Trachurus trecae and Northern stock for Decapterus spp.)

For the southern stock of horse mackerel, in spite of some doubts about the data series used to fit the model, the Working Group recommends a moderate and sustained increase in catch to optimise exploitation in this stock.


Biological State and Trend
 
Trachurus trecae: Northern stock (Guinea and Sierra Leone)
 Exploitation state: Fully exploitedExploitation rate: Fcur/F0.1: 98 %
Abundance level: Bcur/B0.1: 56 %

Stock fully exploited, but level of current biomass too low.


Trachurus trecae: Southern stock (Congo and Angola)
 Exploitation state: UnderexploitedExploitation rate: Fcur/F0.1: 18 %
Abundance level: Bcur/B0.1: 115 %


Decapterus spp: Northern stock (Guinea and Sierra Leone)
 Exploitation state: Fully exploitedExploitation rate: Fcur/F0.1: 109 %
Abundance level: Bcur/B0.1: 54 %

Stock fully exploited, but level of current biomass too low.


Source of information
 
FAO Fishery Committee for the Eastern Central Atlantic/Comité des pêches pour l’Atlantique Centre-Est. “Report of the FAO/CECAF Working Group on the Assessment of Small Pelagic Fish – Subgroup South” Limbe, Cameroon, 25 September–1 October 2006 “Rapport du Groupe de travail FAO/COPACE sur l’évaluation des petits poissons pélagiques – Sous-Groupe Sud” Limbé, Cameroun, 25 septembre-1er octobre 2006 Rome, FAO . 2009 .
Bibliography
 

The bibliographic references are available in the report included in "Source of Information".
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