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Bonga shad - Southern Area of Eastern Central Atlantic, 2005
Marine Resource  Fact Sheet
Status of stocks and resources 2006
Bonga shad - Southern Area of Eastern Central Atlantic, 2005
Fact Sheet Citation  
Ethmalosa fimbriata - Southern Area of Eastern Central Atlantic
Owned byFood and Agriculture Organization (FAO) – More
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Species:
FAO Names: en - Bonga shad, fr - Ethmalose d'Afrique, es - Sábalo africano, ru - Бонга
Geographic extent of Bonga shad - Southern Area of Eastern Central Atlantic
Main Descriptors
Considered a single stock: No        Spatial Scale: Sub-Regional
 
 
Habitat and Biology
Climatic zone: Unspecified.   Bottom type: Unspecified.   Depth zone: Coastal (0 m - 50 m); Shelf - Uppershelf (up to 100 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


Bonga (Ethmalosa fimbriata), otherwise called shad is found along the West African coast and it is an important species as it is mainly found in coastal waters, estuaries and sometimes in rivers of 15-45 metres depth. It is largely targeted by the artisanal fisheries sector. Bonga in the southern region was grouped into four stocks. Since there was no study on molecular biology of this stock, the stocks were mainly based on the catch and effort trend of the fisheries. The stocks were grouped into 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
 

Bonga has been intensively exploited for a long time in the subregion. As a coastal and estuarine species, bonga is mainly exploited by the artisanal fisheries and is a very important species in Guinea.

Northern stock

A range of fishing gears such as ring gillnets, purse seines, beach seines and bottom driftnets are utilized. The canoes used vary from 6–18 m and means of propulsion vary from sails, paddle and 15–40 hp outboard motors. Over 70 percent of the total artisanal landing in Guinea and Sierra Leone is bonga.

Western stock

Fishing gears such as ring gillnets, purse seines, beach seine and surface driftnets are utilised. The canoes used vary from 12–18 m with 25-40 hp outboard motors as a means of propulsion. Bonga constitutes a low percentage in the total artisanal landings for Ghana and Benin.

Central stock

These gears used include purse seines, surface drift gillnets, encircling nets and beach seine. The canoes used vary from 5–9 m and 12–20 m. The means of propulsion range from sails, paddles and 8–40 hp outboard motors. Bonga constitutes about 15–20 percent of the total artisanal landings in Nigeria and Cameroon.

Southern stock

This species is largely caught in Lobito, Angola and is targeted by the artisanal fisheries exclusively employing surface drift gillnets. The canoes are 6–7 metres in length and driven by paddles and 15–25 hp outboard motors. No data on catches were provided by Angola. Bonga constitutes about 6–10 percent of the total artisanal landings for Congo.

Catch

Total annual catch of bonga by countries, fleet and stocks are presented in Table 3.2.1. No catch data were provided by Angola and Togo, which was due to the insignificant quantity landed. Figure 3.2.1 shows the total catch of bonga per stock (north, central, west and south). There has been an increasing trend in catches of bonga in the northern stock (except for year 1999), with interannual fluctuations, reaching a total of 83 000 tonnes in 2004. The total catch in the central stock has an increasing trend with some fluctuations from 1990 to 1995 but a decreasing trend since then. The total catch in the other stocks have been maintained at a constant production level. This species is a bycatch in the industrial shrimp fishery in Sierra Leone.

Effort

Effort data for bonga are presented in Table 3.2.2 and Figure 3.2.2 as the number of trips or number of days fishing. It should be noted that the effort presented here was total effort targeting small pelagic fish except for Congo and Cameroon.

The effort data for the artisanal fishery in Sierra Leone and Ghana were measured as number of fishing trips, the rest of the countries provided effort data in number of days fishing for all fleets. No effort data were provided by Nigeria, Togo and Angola. The overall trend for all countries was fairly stable from 1995 to 2005, except for Guinea which seems to have increased since 1999.

Assessment
 
Assessment Model
Type:  Biomass-aggregated

The Schaefer logistic production model was used on an Excel worksheet.
Data

In order to test the quality of the data available for the assessment, the sub-group carried out an exploratory analysis of the data. At a stock level (north, west, central and south) the data did not indicate a good correlation. However, the CPUEs calculated from the catch and effort data of the Guinea and Congo artisanal fleets targeting bonga were used to carry out an exploratory assessment separately.

The model requires complete time series of data in total catch as well as an index of stock abundance. The estimates of total catch obtained by adding the catch estimates of all the fleets in one country were used as total catch series. For an abundance index, one time series was used. The CPUE from Guinea (1995–2004) and Congo (1998–2005) artisanal fisheries were chosen to represent the northern and southern stocks because the data were considered good enough to track bonga abundance better than the industrial series.
Overall Assessment Results


The results obtained from this trial were satisfactory.

The fitted model for Guinea (northern stock) to the available data was considered satisfactory. It manages to follow the main trends in abundance indices, reacting to the variation in catches (Figure 3.6.1).

The fitted model for Congo (southern stock) indicates that the stock fluctuated markedly during the period considered (Figure 3.6.2) but that it was not overexploited during most of this period.

The model results indicate that the current biomass for Guinea (northern stock) is below the biomass at B0.1, on average, that the current fishing mortality is about 10 percent above the sustainable fishing mortality at the current biomass levels (Table 1).


Table 1: Summary of the state of the stock

Stock/abundance index Bcur/B0.1 Fcur/FSYcur Fcur/FMSY Fcur/F0.1
Ethmalosa fimbriata/North/CPUE Guinea artisanal 50% 111% 179% 161%
Ethmalosa fimbriata/South/CPUE Congo artisanal 73% 15% 19% 18%

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 Guinean (North) stock, one may deduce that the population has been overexploited. However, the result should be viewed with caution given doubts about the abundance index used to fit the model and the possibility that there has been a change in the relative exploitation pattern.

The model result for Congo (southern stock) indicates that the current biomass is below the biomass at B0.1, on average and that the current fishing mortality is about 85 percent below the sustainable fishing mortality at the current biomass level (Table 3. 6.1).

For Congo (South), it can be considered that the population is not yet fully exploited. However, there are still uncertainties regarding, on the one hand the level of discards or changes in the relative exploitation pattern. Given these uncertainties, the result should be viewed with caution.

Discussion

The lack of contrast in the catch time series and abundance indices decreases appreciably the reliability of the result obtained in the modelling. The reliability was also affected by the use of the commercial CPUE data from Guinea and Congo in the fitting, since it is well documented that the CPUE from fisheries of pelagic schooling fish may often fail completely in tracking stock abundance. Therefore, great care must be taken when interpreting these results. However, the modelling results indicate that the stock is overexploited in the north and not fully exploited in the south.

Scientific Advice

Future research
  • To collect effort data for E. fimbriata (Nigeria).
  • To collect catch and effort data (Cameroon).
  • Given the absence of biological sampling for bonga in the subregion, countries are urged to collect biological data on E. fimbriata to enable better analysis of the status of the stock and the effect of the fishery on the stock.
  • It is proposed that countries targeting bonga should carry out research for data/information on the bonga fishery.


for Management considerations

The results from fitting the assessment model, even if they should be considered with caution, have given consistent indications on the current status of the northern and southern bonga stocks. These results show that, at the current biomass levels it is not possible to increase catches in the northern stock while in the southern stock catches can be increased.

This Working Group could not produce results on the assessment of the central stock and recommends therefore that it should not increase over the average of the last five years (19 000 tonnes).

As a precautionary measure, catch level should be reduced for the northern stock and not exceed the average of the last five years (42 000 tonnes). For the southern stock as a precautionary measure catch level should not exceed total year’s catch (1 200 tonnes).


Biological State and Trend
 
Northern stock (Guinea and Sierra Leone)
 Exploitation state: OverexploitedExploitation rate: Fcur/F0.1: 179 %
Abundance level: Bcur/B0.1: 50%
Southern stock (Congo)
 Exploitation state: Moderately exploitedExploitation rate: Fcur/F0.1: 19 %
Abundance level: Bcur/B0.1: 73%

Stock not fully exploited
Central stock (Nigeria and Cameroon)

No assessment made, but catches are stable.
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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