The Schaefer logistic production model was used on an Excel worksheet.
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
|Ethmalosa fimbriata/North/CPUE Guinea artisanal
|Ethmalosa fimbriata/South/CPUE Congo artisanal
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.
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 AdviceFuture research
for Management considerations
- 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.
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).