Assessment Model
Type: Biomass-aggregated
The Schaefer logistic production model was used on and Excel spreadsheet (Appendix II) to assess the two stocks, A+B and C. The Working Group produced a forecast of abundance and catch for the next five years following different management scenarios using the same model on another Excel spreadsheet (Appendix II).
Overall Assessment Results
The fit of the model was judged to be satisfactory for the two zones, A+B and C (Figures 2.6.3a and b). For zone A+B, the results indicate that the current stock biomass is slightly above the target biomass B0.1. The level of exploitation remains below F0.1. The Bcur/B0.1 ratio shows that the stock is fully exploited.
These results show a different situation for zone A+B than that of last year. Current biomass is greater than the biomass balance. This change is due to the recovery of the biomass in 2007.
The results for zone C show that estimated biomass in 2007 is well above the target biomass of B0.1 and the current level of exploitation is below F0.1. The Bcur/B0.1 ratio shows that the stock is under-exploited.
Table 1. Summary of the results of fitting the logistic production model
|
Stock/Abundance index
|
Bcur/B0.1
|
Fcur/FSYcur
|
Fcur/FMSY
|
Fcur/F0.1
|
|
Sardine, zone A+B/Nansen
|
110%
|
67%
|
53%
|
59%
|
|
Sardine, zone C/Nansen
|
142%
|
37%
|
16%
|
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.
Projection
The Working Group proceeded with a projection of catch and abundance over the next five years following different scenarios for each of the stocks A+B and C.
For zone A+B, two scenarios were used. Maintaining fishing effort at its present level leads to an increase in catch in 2008. But this catch remains, nonetheless, below F0.1, followed by a stability in the subsequent years. As for the biomass, this tends to stabilise itself around the current 2007 level. An increase of 10 percent in current effort would allow for an increase in catch in 2008, followed by stability in the catches at a slightly lower level. As far as abundance is concerned, the projection shows a slight decrease in 2008, stabilising itself over the following years and maintaining a level above the target biomass B0.1 (Figure 2.7.1a).
For zone C only one scenario was used, that of maintaining effort at current levels. Catch increases but remains at a relatively weak level compared to F0.1. Abundance increases, stabilising over the following years to a higher level than the target biomass B0.1 (Figure 2.7.1b).
The projections should be considered with a great deal of precaution taking into account the impact of the environment on abundance and stock dynamic. Large variations in abundance could in fact be unrelated to the fishery.
Scientific Advice
Future research
Follow up on last year’s recommendations
- An improvement in the sampling intensity of the different Moroccan fisheries can be seen in 2007. For Mauritania, even though sampling intensity increased on board the Russian fleet, sampling did not cover the whole year.
- The recommendation that sardine be measured in total length to the ½ cm below was not respected by a certain sector of the Mauritanian and Senegalese fisheries which continue to measure to the 1 cm below.
- The programme of exchanges of age reading was continued in 2007.
- Estimates of abundance indices were continued throughout the whole region in October–December 2007 by the national research vessels.
- Length frequency analysis with a view to assessment by structural models was not carried out.
Future recommendations
- Improve sampling of the different fisheries covering all quarters of the year, especially in Mauritania and Senegal.
- Sardine length should be measured to the ½ cm below for all fisheries.
- Continue the programme of age reading exchanges.
- Continue the estimation of abundance indices for the whole region in October–December.
- Organize a Working Group to analyse the length frequency data.
for Management considerationsStock A+BThe results show that the A+B stock is fully exploited. Taking into account the fluctuations that this stock has witnessed and as a precautionary approach, sardine catch in this zone should not exceed 400 000 tonnes in 2009.
Stock CThe results show that the stock is underexploited. This stock offers the potential to increase catch. However, as a precautionary approach, given the instability noted in pelagic stocks, the structure and abundance of the stock should be continually monitored, independently of the data on commercial catches, so as to be able to adjust levels in case of unforeseen changes in fishing effort.
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