1 See Section 10, PostscriptIt is still not clear from the preceding analysis whether there are one or more separate stocks of sardine in the fishing zones between the Strait of Gibraltar and Cape Blanc in Mauritania. The results must therefore be considered in the light of the two hypotheses, the existence of one stock, or of several stocks, in the area.
The catch effort analysis indicated that, if each zone has its separate stock, in Zone A the stock is fully exploited and in Zone C the stock is exploited at a level which produces a catch fairly close to the maximum potential yield. No estimates could be obtained for Zone B.
The cohort analysis indicated that the fishing mortality rate is roughly at the same level in all three zones. If, therefore, the stocks in Zone A and C are exploited close to their maximum level, the same would be the case in Zone B.
The application of a yield per recruit model showed that the yield per recruit could increase substantially by heavier fishing. If, therefore, the conclusion from the catch and effort analysis that fishing takes a quantity of fish close to the potential yield of the stocks is correct, fishing must have caused a decline in recruitment. The information on year-class strength available so far suggest that such a decline may indeed have taken place.
Thus, the results obtained so far are not inconsistent, and suggest that the stocks were about fully exploited in 1976. In view of the observation that the fishing mortality is estimated to be at roughly the same level in all zones, the conclusion would apply both in the case that only one stock exists in the area and in the case of a number of separate stocks.
The acoustic survey provided an estimate of the biomass of sardine in Zones A and B in January 1977. Biomass estimates can also be obtained from the results of the cohort analysis by multiplying the numbers of fish at each age by the average weight of a fish of that age. In addition, an estimate of the biomass of the exploited stock of sardine can be obtained by applying the equation
B = C/F
(B = biomass, C = catch and F = fishing mortality rate).
Data on the average weight of sardine at different ages in the first trimester of 1977 are given by Rami (unpublished), calculated from the average size by age group and the length-weight relationship in the first trimester in Zone A. The data are given in Table 10. These data were used in the biomass estimates in each zone. For the calculation of the exploited stock size, F has been taken to be 0.8 in all three zones.
The results of the various estimates are given in Table 11. The table shows that there are substantial differences between the estimates obtained with the various methods. However, it should be taken into account that the method using the catch and fishing mortality rate data determines the exploited stock biomass only, and that the other methods estimate all year-classes, also those which are not yet fully recruited. Using the cohort analysis data for calculating the exploited stock size, figures of 311 000, 206 000 and 267 000 tons are obtained for Zones A, B and C respectively, which are rather closer to the estimates in the last column of Table 11. It should be realized that the cohort analysis and the catch and fishing mortality rate method are not independent, the fishing mortality rate having been obtained from the cohort analysis and that, therefore, similarity of the results could have been expected.
The large difference which remains is that between the estimate from the acoustic survey and the one from the cohort analysis. There may be several reasons for this difference. The cohort analysis estimates the size which the whole stock, exploited in a certain area, has on a certain date, whether or not the fish at that date are in the area. The acoustic survey measures the fish actually in the area at that time. If part of the stock of Zone C moves into Zone B in winter, this would have been included in the acoustic survey estimate for Zone B. Also, the acoustic survey results may give an overestimate due to inclusion of other pelagic fish, to bias in the calibration estimates, or otherwise.
On the other hand, the estimates of fishing mortality rate may be too high due to mistakes in the assumptions on terminal fishing mortality rate or in natural mortality rate. That, in its turn, weakens the conclusion on the similarity of the level of fishing in the three areas.
