9.1 Apparent Effects of Fishing on the Abundance of Stock
9.2 Other Methods of Evaluation
9.1.1 Sardinella
9.1.2 Mackerel
9.1.3 Horse mackerel
Data on catches and effort were examined to determine to what extent changes in cpue could be related with changes in total fishing effort. It was hoped to plot a Schaefer type production curve but it turned out that the data were not sufficiently detailed to permit more than qualitative stock estimates.
The only adequate abundance index is that for young fish in the Senegalese fishery (medium tonnage sardine vessels).
Since these fish have not so far been recruited to the offshore fishery, their abundance will not be directly affected by fishing and any change will only reflect directly the effects of local fishing which represents about one-fourth of the total sardinella catch in the Senegal-Mauritanian zone. These tendencies may, however, include the indirect effects of heavy, offshore fishing if it should reach a level such as to affect recruitment. In this respect, the recent decline in cpue is a matter of concern, especially if one thinks of the collapses observed in other stocks of clupeids which were massively harvested. It is the opinion of the Working Group that recruitment must be carefully controlled and that,, if possible, routine information should be collected on recruitment in the northern nursery.
Available data do make it possible to undertake an analysis of catch and effort for the Senegalese fishery alone. By way of an example, in the case of Dakar sardine fishery for which statistics are complete, the ratios between cpue and effort, and between catch and effort for S. aurita are given in Fig. 3, where it can be seen that fishing has a considerable effect on the section of this stock caught by this fleet. As to S. maderensis, the data on Dakar sardine fishery alone (Fig. 4) are hardly usable because of interactions between the various sections of the fisheries (Fréon et al., 1978).
Different estimates of total fishing effort were obtained by dividing total catches by the cpue of individual fleets or by average indices for the total of documented fleets. The ratios between cpue and effort were then examined (Table 11, Figs. 6A and 7A. They show a sharp decrease in cpue as effort increases (Fig. 3). It was however admitted that this apparent ratio between cpue and effort index for the same year could be an 'artificial' result due to the method of calculating the index for total effort (it is an essential ratio between x and 1/x). Consequently, the ratio between the effort index for one year and the cpue index was also examined. These figures were calculated in a wholly independent manner so that the method of calculation should not produce any apparent ratio. In addition, for fish remaining in the fishery for more than one year, it may be expected that the fishing effort during one year may have an effect on stock abundance the following year. This correlation for different cpue indices appears in Table 11, Fig. 6B, where one will again find an evident relationship between the cpue decrease and the increase in effort.
The ratio between the indices of cpue and total effort was examined in the same manner as in the case of mackerel. The results appear in Table 12 and Fig. 8. These figures suggest that a rise in fishing effort is followed by an increase in cpue and in stock abundance. In fact the evolution of mackerel and horse mackerel populations should be studied simultaneously, bearing in mind that a change of target species on the part of most fleets shifting from mackerel to horse mackerel, for which reasons the decrease in mackerel must be overestimated. It is likely that this stock is not exploited as massively as figures would indicate. Similarly fishing must make itself felt on stocks of horse mackerel. Unfortunately, until one can count on better abundance indices for each species no estimates of these effects can be given in figures.
The data available were not sufficient for the application of other methods of resource evaluation, for example, the Beverton and Holt model for the calculation of production yield per recruit. An estimate of mortality by fishing (F) was obtained on the basis of biomass values resulting from acoustic surveys, using the following ratio:
The application of this ratio to available estimates on horse mackerel, integrating the data for both species gave a figure of F = 0.11. For fish of moderate longevity this is a low but not insignificant value which suggests that stocks are lightly to moderately exploited.
