7.1 Catch and effort analysis
7.2 Cohort analysis
7.3 Yield per recruit
7.4 Recruitment
7.5 Biomass estimates by acoustic surveys
7.1.1 General
7.1.2 Zone A
7.1.3 Zone B
7.1.4 Zone C
Data on catch by fishing zone are given in Table 6 of Annex III. Data on fishing effort are available for the Moroccan fleet fishing in Zone A, the Spanish fleet in Zone B, and the Polish fleet fishing in Zone C. The Group decided to make catch/effort analyses for the fishery in each zone separately, which analyses would provide information on the state of exploitation and potential yield by zone if each zone contains its independent sardine stock. Unfortunately, the available data did not allow a catch/effort analysis for the sardine fisheries in all the areas together, on the assumption that all sardines belong to the same stock.
Table 3 gives Moroccan catch and effort data for this zone. It should be noted that the catches are different from those given in Table 3 of the Report of the Third Session of the CECAF Working Party on Resource Evaluation, due to the exclusion of catches made outside the Safi and Agadir area, because of the indications that the sardine in the Casablanca area may belong to a separate stock (section 4.8). The effort has been calculated for the fleets in Safi, Essaouira and Agadir and is expressed as the sum of the products of the number of trips and the gross tonnage of all vessels in the fleet.
As has been noted in earlier reports, the effort has increased considerably during the period 1969-1976, whereas the catches have remained fairly constant, with some large irregular fluctuations which are believed to have been related with fluctuations in fish availability and abundance due to climatic factors.
If Zone A contains a separate stock, Figure 2 shows the relation between effort, catch per effort and total catch. The points on the graph show a fairly large dispersion, probably due to the variability in fish availability. However, it has been possible to calculate the regression line between the catch per unit of effort and the average effort of the year of observation and the previous year (Gulland model), and also between the logarithm of the catch per effort and the average effort of two years (Fox model). The models suggest that if the fish in Zone A belong to a separate stock, the average potential yield of this stock is between 204 000 and 250 000 tons, which will be obtained with an effort of between 1 650 and 2 170 units. According to these results the present fishing effort of 1 845 units has already reached, and perhaps passed, the level of effort which gives the average production of the stock.
The graphs may have been somewhat affected by the fact - mentioned before in section 5.1 - that the catch does not only depend on the effort, but that also the effort tends to be higher in years of high fish availability. Whereas, therefore, the conclusions on the optimum catch and optimum effort have to be considered with some caution, at any rate the fact that the catch has not shown a definite trend with a continually increasing level of effort suggests strongly that the stock in this zone cannot produce, on the average, more than it has produced in the past ten years.
Table 4 shows the catches in this area and the catch per unit of effort of the fleet of the Canary Islands. The table shows that the catches in this area have continuously increased, as did the catches per day fishing. It is believed, however, that this increase in catch per day fishing does not reflect an increase in the stocks in this zone, but rather an increase in the size and efficiency of the fishing vessels.
A better estimate of the change in stock size would probably be obtained from the data of catch per day fishing per ton of fishing vessel. It is therefore suggested that the Spanish scientists try to obtain information on the size of the fishing vessels in the past years to make the necessary corrections. Until such data are available, no reliable catch/effort analysis is possible for this zone.
1 See Section 10, PostscriptThe data for this zone are given in Table 5. The catches have increased very rapidly from about zero to over 200 000 tons in six years. Data on catch per day fishing are available for the type B23 trawlers of the Polish fleet for the years 1972 to 1977. As has been mentioned in the 1976 CECAF working party report, the catch per day fishing of these vessels was substantially affected by saturation phenomena and therefore reflects more the quantity of fish which the ships could process per day than the quantity they could catch. New Polish data on the catch per hour trawling of the type B18 trawlers were not believed to be affected by vessel saturation and were therefore believed to better reflect the fish densities. These figures are given in Table 5 and have been used to estimate the total effort. Analysis of these data suggests that the maximum yield of the stocks would be in the range of 190 000 to 220 000 tons, to be obtained with between 45 000 and 70 000 Polish effort units (hours trawling). These data suggest that if the sardine in Zone C belong to a separate stock, this stock is nearly fully exploited (Fig. 3).
An important improvement in the data base for population studies of the sardine resources, as compared with the 1976 CECAF working party meeting, has been the provision of age composition data for the catches by Morocco, Poland and Spain. Uniformity of and agreement on the age determination of the sardine had been obtained between the scientists concerned of Morocco and Spain, by previous exchange of age reading material.
The available data included:
(a) catch of S. pilchardus by the Moroccan fleet in Zone A, expressed in numbers (x 103) of each age group by three-monthly periods, from the third trimester in 1973 up to the third trimester in 1977 (Annex IV, Table 1);on the hypothesis that each zone contained an independent stock of sardine. Also, an analysis could be made for Zone A and B together, assuming that these zones contained the same sardine stocks. Unfortunately, the available data did not allow carrying out an analysis for all three areas together.(b) catch of S. pilchardus by the Spanish fleet in Zone B, expressed in numbers (x 103) of each age group by three-monthly periods, from the third trimester of 1975 up to the first trimester in 1977 (Annex IV, Table 2);
(c) total annual catch of S. pilchardus in Zone C expressed in numbers (x 103) by year class (Annex IV, Table 3)
The Moroccan data covered practically all fishing in Zone A in the period under study. For the analysis, an annual natural mortality rate (M) of 0.8 was assumed (M = 0.2 for a three-monthly period) on the basis of data in the literature on the natural mortality of this species. Terminal annual fishing mortality values (Ft) of 0.8 and 1.2 (0.2 and 0.3 for a three-monthly period) were applied. The two assumed F values gave rapidly converging F estimates for the preceding trimesters, and also gave very similar F values for the same quarters in successive years. On this basis, and taking into account that the terminal quarter used in the analysis is a period of rather heavy fishing, the value for Ft of 0.2 for the three-monthly period was accepted. The sum of the quarterly mortality data obtained from the analysis gave the estimate of the annual mortality rates shown in Table 6. Table 7 shows the number of fish of each age present at the beginning of the years 1974, 1975 and 1976.
Bravo de Laguna et al. (1977) applied the same method for the Spanish catches in Zone B. No other fleets fished in this zone during the period. The authors used annual natural mortality rates of 0.6 and 0.8 but the results were rather similar in both cases. Three values for terminal fishing mortality rate were used. Comparison of the results obtained with these values for the successive years and the consideration that the terminal quarter in this study, the first quarter of 1977, was a period with considerably less fishing than the year before, led to the conclusion that an Ft = 0.1 would be the most likely value for that period. The results of this analysis are included in Tables 8 and 9.
In the cohort analysis for Zone A and B together, on the assumption that these two zones together contain one stock, the same reasons as above led to the assumption of the following values:
Ft = 0.1 or 0.2 (three-monthly rate), M = 0.8 (annual rate).Tables 8 and 9 contain the results of this analysis.
Finally, a document presented to the working group by Krzeptowski (in print) describes a similar analysis based on the annual catch data for Zone C. In this case a natural mortality rate was assumed of M = 0.5, and three different annual mortality rates were assumed for Ft, 1.0, 1.4 and 1.6. Again, the different values of Ft gave rather similar results. Table 8 combines all the data on annual fishing mortality rates obtained in these studies for 1976 and Table 9 gives the numbers of sardines in the stock at the beginning of 1976. The Polish values given in these tables are obtained with Ft = 1.0.
Table 8 shows that in Zone A fishing mortality is heavy from age groups I onward, whereas in areas B and C the fish is not fully recruited until they reach age group III. It would appear that in 1976 the fishing mortality rate for the fully recruited age groups has been of the same order of magnitude in all three zones.
In considering these results, it should be taken into account that the value of M used in the analysis for Zone C was lower than that used for the other areas. If, in the calculations for Zone C, a natural mortality rate of 0.8 had been used, the same value as used in Zone A and B, the estimated fishing mortality rate in this zone would have been lower. Notwithstanding this reservation, it can be concluded from this analysis that in all areas the fishing mortality rate has been rather high, of about the same magnitude as the natural mortality rate. The average value found for all fully recruited age groups in 1976 was for Zone A (age groups II-IV) 0.73, for Zone B (age groups III-VI) 0.78 and for Zone C (age groups III-VI) 0.97.
The fishing mortality rates found by the cohort analyses showed that the fishing mortality rate was about equal to the natural mortality rate and the exploitation rate E can therefore be taken to be about 0.5. This observation has been used to study the effect of changes in the total fishing effort on the yield per recruit. K has been estimated at between 0.35 and 0.98 (section 4.4), M at 0.8, and hence M/K will lie between 0.81 and 2.3. The growth equation shows that fish of two years old have already completed 70 percent or more of their growth in length.
The yield tables show that for all values of M/K higher than 0.75, C = 0.70 or higher and E = 0.5, the yield per recruit can be substantially increased, although this would require a considerable increase in effort. Thus, if M/K = 0.1, C = 0.70, an increase in yield per recruit of 32 percent can be obtained by an increase in fishing effort of 80 percent.
The cohort analysis provides an estimate of the number of fish of each year-class at the beginning of the recruitment of that age group to the fishery, and therefore allows determining changes in recruitment. Unfortunately, the number of years of observation in the Moroccan and Spanish fisheries is too low to determine any trend in the recruitment in the successive years.
Provisional Polish data suggest a substantial decline in the recruitment in the last five or six years but these data require confirmation.
There is need for further studies on the changes in recruitment in the area.
An acoustic survey carried out in November 1974 between Pointe Stafford (27° 40’ N) and Cape Cantin (32° 35’ N) gave an estimate of biomass in that area of about 1.4 million tons (Johannesson, Villegas and Lamboeuf, 1975). A recent survey carried out in January 1977 (Lamboeuf, 1977) gave an estimate of the biomass between Cape Cantin and Oued Drâa of 325 000 tons and between Oued Drâa and 27° N of 715 000 tons, total 1.04 million tons. Taking into account that the 1977 survey covered a slightly larger area, it would thus appear that the biomass of sardine has decreased between November 1974 and January 1977. However, Lamboeuf (1977) estimated the amount of pelagic fish other than sardine which may have been included in the 1974 survey results and arrived at an estimate for sardine from that survey of 1.1 million tons, very close to the 1977 survey estimate.
