6.1 Availability of Biological Data
6.2 Analysis of Biological Data
6.1.1 Length measurements
The only new measurements are monthly (or bimonthly) sardinella data, not extrapolated to the total catch. In the case of Ivory Coast the data relate to catches of seiners during the period from February 1978 to August 1979, and to an experimental trawling survey in 1978 (Tables 10, 11 and 12, Figure 6). The measurements carried out in Ghana correspond only to catches by the industrial seiner fleet and a research trawler (Tables 13, 14, 15 and 16, Figure 6).
Since monthly catch data were not available for Ghana at the time of the meeting, size frequencies have not been extrapolated to the total landings of seiners in Ivory Coast and Ghana. The Ghanaian delegation promised to perform the calculations after the meeting (Appendix 5).
There are no new data on sardinella measurements concerning the artisanal fisheries (Table 17).
6.1.2 Other biological data
Other information concerns studies on zooplankton and ichthyoplankton (the eggs and larvae of S. aurita) collected on the Tema transect from 1976 to 1978 (Tables 18 and 19). Only a small number of samples, particularly for the last two years, are available.
In Ghana, data collected for the calculation of gonadosomatic indices have not been processed and in Ivory Coast collection of such data has only recently commenced.
6.2.1 Analysis of length measurements
Cohort analysis cannot be undertaken with available data. The evolution in sizes within the samples obtained from the catches of Ghanaian seiners (Table 20) indicates that, contrary to what might be expected, the catches, following recovery of the stock, did not contain juvenile or young individuals as caught in the preceding years. On the contrary, the modal size of the predominant age classes in catches since 1975 is generally higher than that of the 12 preceding years (18 to 19 cm now as against 14 to 18 cm). However, these data only concern the industrial fishery and it is likely that the Ghanaian artisanal fisheries (beach seines and poli net) caught the young individuals.
Comparison of modal sizes of S. aurita caught by Ghanaian and Ivorian seiners (Figure 6) indicates that sizes of fish landed by the Ivorian seiners from July to October 1978 are on average larger (about 2 cm); this corresponds to about 9 months age difference using Marchal’s (1974) growth curve. It is difficult to explain this phenomenon if we admit that there is only one S. aurita stock off Ivory Coast and Ghana and also the usual migratory pattern (Ansa Emmim in ORSTOM, 1976). Two possible explanations were proposed by the working group:
(i) biased measurements (taken as fork length and to the centimetre below in both countries) or unreliable data;6.2.2 Analysis of plankton data(ii) three possible causes for a segregation between the fish caught by Ivorian and those caught by Ghanaian fleets:
- exploitation of two different sub-stocks by the two fleets which during the period under study worked respectively in the Ivorian sector of Bassam-Axim and in the Ghanaian sector off Tema-Winneba (unlikely hypothesis);- exploitation of two different cohorts in the two sectors, though using same gear and probably fishing the same depths. It will be noted in this context that the Ghanaian research trawler operating further out at sea than seiners caught bigger individuals than those landed by the seiners of the two countries; this agrees with the normal bathymetric distribution of S. aurita;
- exploitation of the same cohort within the two regions but having different growth rates for reasons unknown.
No firm conclusions may be drawn from the data obtained from plankton surveys due to inadequate coverage, particularly with regard to ichthyoplankton. In fact, studies on S. aurita conducted in the Congo (FAO, 1979d) indicate a very heterogeneous distribution of eggs and larvae in time and space and a large variability in the localization of concentrations between years. Despite these reservations, if the available data were representative they would indicate on the one hand that the low zooplankton abundance observed from 1969 to 1975 has not persisted (Table 21) and on the other that spawning has remained low since 1974. This last point contradicts the appearance in 1978 of a strong yearclass in the Ghanaian industrial fishery of individuals of about 2 years old. The difference observed in the localization of eggs and larvae of the S. aurita suggests that spawning takes place near the coast and that the larvae are moved offshore by currents. Recent work carried out in Ghana on phytoplankton population abundance, primary production and chlorophyll measurements has shown that these parameters vary in the same way as upwelling intensity (Anang, personal communication).
6.2.3 Recommendations for biological studies
The working group strongly recommends that:
(i) routine length measurements be carried out or continued for all fisheries in the three countries and that length frequencies be extrapolated to the total catch before meetings;(ii) routine collection of biological data on reproduction be undertaken to avoid the existence data which often cannot be used; the minimum requirement is a study on gonadosomatic relationships;
(iii) further ichthyoplankton studies be combined with a broader coverage of spawning period and areas;
(iv) It would be useful to study the relationship between the abundance of phytoplankton and feeding of juvenile S. aurita.
