5.1 Hydrological Description
5.2 Influence of Hydrological Conditions on the Biological Cycle of the Species
The zone under study belongs to two great hydrological systems, one of a temperate nature covering the northern part as far as 10°N, the other equatorial, to the south of the Bissago Islands.
The northern system is governed by the upwelling connected with the system of the Canary currents and marked by clear distinction between a cold season and a warm season at the level of the continental shelf. The first season, moving from north to south, begins between October and December and ends up moving from south to north in May and June. It coincides with the period of maximum southward expansion of the upwelling involving the displacement of a thermal front from Cape Blanc to the Bissago Islands. During the hot season the trade winds subside, giving place to a west to southwest wind pattern, bringing about the cessation of upwelling. The thermal front then moves as far as Cape Blanc (Rébert, Annex 9, Figs. 1 and 2).
Variations in upwelling intensity have been known since 1959; between 1959 and 1963 upwelling was intense, weak from 1964 to 1970 and once again strong between 1971 and 1974. It decreased between 1975 and 1976 (Sedykh et al., Annex 10).
The southern system shows very marked thermal contrasts and is characterized by the occurrence of a four hydrological seasons (two warm and two cold seasons) of varying duration. The importance of the second cold season, centered in the month of August, gradually increases toward the east (Annex 9 Fig. 3) and becomes dominant after Cape Trois Pointes (Berrit and Rébert, 1977).
Soviet researchers (Sedykh et al., Annex 10) have studied the relationships between the hydrological conditions and the biological cycle of various species. Among fish, close relationships have been demonstrated in various aspects of the biological cycle. In particular, the variations in the intensity of upwelling have been correlated with variations in the period of sexual maturation, rate of growth, abundance of age groups and the total biomass.
Studies of the maturation period have been made on Sardina pilchardus and Scomber japonicus which have shown that with more intense upwelling maturation is more precocious. In the case of the first species growth would seem to be faster when upwelling is stronger. Similarly, good correlations have been demonstrated in recent years between year to year variations in the abundance of age groups and the total biomass of this species, on the one hand, and indices of upwelling intensity, on the other.
Studies have been made in Senegal as to possible relationships between variations in cpue of Sardinella aurita by Dakar sardine boats and various parameters of the physical environment of the Senegal-Mauritanian zone, such as surface temperature, salinity, rainfall, river flow and wind components affecting upwelling. As a result, significant correlations have been found between cpue variations and variations in intensity of winds causing upwelling as measured at Nouadhibou, but their interpretation is difficult. Parameters do not seem to be clearly related to the characteristics of the environment other than, to a certain extent, surface temperatures which probably affect the availability of the part of the stock under study (Rébert, Annex 11).
