The hydrography is characterised by two general phenomena, the northeast trade winds and the current flowing from northeast to southwest (Canary current). The trade winds cause upwelling of water from deeper water layers to the surface at several places along the coasts. The anticyclone of the Azores is particularly marked during the northern summer, whereas the high pressure areas spread out latitudinally in winter (Le Floch, 1974). This causes a seasonal fluctuation in the trade winds which originate near the Gibraltar Strait in summer and further south in winter. Along the desert coast, from Sidi Ifni to Cape Blanc in Mauritania, the trade winds are nearly permanent but become weaker from October to January. These fluctuations affect the zones of upwelling, which appear more southward in winter and more northward in summer, with the result that there is a great variation in the distribution of temperatures and salinities in space and time.
North of Cape Juby the phenomena of upwelling have a marked seasonal character and a rather limited extension. South of this cape, upwelling is observed during the whole year with fluctuations in the seasonal intensity, and a maximum from April to September; the upwelling area may reach a length along the coast of 120 miles, but is restricted to the area close to the coast (A. Cruzado, 1974; Le Floch, 1974).
These coastal upwellings cause an enrichment of the surface water with nutrient salts, which results in an intensive development of phytoplankton, followed by zooplankton. The production in each zone varies with the seasonal variation in the upwelling; the primary and secondary production is much more important south than north of Cape Juby (Grall et al., 1974; León, Braun and Escánez, 1974; Estrada, 1974).
The seasonal character of the upwelling north of Cape Juby affects the migration and the availability of the sardines in Zone A.