Until recent decades, it was believed that the boundary between sardine and sardinelle populations was located in the vicinity of Cape Blanc (22 degrees N). To the north of this cape, strong upwellings occur throughout the year and although primary production is high, the zooplankton remains low, due to the speed with which it is horizontally transported away. To the south, the upwellings are seasonal and shelf circulation consists of two layers circulating in opposite directions. A short time lag exists between primary and secondary production, as a result of zooplankton migration between these two currents. Sardines and sardinelles arc both plankton feeders. Phytoplankton can, in some cases, make up the bulk of the sardine's diet. On the contrary, sardinelles feed mainly on zooplankton. These feeding habit differences favour sardine settlement in the strong upwelling areas. During the 70's, the sardine fishery moved southwards. This can be related to an intensification of the trade winds, occurring over the same period, at their most southern boundary. This led to a strengthening of upwellings along the Mauritanian and Senegalese coastlines and consequently caused a modification in the biotope favouring sardine settlement. Secondly, undercurrents along the shelf were altered due to the decrease in those moving northwards and an increase in those moving towards the south. As a result, a new distribution pattern of larvae can be expected, giving rise to more sardines drifting towards southern waters, and fewer sardinelles being carried to the north.
