An upwelling-induced retention area off Senegal: a mechanism to link upwelling and retention processes

Abstract

In upwelling ecosystems, fish tend to avoid spawning in areas dominated by strong offshore transport and strong wind-mixing. Spawning grounds are usually located downstream of the main upwelling centres and spawning is mostly tuned to avoid the peak of the upwelling season. However, in areas such as Peru, Senegal and Côte d'Ivoire-Ghana, spawning and upwelling occur simultaneously. What are the mechanisms that allow such reproductive strategies to be successful? To attempt to answer this question, some environmental characteristics of the spawning ground of Sardinella aurita in the Senegalese upwelling system are investigated. S. aurita adults migrate from Mauritania to Senegal in early winter, following the southward extension of upwelling along the coast. Intense spawning in late spring is concentrated south of Cap Vert. There, the surface distribution of several environmental parameters is quite unusual for an upwelling area. Sea surface temperature is minimum over the shelf and increases towards and away from the coast. Chlorophyll distribution peaks nearshore. This unique surface structure is interpreted as the result of a "double cell" structure in the upwelling vertical circulation: a first cell located at the shelf break, the main upwelling cell that brings cold and nutrient-rich subsurface water to the surface, and a second cell located inshore of the shelf break. A nearshore convergence tends to trap and retain phytoplankton and other biological components. In an upwelling system with a double-cell circulation structure, the coastal cell represents a very favourable environment for fish to reproduce: eggs and larvae do not spread offshore, but are retained in the productive and relatively stable coastal waters. Clearly, therefore, upwelling is not always detrimental for larval survival. Rather, upwelling and retention can occur simultaneously and together provide a favourable reproductive habitat.