Assessment of an environmental barrier to transport of ichthyoplankton from the southern to the northern Benguela ecosystems

The Lderitz upwelling cell and Orange River cone (LUCORC) area, a transboundary region between South Africa and Namibia, is considered to be an environmental barrier to transport of ichthyoplankton from the southern to the northern Benguela upwelling ecosystems. We use environmental data and modelling to assess the potential mechanisms responsible for this barrier: environmental data were extracted from the 1 1World Ocean Atlas 2001 database and used to build maps of annual mean salinity, temperature, chlorophyll, dissolved oxygen and nutrient concentrations; outputs of a regional circulation model were used in an individual-based model to assess the transport of passive particles from the southern to the northern Benguela. The data show no clear environmental barrier at sea surface, but the model results suggest that particles released there would be largely transported offshore. The model also shows that particles released below the surface could be transported alongshore from the southern to the northern Benguela, but low subsurface temperatures would increase ichthyoplankton mortality and hence be a strong limiting factor to northward transport. We conclude that the combination of a surface hydrodynamic and a subsurface thermal barrier could limit the possibility for ichthyoplankton of epipelagic species to be transported from the southern to the northern Benguela, but that ichthyoplankton of mesopelagic species, having a wider tolerance to low temperatures, would be less affected.