CROSS-SHELF CIRCULATION, ZONATION AND MAINTENANCE MECHANISMS OF NYCTIPHANES-CAPENSIS AND EUPHAUSIA-HANSENI (EUPHAUSIACEA) IN THE NORTHERN BENGUELA UPWELLING SYSTEM

Horizontal and vertical cross-shelf distribution of developmental stages of the inshore euphausiid species Nyctiphanes capensis and the shelf-break species Euphausia hanseni during different phases of the upwelling cycle in the northern Benguela upwelling system are analysed, in order to provide an explanation for the consistent distribution patterns observed. A two-cell cross-shelf circulation model is proposed, based on upwelling in inshore areas as well as at the shelf break, probably reinforced by the possible existence of a strong equatorward current at the shelf break. It is concluded that both euphausiid species complete their entire life cycle within discrete zones across the shelf and slope regions. It was observed that the interaction of biological processes, especially ontogenetic migration, with the physical processes during different phases of the upwelling cycle resulted in differential probabilities of cross-shelf and alongshore transport by N. capensis and E. hanseni, and the mechanisms involved in the segregation and maintenance of the species in discrete zones of the northern Benguela upwelling system outlined.