Population structure, biomass and distribution of <i>Nyctiphanes capensis</i> (Euphausiacea) in the vicinity of Algoa Bay, South Africa

Abstract

Samples taken on three research cruises along the east coast of South Africa in 1989 contained large numbers of Nyctiphanes capensis (Hansen), a euphausiid species commonly found in the cold waters of the west coast. Size-frequency distributions were determined for euphausiid populations at each station, and density and biomass calculated. Densities were found to be low close inshore and offshore in the warm Agulhas current and highest on the shelf and shelf-edge. A dense aggregation of N. capensis was analysed in greater detail to ascertain the reasons for the accumulation of euphausiids in this shelf area. This sample was dominated by females (many with brood pouches containing nauplii) with Stage 2 ovarian development. Stomach analyses showed that the majority of specimens had stomachs full of phytoplankton-derived material. It is possible that aggregations of euphausiids form to feed on high primary production associated with upwelling on the shoreward edge of the Agulhas current.