Zooplankton spatial distribution along the South African coast studied by multifrequency acoustics, and its relationships with environmental parameters and anchovy distribution

Abstract : The Central Agulhas Bank (CAB) is an important component of the southern Benguela ecosystem. Despite relatively low primary production, secondary production supports large populations of mid-trophic-level pelagic fish. Exhaustive sampling of the Agulhas Bank ecosystem was performed during a routine acoustic biomass survey in November 2006. A TAPS-6 was deployed with near-simultaneous, plankton-net sampling. Concurrent collections of fish-school and environmental data permitted a detailed study of the water column. Zooplankton was classified by equivalent spherical diameter (ESD; mm): 0.0-0.35 (Class I), 0.35-0.8 (Class II), 0.8-2 (Class III), and >2.0 mm (Class IV). Clear relationships with environmental parameters were only evident at night on the CAB (west of 23 degrees E) for Classes II and III. Class III exhibited pronounced diel vertical migrations, whereas Class I exhibited a reverse pattern. Also observed were the effects of anchovy predation on small zooplankton aggregated in areas of high chlorophyll a.