Long-term increase in crustacean zooplankton abundance in the southern Benguela upwelling region (1951-1996): bottom-up or top-down control?

Since 1951, zooplankton samples have been collected on the shelf along the west coast of South Africa, where pelagic fish recruit each year. Retrospective analysis of collections made during peak recruitment in austral autumn showed a significant increase in the abundance of planktonic crustaceans by two orders of magnitude (from 3.74 x 10(3) ind.m(-2) in 1951 to 7.03 x 10(5) ind.m(-2) in 1996). This long-term increase was accompanied by a shift in the community size structure, which parallelled altered regimes of anchovy and sardine, two size-selective planktivores. The increase in crustacean zooplankton could be related to the observed long-term intensification of coastal upwelling in the region and a reduction in predation by declining stocks of pelagic fish over the past four decades.