There have been marked shifts, influenced by the environment, in the distribution and abundanceof middle-sized, and smaller, pelagic fish in the South-east Atlantic Ocean. The changes have often been sustained over several years, giving rise to regimes in the structure of ecosystems. Global environmental change may initiate and sustain such regimes through a steady modification of habitat, or through episodic events that cause large changes in populations. Preliminary simulation studies indicate that altered advective processes off South Africa have little influence on mean year-class strengths of fish that spawn above the thermocline, as advective losses account for only a small fraction of the overall mortality of young stages. However, changed flow may alter the geographic distribution of young of the year.