Simulating anchovy–sardine regime shifts in the southern Benguela ecosystem

Trophic models of the southern Benguela ecosystem have been developed to represent average ecosystem structures for two periods: 1980–1989 and 1990–1997. Ecopath with Ecosim software is used to simulate changes from the 1980s to the 1990s ecosystem structure. Two hypotheses are tested of mechanisms that could cause the changes. First, using the model of the 1980s, four scenarios are considered in which different combinations of fishing mortality rates of sardine, anchovy and horse mackerel are changed to mimic the situation in the 1990s model. Results show that it is unlikely that observed changes in pelagic fish catches between the 1980s and 1990s played a large role in driving the changes in abundance of anchovy and sardine. Second, changes in the susceptibility of phytoplankton and zooplankton prey to feeding by anchovy and sardine are simulated for the two decades. Results show that shifts between anchovy and sardine regimes in the southern Benguela ecosystem may be caused by changes in the availability of mesozooplankton prey to anchovy and sardine, mediated through changes in environmental conditions. The magnitudes and duration of environmental changes are important in understanding changes in the ecosystem.