A frame-based modelling approach to understanding changes in the distribution and abundance of sardine and anchovy in the southern Benguela

A number of ecologically and economically important species in the southern Benguela, including the forage fish sardine Sardinops sagax and anchovy Engraulis encrasicolus, have undergone southward/eastward shifts in their distribution in the 1990s/early 2000s. In addition to the effects of changes in prey availability to top predators, the spatially-distinct nature of the system means the location of a stock has implications for its productivity. The spatial dynamics of small pelagic fish are of particular importance because they are thought to exert wasp-waisted trophic control on the system. An objective-driven frame-based model was constructed to investigate the ability of the approach to represent spatial and population dynamics of sardine and anchovy, and to explore the implications of possible management strategies. Climate variability and fishing pressure were assumed as drivers. Sensitivity analyses were performed and a number of scenarios tested. A frame-based approach appears to be useful within this context. Results suggest that the productivity of the sardine resource within the model is highly dependent on the spatial characteristics of fishing pressure. The role of anchovy within the model system has not yet been fully developed. Increasing our understanding of the relative suitability of environmental conditions of different regions is also important if we are to increase our capacity to predict trends in abundance and distribution. This study shows support for continued careful consideration of spatialized management approaches to the South African sardine fishery.