Modelling environmental effects on the early life history of the South African anchovy and sardine: a comparative approach

Abstract

By means of modelling, the South African anchovy Engraulis capensis and sardine Sardinops sagax are compared in terms of spawning, transport and survival of young stages. As there is variability in the areas across which adult anchovy and sardine spawn, two contrasting egg distritutions were modelled for each species. Dissimilarities found between the two species can be attributable to (i) differences in egg distributions, and hence in the paths along which eggs and larvae are transported passively by currents, (ii) differences in survival rates of eggs and larvae, (iii) differences in duration of stages until swimming age and (iv) differences in the duration of the spawning season. In the absence of advective losses, more anchovy reached swimming age than did sardine. However, anchovy spawned in regions that are more susceptible to advective losses. Adult biomass levels were low in the years when anchovy and sardine spawning was restricted to the South Coast. Also, advective losses of both species were greater when spawning distributions were contracted. Higher proportions of young reached the West Coast when spawning extended over the South and up the West coasts. Sardine accumulated farther west than anchovy. Anchovy eggs and larvae were lost to advection mainly on the South Coast between Cape Agulhas and Mossel Bay, whereas advective losses of sardine were restricted to areas west of the Cape Peninsula.