Influence of feeding conditions on breeding of African penguins—importance of adequate local food supplies

Abstract

Off the coast of South Africa, the 2 most important prey items for African penguins Spheniscus demersus are sardine Sardinops sagax and anchovy Engraulis encrasicolus, species heavily exploited by commercial fisheries. At Dassen Island, the proportion of burrows occupied for breeding by African penguins from 1995 to 2008 was affected positively by the interaction between the overall biomass of sardine and anchovy. However, sardine had a positive effect while anchovy had a negative one. This unexpected negative relationship may result from a reduced local availabil- ity of anchovy linked to the fishing effort that continued to be focussed in the vicinity of Dassen Island, while the growing anchovy stock experienced an eastward shift away from Dassen Island in recent years, creating a spatial mismatch between penguin and available anchovy. A decrease of the Southern Oscillation Index tended to advance the onset of penguins' breeding, as did an increase in sea surface temperature (SST) at the sardine and anchovy spawning grounds. An increase in SST at the nursery and spawning areas tended to decrease the penguin per capita growth rate. Earlier breeding resulted in decreased chick production, possibly because it created a temporal mismatch between chick rearing and winter availability of young-of-the-year anchovy and sardine along South Africa's west coast. The local abundance of prey is more important for breeding African penguins than overall fish abundance. Consequently, management of the purse-seine fishery should be adjusted spatially in order to ensure adequate local food supplies for breeding African penguins.