High adult mortality of African Penguins Spheniscus demersus in South Africa after 2004 was likely caused by starvation

Abstract

From 2004–2011, following the collapse of sardine Sardinops sagax, a main food for African Penguins Spheniscus demersus, to < 25% of its maximum recorded abundance, survival of penguins breeding at Dassen and Robben islands, north of Cape Town, South Africa decreased markedly. Based on numbers of penguins breeding at the two islands and observed increased levels of mortality, it was estimated that ∼62 000 breeding individuals (∼95% of the birds that elected to breed in 2004) died as a result of food scarcity in this eight-year period. African Penguins moult annually, coming ashore and fasting for 21 days, when they shed and replace all their feathers. Failure to fatten sufficiently to moult, or to regain condition afterwards, results in death. At both islands, annual survival rates of adult penguins, and proportions of breeders that failed to return to their colonies to moult, were significantly related to an index of prey availability developed for the region. Although some adults moulted at a colony to the southeast, where food may have been more plentiful, much of the mortality likely resulted from failure of birds to fatten sufficiently to moult. The fishery exploitation rate of sardines west of Cape Agulhas was consistently above 20% between 2005 and 2010, peaking at 80% in 2006. In 2024, African Penguins were uplisted to Critically Endangered on the IUCN Red List. Management that helps to promote the long-term recovery of sardine biomass in the main foraging areas of African Penguins throughout their annual cycle will be important in securing their future survival.