Implications for seabirds off South Africa of a long-term change in the distribution of sardine

Abstract

From 1997 to 2005, the distribution of sardine Sardinops sagax, an important prey item for four seabirds off South Africa, shifted 400 km to the south and east, influencing its availability to breeding birds. It became progressively less available to seabirds in the Western Cape Province, where the number of African penguins Spheniscus demersus breeding decreased by 45% between 2004 and 2006, survival of adult penguins decreased and penguins established a new eastern colony in 2003. In that province, the number of Cape gannets Morus capensis breeding decreased by 38% between 2001/2002 and 2005/2006 and the contribution of sardine to the diet of gannets fell from an average of 40% during the period 1987–2003 to 5–7% in 2005 and 2006. The proportions of Cape cormorants Phalacrocorax capensis and swift terns Sterna bergii breeding in the south of the province increased as sardine moved south and east. In the Eastern Cape Province, the number of penguins breeding halved between 2001 and 2003, whereas after 2002 there was an increase in the number of Cape gannets that bred and in the contribution of sardine to their diet. It is likely that in that province sardine became increasingly available to gannets but remained beyond the shorter feeding range of penguins. Management measures that may mitigate the impacts on seabirds of an unfavourable, long-term change in the distribution of their prey include the provision of breeding habitat where prey is abundant, spatial management of fisheries competing
for prey, and interventions aimed at limiting mortality.

Keywords: African penguin; Cape cormorant; Cape gannet; displacement of food; environmental change; mitigation; sardine; seabird; swift tern

African Journal of Marine Science 2008, 30(1): 177–184