Comparison of moult phenology of African penguins Spheniscus demersus at Robben and Dassen islands

Abstract

Dassen and Robben islands are approximately 50 km apart, and currently support the largest and third largest populations of African penguins Spheniscus demersus respectively. At both islands, moult is the most synchronised and seasonal activity of the annual cycle. The main difference in moult phenology between the two islands is the degree of synchronisation. The peak moult period at Dassen Island is broader (less synchronous) than at Robben Island. At both islands, juvenile penguins moulted less synchronously than adults. Juvenile moult was less synchronous at Dassen Island than at Robben Island. Interannual variation in moult phenology was similar at Robben and Dassen islands. Islandlevel moult phenology was more synchronous in the year following the Treasure spill than was the case after the Apollo Sea spill.  This was attributed to the larger number and  proportion of birds that were affected by the Treasure spill. The temporal duration of the  disruption to moult phenology was also greater following the Treasure spill, especially at Robben Island. Moult counts provide a relatively reliable method of estimating the size of the adult population at Robben Island, but not at Dassen Island where the large number of birds moulting away from the shoreline results in the population size being significantly underestimated. Although  the number of active nest sites continued to increase up until 2004 at both islands, the numbers of adult moulters counted declined by 55% between 2003 and 2005 at Robben Island,  and by 50% between 2002 and 2005 at Dassen Island. The decrease in the number of moulters coincided with a decreased availability of food in the region.

Keywords: African penguin, Dassen Island, moult phenology, population trend, Robben Island, Spheniscus demersus

African Journal of Marine Science 2009, 31(1): 19–29