Biological Conservation 40 (1987) 117-126 Population Dynamics of the African Penguin Spheniscus demersus at Marcus Island in the Benguela Upwelling Ecosystem: 1979-85 Graeme D. La Cock, David Cameron Duffy & John Cooper Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, South Africa (Received 4 June 1986; revised version received 19 September 1986; accepted 29 September 1986) ABSTRACT Breeding attempts by the African penguin Spheniscus demersus at a colony on Marcus Island, Saldanha Bay, South Africa, decreased by 14% per year during 1979-85. Annual adult survival was 33"3-70.4%. Nesting success at Marcus Island was similar to that at other African penguin colonies of St Croix and Dassen islands and at colonies in other areas of other penguin species not undergoing population decreases. Minimum survival o f juveniles in the first year of life was only 4% of the level needed to sustain a stable population. INTRODUCTION Populations of the African or jackass penguin Spheniscus demersus, the only penguin species breeding in Africa, have decreased by 59% off Namibia and by 38% off South Africa between 1956 and 1978-79 (Shelton et al., 1984). The decrease has not been uniform: in South Africa, the population remained stable on the east coast, increased on the south coast, and decreased on the west coast (Shelton et al., 1984), the area with the greatest concentration of breeding penguins in the late 1950s (Rand, 1963a,b). In an attempt to assess long-term population trends on the west coast of South Africa, and reasons for such trends, a long-term study site was established at Marcus Island (33°03'S; 17°58'E), in Saldanha Bay, Cape Province, in 1977. 117 Biol. Conserv. 0006-3207/87/$3"50 © Elsevier Applied Science Publishers Ltd, England. 1987. Printed in Great Britain