Population Dynamics of the African Penguin Spheniscus Demersus at Robben Island, South Africa

Abstract

SUMMARY CRAWFORD, R.J.M., SHANNON, L.J. & WHITTINGTON, P.A. 1999. Population dynamics of the African Penguin Spheniscus demersus at Robben Island, South Africa. Marine Ornithology 27: 139–147. African Penguins Spheniscus demersus recolonized Robben Island in 1983 when about nine pairs bred at the island. By 1996, the colony had grown to about 3100 pairs. Adult survival was probably between 0.82 and 0.90 in 1993/94, but fell to 0.75 in 1994/95 when many birds at the island were oiled following the sinking of the Apollo Sea in June 1994. Some penguins initiated breeding when two years old, and all were assumed to be breeding at age five. The proportion of mature birds that bred in a year varied between about 0.70 and 1.00. During a breeding season, pairs laid their first clutches between January and August, mostly in February and March. The average clutch was 1.86 eggs. Of lost clutches 32% were replaced, whereas 23% of pairs losing broods relayed and 21% of pairs that successfully fledged chicks relayed. On only one occasion was the laying of a third clutch during a breeding season recorded, and this was unsuccessful. The mean number of chicks fledged per breeding pair varied between 0.32 and 0.59 per annum. Both fledging success and immigration of immature birds to the colony were significantly related to the spawner biomass of Cape Anchovy Engraulis capensis, the most important prey item of penguins at the island. Growth of the colony has been driven by immigration. Depending on the values assumed for survival of adults and first-year birds, 59–87% of new adults in the colony resulted from immigration. Several birds banded as chicks at Dassen and Dyer islands were recorded breeding at Robben Island.