Factors influencing growth of the African penguin colony at Boulders, South Africa, 1985–1999

Abstract

This paper reports on growth of the Boulders colony of African penguins Spheniscus demersus from inception in 1985 to the present. More than 900 pairs now breed there. Growth of the colony slowed in 1995 and 1996 and reversed in 1998, coinciding with periods of low abundance of Cape anchovy Engraulis capensis off South Africa. In December 1996, penguins were excluded from a portion of land where they had formerly bred. They responded by increasing the density of their nests in other areas and expanding their area of breeding longshore. These patterns indicate that food and not space are currently controlling colony growth rate. Much of the colony growth probably results from immigration of first-time breeders from other colonies. Of immigrants, 70-80% may be fr0111Dyer Island to the south-east, where numbers of penguins have decreased. Boulders also is frequently visited by penguins from other colonies, and by rehabilitated birds.

Prognostic Indicators of Rehabilitation Outcomes for Adult African Penguins (Spheniscus Demersus)

“…The clutch is usually 2 eggs, sometimes 1, rarely 3 (Crawford et aL , 2000b. Eggs are rounded oval, white, becoming stained as incubation proceeds.…”

African penguins as predators and prey – coping (or not) with change

“…Sporadic years of food scarcity slowed or reversed growth of the penguin population at Boulders when it numbered fewer than 650 breeding pairs, but when food attained adequate levels the population again expanded (Crawford et al 2000b). Conversely, where larger populations competed with commercial fisheries for food in the 1950s and 1960s, severe reductions in penguin populations were recorded within 10 years (Figs 2, 3).…”

“…Therefore, it is possible that this population was not initially impacted by food scarcity to the same extent as the other localities. Growth of the mainland population at Boulders slowed or reversed in years of reduced abundance of food when it was larger than 366 pairs, but not when it was 54 pairs or less (Crawford et al 2000b). A reduced density of food within the foraging range of colonies can be expected first to limit large populations.…”

“…Other predators continued to restrict growth of that population until 1995 (Crawford et al 1995b, Whittington et al 1996. Growth of the populations at Stony Point, Robben Island and Boulders is largely attributable to immigration of first-time breeders, most of which are from Dyer Island (Whittington et al 1996, Crawford et al 1999a, 2000b. More recently, the breeding populations at Dassen and Vondeling islands also have increased, despite the loss of several thousand birds at Dassen Island during the Apollo Sea oil spill in 1994 , Underhill et al 1999).…”

“…By 1995, breeding had stopped at seven or more localities (Crawford et al 1995c). Although numbers at some colonies increased, and three new colonies were established in the 1980s (Crawford et al 1995a, Whittington et al 1996, Crawford et al 2000b, the increases were insufficient to offset the downward trend. By the early 1990s the number of birds in adult plumage was about 179 000 (Crawford et al 1995b), some 10% of the number in 1910.…”