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

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 from Dyer 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.     

Density-dependent changes in reproductive parameters and condition of southern Benguela sardine Sardinops sagax

The sardine Sardinops sagax population in the southern Benguela has undergone substantial fluctuations in size over the past 50 years, collapsing from an apparently large population in the 1950s to low levels in the mid-1960s, remaining low for the next two decades, and recovering from the late 1980s to a population size that is now similar to or larger than that which occurred during the 1950s. Marked changes in condition and reproductive parameters of sardine have also occurred during this period; condition and standardised gonad mass are higher and length-at-maturity is lower at low population size compared with high population size. The correspondence between the temporal patterns in condition, reproductive parameters and population size are strongly suggestive of density-dependence, and indicate a compensatory response arising from reduced intra-specific competition. This is likely to have resulted from greater per capita food intake, improved body condition and hence faster growth, thus enabling fish to achieve maturation at a presumably younger age and smaller size. Biological parameters did not vary in or out of phase with time-series of sea surface temperature in the southern Benguela, weakening the hypothesis of environmentally mediated changes in these parameters and hence providing support for the hypothesis of a direct density-dependent response by sardine.