Processes influencing the population dynamics and conservation of African penguins on Dyer Island, South Africa

Abstract

Dyer Island, South Africa, supported the largest African penguin Spheniscus demersus colony in 1979 (22 655 breeding pairs), but population dynamics of the species have not followed the trends of adjacent colonies in years of high fish abundance or shifts in prey distribution. Less than 1 500 pairs were breeding on Dyer Island in 2013. Available knowledge on demographic parameters was collated and ecological processes were quantified. Juvenile and adult survival probabilities estimated for birds ringed on Dyer Island between 2005 and 2011 were 0.247 (SE 0.06) and 0.545 (SE 0.05) respectively. Juveniles had a high probability (0.918; SE 0.077) of moving away from Dyer Island. Predation by seals and kelp gulls, as well as oiling, are currently important top-down pressures on penguins on Dyer Island. Numbers of birds breeding on the island were negatively correlated with purse-seine catches 20 nautical miles around it. However, it seems that once the colony size is <3 500 breeding pairs, the importance of fishing pressure diminishes, suggesting that once a colony has been reduced to a relatively small size, its growth is limited by pressures other than fishing. From the review of available knowledge presented here, we recommend a number of management options that should be tested in a suitable model.