Investigation of the impact of fur seals on the conversation status of seabirds at islands off South Africa and the the Prince Edward Islands

• Main Author: Makhado, Azwianewi Benedict
• Other Authors: Underhill, Les, Crawford, RJM, Bester, MN
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Cape Town 2009
• Subjects: Zoology; Antarctic; Bird Island; Dyer; Dyer Island; Edwards Islands; Antarc*; Antarctic Fur Seals; Marion Island; Prince Edward Islands
• Online Access: http://hdl.handle.net/11427/6125; https://open.uct.ac.za/bitstream/11427/6125/1/thesis_sci_2009_makhado_ab.pdf

This research project investigated the impact of predation by the Cape fur seals Arctocephalus pusillus pusillus on the conservation status of seabirds at islands off the coast of South Africa and at the Prince Edward Islands. At Malgas Island, the impact of predation was investigated over three breeding seasons (2000/01, 2003/04 and 2005/06), and at Dyer Island over two breeding season (2004 and 2006/07). At Marion Island (Prince Edwards Islands) and at other South African islands, such as Dassen Island, Bird Island at Lambertâs Bay and Robben Island, observations were collected opportunistically. Cape fur seals were estimated to have killed some 6 000 Cape gannet Morus capensis fledglings around Malgas Island in the 2000/01 breeding season, 11 000 in 2003/04 and 10 000 in 2005/06. This amounted to an estimated 29%, 83% and 57% of the overall production of fledglings at the island in these breeding seasons respectively. Preliminary modelling suggests this predation is not sustainable. There was a 25% reduction in the size of the colony, the second largest of only six extant Cape gannet colonies, between 2001/02 and 2005/06. At Dyer Island, it was estimated that seals killed up to 7% of African penguin Spheniscus demersus adults annually. The present mortality attributable to seals is considered unsustainable. Seals also killed a substantial proportion, 4â8%, of Cape cormorant Phalacrocorax capensis fledglings as they left the island. Although this level of mortality may be sustainable for Cape cormorants, this species have also been affected by disease. iv Predation on seabirds by Cape fur seals was demonstrated to occur at several other localities in southern Africa but observations were insufficient to enable the effect of such predation to be quantified. Several hundred Cape gannet fledglings were killed annually at Bird Island (Lambertâs Bay) and threatened penguins and cormorants were attacked at other localities. At subantarctic Marion Island, Antarctic fur seals A. gazella fed on king Aptenodytes ...