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

Doctoral Thesis


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University of Cape Town

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 patagonicus and macaroni Eudyptes chrysolophus penguins. However, predation there on seabirds by subantarctic fur seals A. tropicalis has not yet been observed. King penguins were mostly killed in winter and macaroni penguins (which are absent from the island in winter) in summer. Predation of seabirds by seals has recently increased in southern Africa and may be doing so at Marion Island. In both regions fur seal populations are expanding. The influences of environmental factors on the rate of predation were investigated at both Malgas and Dyer Islands. Of environmental factors considered, time of the day had the most important influence on predation by Cape fur seals on fledglings of Cape gannets and Cape cormorants and adult African penguins. Fledglings of gannets and cormorants were mainly killed between mid morning and late afternoon, coinciding with the time they left the islands and were in the water. Most adult African penguins were killed as they returned in the evening from foraging at sea. Wind speed and direction, sea state and tide had a variable and lesser influence on predation rates. Although these variables might be used to interpolate predation rates through periods when observations are not conducted, their contribution is limited and there will remain considerable uncertainty in the estimate of actual numbers of fledglings killed. Uncertainty will best be decreased by extending the period of observations so as to reduce the amount of days for which predation rates are interpolated. v In six years between 1999 and 2008, 141 Cape fur seals that were feeding on, or thought to be feeding on, Cape gannet fledglings as they left Malgas Island, South Africa, were shot and collected. Examination of the stomachs of 93 of these seals showed that Cape gannets contributed an average of 70% by mass to the diet, and known prey items of Cape gannets, which may have been obtained from the alimentary tract of the fledglings, a further 5%. Hence, when the seals were culled, they were subsisting mainly on the fledglings. Other prey items of the seals included rock lobster Jasus lalandii and common octopus Octopus vulgaris. All the collected seals were bulls. Their ages were estimated from measures of their total length and previously published information on ages at length. All were 10 years old or less, and their mean age was 4.5 years. The hunting behaviour of Cape fur seals feeding on seabirds at Malgas and Dyer Islands was investigated in 2003/04 and 2005/06; and 2004 and 2006/07 respectively. At these islands, attacks on seabirds were identified mainly through the presence of other birds overhead and sometimes by the thrashing of a victim in, or throwing of it from, the water. Most attacks occurred beyond the surf zone at distances of 20â100 m from the island. Seals hunted in groups or solitarily and usually attacked birds from underneath or behind. Usually most, or a substantial portion of the carcass was utilized but some surplus killing was observed. On average, attacks lasted 11 min for both Cape gannets and Cape cormorants and 16 min for African penguins. Due to the increase in seal predation, interventive management such as culling individual fur seals seen preying on seabirds was implemented at Malgas Island. It was anticipated that removal of the small number of individual seals which target seabirds would reduce vi mortality on the seabirds because this is a learned behaviour. In the 2006â07 breeding season of Cape gannets at Malgas Island, the removal of 61 Cape fur seals that preyed on gannet fledglings when they left to sea significantly reduced the mortality rate of these fledglings in the short term. However, because seals learned to avoid the boat used for their removal, it was not possible to remove all the seals that killed gannet fledglings and some mortality continued. There was a decrease over time in both the maximum and the mean age of seals culled. Sustained removal of these animals may reduce this feeding behaviour.