Natural and human-induced predation on Cape Cormorants at Dyer Island

Abstract

SummaryTo develop conservation strategies for vulnerable seabird species that need attention, it is important to know which factors influence their breeding productivity. Predation of eggs and chicks can have large influences on seabird reproduction, especially when human disturbance facilitates predation. On Dyer Island, Kelp GullsLarus dominicanusprey on Cape CormorantPhalacrocorax capensiseggs and chicks, whereas Cape fur sealsArctocephalus pusillus pusillusprey on Cape Cormorant fledglings in the waters surrounding the island. Kelp Gulls were estimated to predate 3.8% of the total number of Cape Cormorant eggs and 2.0% of the chicks on the island. These percentages can be expressed as a loss of 4.8% of Cape Cormorant fledglings, which is low compared to the estimated 24.3% mortality of Cape Cormorant fledglings by Cape fur seal predation. Human disturbance facilitated Kelp Gull egg and chick predation and increased the mobbing of cormorant fledglings by Kelp Gulls. Cormorant egg predation by gulls was more frequently reported in the late afternoon. Seal predation was more abundant at the northern side of the island compared to the southern side, was recorded more frequently in the morning, and increased through the breeding season. The altered abundance and distribution of prey, the availability of suitable breeding habitat and mortality from avian cholera, have also influenced the Cape Cormorant’s population size. Hence, the possibility that Cape Cormorants may be locked in a predator-pit, where seals and gulls prevent the population from increasing in size, needs further attention.