Eleven species of seabird breed in South Africa's Eastern Cape Province. Numbers of African Penguin Spheniscus demersus and Cape Gannet Morus capensis in the province increased in the 20th century, but penguins decreased in the early 21st century. A recent eastward displacement of Sardine Sardinops sagax off South Africa increased the availability of this food source to gannets but did not benefit penguins, which have a shorter foraging range. Fishing and harbour developments may have influenced the recent decrease of penguins. Cape Cormorant Phalacrocorax capensis and Swift Tern Thalasseus bergii also feed on sardine and increased in the Eastern Cape in the early part of the 21st century. There has been a recent increase in the Kelp Gull Larus dominicanus population in South Africa's Indian Ocean sector, which includes the eastern portion of the Western Cape Province. Hartlaub's Gull L. hartlaubii and Damara Tern Sterna balaenarum, which have their main populations in the Benguela system off western southern Africa, were first recorded in the Eastern Cape in the late 1970s and later shown to be breeding there. Numbers of White-breasted Cormorant P. lucidus and Caspian Tern Hydroprogne caspia breeding in this province have been stable, but those of Grey-headed Gull L. cirrocephalus and Roseate Tern St. dougallii have increased since the early 1990s. The former three species also breed at inland localities; the Roseate Tern is at the western extent of its distribution in the Indian Ocean. Although not yet breeding in the Eastern Cape, the Crowned Cormorant P. coronatus, another western species, has extended its range to this province. Congruent changes in the distributions and abundance of several marine species off South Africa, and similar changes elsewhere in the Indian Ocean, suggest that large-scale environmental change may be influencing the distribution and abundance of species. Conservation will need to account for these as well as for anthropogenic impacts. Keywords: Cormorants, gannets, gulls, penguins, terns, population trends, range expansion, eastern South Africa.