Trends in numbers of Cape Cormorants (Phalacrocorax capensis) over a 50-year period, 1956–57 to 2006–07

Abstract

The population trend of Cape Cormorants (Phalacrocorax capensis), a species endemic to southern Africa and that feeds mainly on shoaling pelagic fish, is described for a 50-year period, from 1956–57 to 2006–07. The main breeding localities for the species are grouped in three regions in the Benguela upwelling ecosystem: guano platforms off central Namibia, islands off southern Namibia and islands off South Africa's Western Cape Province. From 1956–57 to 1978–79, the numbers breeding off Namibia increased, as a result of increased availability of breeding space and adequate supplies of food. In the same period, numbers remained stable in the Western Cape. Numbers decreased off southern Namibia in the early 1980s and off central Namibia in the early 1990s, when environmental perturbations reduced the availability of food. Numbers decreased in the Western Cape in the early 1990s, following periods of scarcity of anchovy (Engraulis encrasicolus), an important prey item, and an outbreak of avian cholera caused by the bacterium Pasteurella multocida. They remained low as cholera outbreaks continued and some pelagic fish were displaced to the east beyond the foraging range of breeding Cormorants. The overall population of Cape Cormorants was of the order of 100000 pairs in 1956–57, increased to ∼250000 pairs in the 1970s, but reverted to ∼100000 pairs in 2005–06.

Additional keywords:

• avian cholera
• environmental perturbation
• food
• guano
• population trend