Cape Gannet on the Sardine Run
The Cape gannet (Morus capensis) is listed as vulnerable since it has a very small breeding range on just six islands, and over-exploitation of its prey by human fisheries – compounded by pollution – is causing a continuous decline in the quality of surrounding waters for foraging.
It’s a 84-94 cm, mainly white, sleek seabird with a pale yellow head, black tail, primaries and secondaries. Immature gannets are dark brown, mottled paler, and shows increasing amounts of adult plumage after its first year. Similar spp: Adult Masked Booby (Sula dactylatra) has a white head, adult Northern Gannet (Morus bassanus) has a white tail and secondaries while adult Australasian Gannet (S. serrator) has only four, occasionally more, black central tail feathers. Voice: Usually silent at sea, rasping arrah arrah is the most common call at colonies.
Distribution and population
Morus capensis breeds at just six islands: Bird (Lambert’s Bay), Malgas and Bird (Algoa Bay), South Africa, and Mercury, Ichaboe and Possession, Namibia. Historically it bred on four more islands (Kemper et al. 2007). Outside the breeding season, adults are generally sedentary but young range east to KwaZulu-Natal, Angola, Mozambique and Tanzania, and regularly north as far as Nigeria, but usually within 100 km of land. In 1996, the global population numbered c.173,000 breeding pairs: 153,000 (88.4%) in South Africa, the balance in Namibia. The total breeding population in 2004-2006 was c.150,000 pairs (Kemper et al. 2007). Exchange occurs between breeding localities. Although the numbers breeding at South African islands increased between 1956 and 1996, the Namibian population declined massively. The total breeding population has decreased by 1.14% per year over the 49 years between 1956-1957 and 2005-2006, equivalent to 36% over 39 years (Kemper et al. 2007). Over a 50 year period, numbers at the three Namibian colonies fell by 85-98%, with greater proportional decreases in the south (Crawford et al. 2007). The colony at Lambert’s Bay increased between 1956-57 and 2003-04, but attacks by Cape fur seal (Arctocephalus pusillus) on birds on nests caused abandonment of the whole colony in 2005-05 (Crawford et al. 2007). The colony at Possession Island now numbers only 750 pairs, and may soon be lost.
Behaviour
This species is not strictly migratory and the majority of birds remain within 500km of their breeding site year round (del Hoyo et al. 1992), some (mainly adult males) continue to use the breeding grounds as roosting sites throughout the non-breeding season (Nelson 2005). However some adults disperse up to 3300 km from the breeding colonies, moving along the African coast for about 3 months during the non-breeding season (Hockey et al. 2005; Nelson 2005). Juveniles disperse northwards in April (del Hoyo et al. 1992, Nelson 2005), travelling up to 4000 km towards the equator (Nelson 2005) where they may remain for over a year (Nelson 2005), returning to breed around four years of age (Makhado et al. 2006). Breeding occurs between September and April in large colonies of up to 5000 pairs (Nelson 2005), although it will also nest in much smaller groups (Nelson 2005). Large foraging aggregations occur around trawling vessels (del Hoyo et al.1992). Individuals can travel as much as 450 km in a day in search of food (Mullers 2009).
Habitat This species is strictly marine. Breeding It prefers to nests on flat or gently sloping open ground on offshore islands (Hockey et al. 2005), but will also use island cliffs as well as man-made structures such as guano platforms (Nelson 2005). Non-breeding It most often forages within 120km of the shore (Adams and Navarro 2005), particularly frequenting areas where purse-seine netting occurs (Nelson 2005), but occasionally wanders further offshore over the continental shelf (del Hoyoet al. 1992) where it benefits from the discards of deep-water stern trawlers (Nelson 2005).
Diet It feeds mainly on shoaling pelagic fish (del Hoyo et al. 1992) such as anchovy (Engraulis capensis), sardine (Sardinops sagax) or saury (Scomberesox saurus), as well as offal discarded by fishing boats including demersal fish (Hockey et al. 2005). In South Africa fluctuations in the contribution of E. capensis and S. sagax in the diet match the changing abundance of the species (Crawford and Dyer 1995, Crawford et al. 2007).
Breeding site The nest consists of a mound with a cup-shaped depression in its center (Nelson 2005). It is made from guano, vegetation and other matter that can be scraped together (Hockey et al. 2005). Where no such material is available, eggs are laid on bare ground (Hockey et al. 2005).