Justification of Red List Category
This species has undergone a large population reduction over the past three generations and is projected to continue to decline rapidly over the next three generations. For these reasons it is listed as Endangered.
The most recent population estimate is made up of 10,500 pairs at Ichaboae Island, 2,200 pairs on Mercury Island and 380 pairs on Possession Island, all in 2010 (Kemper 2015). 81,000 pairs were estimated at Bird Island (Algoa Bay), 21,000 pairs at Malgas Island and 8,000 pairs at Bird Island (Lambert’s Bay) in 2015 (Crawford et al. 2015 updated by R. Crawford in litt. 2016). This gives a global total of 123,080 pairs, which equates to 246,160 mature individuals, rounded here to 246,000.
The total breeding population was previously estimated to have declined by 1.14% per year over the 49 years between 1956-1957 and 2005-2006 (Kemper et al. 2007). Recent data, however, shows that the species may in fact be declining at a faster rate than this. Historically, the global population numbered c.254,000 breeding pairs in 1956, but has subsequently decreased to c.249,000 pairs in 1968, c.179,000 in 1989 and c.145,000 pairs in 2005 (Crawford et al. 2007). The most recent population estimate is made up of 10,500 pairs at Ichaboae Island, 2,200 pairs on Mercury Island and 380 pairs on Possession Island (Kemper 2015), with 81,000 pairs at Bird Island (Algoa Bay), 21,000 pairs at Malgas Island and 8,000 pairs at Bird Island (Lambert’s Bay) (Crawford et al. 2015 updated by R. Crawford in litt. 2016), which gives a global total of 123,080 pairs. This gives an overall decline of 51.5% between 1956 and 2015, and with only minor extrapolation this would equate to a c.52.4% decline over 3 generations (60.6 years). An analysis that used a Bayesian state-space model to nests counts made between 1956 and 2016 at the species’s six breeding colonies indicated a decline of 51.5% (95% credible intervals: 39.5–62.5%) over three generations. Overall, 61.6% of model iterations fell within the 50–79% decline band and 38.3% fell within the 30–49% band (R. Sherley in litt. 2017). When comparing the most recent population estimate to the 1968, 1989 and 2005 estimates and projecting these rates of decline into the future, the rate of decline over three generations would be c.60%. Therefore, rates of decline over three generations likely fall within the range of 50-79%.
Morus capensis breeds at 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 (Crawford et al. 1983). Outside the breeding season, adults may disperse up to c.1,000 km from colonies, whereas juvenile birds may move >2,000 km (Klages 1994). Birds range east to KwaZulu-Natal and Mozambique and as vagrants to Tanzania (Crawford et al. 1983, Klages 1994). On the west coast of Africa, birds formerly moved north as far as Nigeria, and as vagrants to Western Sahara (Crawford et al. 1983). However, there are few recent records of birds moving north of southern Angola, suggesting possible change in dispersal pattern (Oatley et al. 1992, Klages 1994). The species usually remains over the continental shelf, often within 100 km of land (Hockey et al. 2005). Limited exchange occurs between breeding localities (Crawford et al. 1983). There has been a recent shift in the breeding distribution of the species around the southern African coast from the northwest to the southeast, which has matched a shift in the distribution of epipelagic forage fish, the Cape Gannet’s main prey, that may have been environmentally driven (Crawford et al. 2007, 2015).
Behaviour This species is not strictly migratory and the majority of birds remain within 500 km of their breeding site year round (del Hoyo et al. 1992, Klages 1994), some (mainly adult males) continuing 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 used to disperse northwards in April (del Hoyo et al. 1992, Nelson 2005), travelling up to 4,000 km towards the equator (Nelson 2005), returning to breed around four years of age (Crawford 1999). Breeding occurs between September and April in large colonies of >10,000 pairs and in smaller groups (Crawford et al. 2007). 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, but will also use island cliffs as well as man-made structures such as guano platforms (Hockey et al. 2005). Non-breeding It most often forages within 120 km of the shore (Adams and Navarro 2005), particularly frequenting areas where purse-seine netting occurs (Nelson 2005). It occasionally wanders further offshore over the continental shelf (del Hoyo et al. 1992) where it benefits from the discards of deep-water stern trawlers (Nelson 2005).
Diet It feeds mainly on shoaling pelagic fish such as anchovy Engraulis capensis, sardine Sardinops sagax or saury Scomberesox saurus, as well as offal discarded by fishing boats including demersal fish (del Hoyo et al. 1992, 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 centre (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). .
Food shortage, following the collapse of the Namibian sardine fishery and an eastward displacement of epipelagic fish off South Africa, has been the main cause of declines (Crawford et al. 2007, 2014, 2015). In Namibia, anchovies only temporarily and partially replace sardines in the diet when the latter becomes scarce (Crawford et al. 2007). In both Namibia and South Africa, the numbers of Cape Gannets breeding were significantly related to the biomass of epipelagic fish prey (Crawford et al. 2007). Competition with purse-seine fisheries targeting small pelagic fish has been demonstrated in South Africa, with a negative impact on foraging success, adult body condition and chick growth rates (Okes et al. 2009, Cohen et al. 2014, Grémillet et al. 2016), but little evidence for impacts on the local population size or a decrease in adult survival rate (Crawford et al. 2014). However, this may be in part due to an increased reliance on fisheries discards (Tew-Kai et al. 2013), which may impair reproductive performance (Grémillet et al. 2008) and increase the risk of bycatch mortality previously noted as a threat to the species (Du Toit et al. 2002, Peterson et al. 2008).
Storms can have significant impacts on breeding success, with the entire population considered at risk from coastal flooding (Spatz et al. 2017) and some colonies at risk of completely disappearing in the face of rising sea levels and increased storm frequency and severity. The effects from storms may be exacerbated by previous guano collection. Previously, guano mining removed suitable habitat, disturbed breeding birds and shortened the breeding season at some sites, but commercial exploitation largely ceased by the 1990s. Direct chick predation by Cape Fur Seal Arctocephalus pusillus has recurred at several colonies, threatening the viability of that at Malgas Island, while at least two colonies (Hollam's Bird Island and Lambert's Bay in 2005/06) have been physically displaced by fur seals (Makhado et al. 2006, Crawford et al. 2007). Additional native predators that cause chick mortality and may be a concern include Kelp Gull Larus dominicanus and Great White Pelican Pelecanus onocrotalus (Du Toit et al. 2002, Mullers 2009).
Oil spills are thought to be an ongoing minor threat identified in Namibia from vessels that process fish at sea and also on-shore facilities discharging fish oil (Du Toit and Bartlett 2001), but previous large incidents have impacted relatively large numbers of birds (e.g. c.5,000 individuals oiled during an incident in 1993). Historically, large numbers (especially juveniles) were collected for food or for use as bait in Cape Rock Lobster Jasus lalandii traps (Du Toit et al. 2002).
Conservation Actions Underway
In South Africa, Lambert's Bay and Bird Island are nature reserves and Malgas Island is within West Coast National Park. In Namibia, the three breeding islands are administered by the Ministry of Fisheries and Marine Resources and are part of the Namibian Islands’ Marine Protected Area. All six islands have been identified as Important Bird Areas (IBAs) (Barnes 1998). Oiled birds are rehabilitated with success in South Africa, and the species is protected by law in both countries. Bycatch in trawl fisheries, through collisions with warp cables is being addressed through the use of bird scaring lines in South Africa and more recently in Namibia, although bycatch in the trawl net is more difficult to mitigate due to the birds’ plunge-diving behaviour (Maree et al. 2014).
Conservation Actions Proposed
Carry out surveys to obtain an up-to-date population estimate. Conduct regular surveys to monitor population trends. Identify key foraging hotspots of both breeding and non-breeding gannets from all colonies, in order to protect these in future. Continue research/monitoring the impact of fisheries and predation. Develop and implement a sustainable, coordinated fisheries plan for the region. Develop measures to prevent oil spills from illegal cleaning of ship tanks, and minimise fish oil entering the sea during processing. Consider the potential culling of individual seals that are inflicting excessive mortality on, or causing extensive disturbance to, threatened colonies (Crawford et al. 2007). Reduce competition with purse-seine fishing activities targeting small pelagic fish in South Africa through the implementation of revised fishing quotas and/or no-take zones (Okes et al. 2009, Pichegru et al. 2009, 2010, Cohen et al. 2014, Grémillet et al. 2016).
84-94 cm. Sleek, mainly white seabird. Black tail, primaries and secondaries. Pale yellow head. Immature dark brown, mottled paler, and shows increasing amounts of adult plumage after first year. Similar spp. Adult Masked Booby Sula dactylatra has white head, adult Northern Gannet Morus bassanus has white tail and secondaries, adult Australasian Gannet S. serrator has only four, occasionally more, black central tail feathers. Voice Usually silent at sea. Rasping arrah arrah is most common call at colonies.
Text account compilers
Symes, A., Taylor, J., Wheatley, H., Anderson, O., Fjagesund, T., Malpas, L., Ekstrom, J., Calvert, R., Martin, R., Pilgrim, J., Robertson, P.
Hagen , C., Gremillet, D., Kemper, J., Dowsett, R.J., Crawford, R.J.M.
BirdLife International (2020) Species factsheet: Morus capensis. Downloaded from http://www.birdlife.org on 19/09/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/09/2020.