Page 341 - Atlas Sea Birds Ver1

3.5 Numbers, trends, status, and conservation of Cape Gannet
(Morus capensis)

Authors: AB Makhado, DB Tom, BM Dyer, M Masotla, L Upfold, and RJM Crawford

Abstract:
Cape Gannets (Morus capensis) are endemic to southern Africa, where they bred, or attempted to breed, at ten islands in the Benguela
upwelling system (BUS). Since 1956 they have bred at six localities. Their overall population decreased from c. 254,000 pairs in 1956/57
to c. 145,000 pairs in 2021/22. Given a generation length of 18.3 years, in 2022 they were classified as Endangered by the International
Union for Conservation of Nature. Whereas most Cape Gannets bred in Namibia in 1956/57, by 1978/79 numbers in Namibia and South
Africa were approximately equivalent and thereafter most birds nested in South Africa. In South Africa, after 2000, numbers breeding in
the west decreased but numbers in the east increased. These distributional shifts were influenced by changes in both abundance and
availability of prey. In 2021/22, 70% of the species’ overall population bred at its easternmost colony, Bird Island in Algoa Bay. The main
driver of the decrease in gannets was food scarcity, with other threats including losses to predation by Cape Fur Seals (Arctocephalus
pusillus pusillus), by-catch, oil spills, disease, severe weather, and human disturbance.

Introduction:
Cape Gannets (Morus capensis) are endemic to southern Africa, where they have bred or attempted to breed at ten islands within the
Benguela upwelling system (BUS; Crawford et al. 1983, Sherley et al. 2019). Since 1956 they have nested at six of these islands, three
of which (Mercury, Ichaboe, and Possession) are in Namibia and three (Bird in Lambert’s Bay, Malgas and Bird in Algoa Bay) in South
Africa (Figure 1).
Cape Gannets were a major producer of seabird guano in southern Africa (Crawford and Shelton 1978, Makhado et al. 2021). They
were present at Mercury and Ichaboe islands, but not at Possession Island, in 1828 (Crawford et al. 1983). Extensive exploitation of
accumulated deposits of guano at Ichaboe Island from 1843 to 1845 made it unlikely that gannets would have been able to breed there at
that time. This may have caused them to colonise Halifax and Possession islands. They stopped breeding at Halifax Island by the 1920s
(Crawford et al. 1983) but a small colony persisted at Possession Island (Table 1). Cape Gannets bred at Hollamsbird Island from at least
1828 until at least 1938 (Crawford et al. 1983, Shaughnessy 1984) but did not in 1956 (Rand 1963b). Since then, Hollamsbird Island has
been mainly a rookery for Cape Fur Seals (Arctocephalus pusillus pusillus) (Rand 1963b, Williams and Dyer 1990). Cape Gannets were
present at Malgas Island in 1648 and at Bird Island in Algoa Bay in 1755. They bred at Seal Island in False Bay in 1687, but have not done
so since the late 18 century (Crawford et al. 1983). The earliest record of gannets breeding at Lambert’s Bay was of one or two pairs in
th
1912 (Jarvis 1971). The colony then rapidly increased in size (Crawford et al. 1983). Cape Gannets do not normally come ashore at sites
that are not breeding localities (Crawford et al. 1983, Berruti 1985). However, they were seen ashore at Robben Island in 1968 and at Dyer
Island in 1984 and 1985. One egg was found at Dyer Island in October 1985 (Berruti 1985, Crawford et al. 1986).
Cape Gannets feed extensively on high-energy sardine (Sardinops sagax) and anchovy (Engraulis encrasicolus), two shoaling fish
species, which are also heavily exploited by the purse-seine fisheries of Namibia and South Africa (Crawford et al. 1985, 2019, Klages et
al. 1992, Berruti et al. 1993, Green et al. 2014). Following a collapse of Namibia’s sardine resource in the 1960s and 1970s, there was
a large decrease in numbers of Cape Gannets breeding in that country. In the 1970s, anchovy was plentiful off South Africa and there
was an increase in the number of gannets breeding there (Crawford 2007, Crawford et al. 2007). At the recent turn of the century there
were shifts to the southeast of sardine and anchovy off South Africa (Fairweather et al. 2006, Roy et al. 2007). This was associated with
decreased numbers of Cape Gannets breeding at South African colonies west of Cape Agulhas but increased numbers nesting in Algoa
Bay (Crawford et al. 2014). These changes in the distribution of Cape Gannets are illustrated in Figure 2.
This chapter collates estimates of numbers of Cape Gannets breeding at different localities in southern Africa, and summarises the
species’ conservation status, Important Bird and Biodiversity Areas (IBAs) and threats to it.

Numbers and conservation status The largest number to have bred at an island was c.
175,000 pairs at Ichaboe Island in 1956/57. Maxima
Records of Cape Gannets breeding at localities in different observed at other colonies are shown in Table 1, the lowest
years from 1648–2021 are shown in Table 1. Information being about 9,400 pairs at Mercury Island, also in 1956/57.
was collated from Rand (1963a, b), Jarvis (1971), Jarvis The overall number breeding in 1956/57 was estimated
and Cram (1971), Randall and Ross (1979), Crawford et to be c. 254,000 pairs, of which 69% bred at Ichaboe
al. (1983, 2007), Kemper (2015), Sherley et al. (2019), Island in Namibia in the central BUS. This decreased to c.
and relevant breeding locality chapters in this volume. 145,000 pairs in 2021/22, when 70% bred at Bird Island,
Most values were calculated as the product of the density Algoa Bay, in South Africa at the eastern boundary of the
of nests measured at a locality and the area occupied BUS. Over the same period numbers breeding in Namibia
by breeding gannets, which was determined from aerial decreased from 204,000 pairs to 10,000 pairs, whereas
photographs (Crawford et al. 2007, Sherley et al. 2019). in South Africa numbers increased from 50,000 pairs to
Smaller colonies were sometimes counted on the ground. 134,000 pairs (Table 1).
When multiple counts were obtained at a locality in a given In 2015, Cape Gannets were considered Critically
year, the maximum was used. Midpoints of ranges given Endangered (CR) in Namibia (Kemper 2015) and Vulnerable
for Bird Island, Algoa Bay by Randall and Ross (1979) were (VU) in South Africa (Hagen 2015). In 2019, a model
used for 1755 and 1858. Minima given by Jarvis (1981) indicated that the overall population should be classified
were used for Bird Island, Lambert’s Bay, in 1907 and as either Endangered (EN) or VU (Sherley et al. 2019). In
1912. ‘√’ signifies that breeding occurred at a locality but 2022, its conservation status was listed as Endangered
the numbers were not ascertained; ‘?’ that breeding may (EN) by the International Union for Conservation of Nature
have taken place. (IUCN), on account of a large population reduction over

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