Page 343 - Atlas Sea Birds Ver1

(a) Namibia
South Africa
200

150

100

50
Thousand pairs 100 (b) West of Cape Agulhas

Algoa Bay

80

60
40 Cape Gannet juveniles (photo L Upfold)
2014, Rishworth et al. 2014). Between 2011 and 2014 there
20
was a net deficit in energy expenditure during foraging for
most Cape Gannets tracked from Malgas Island (Grémillet
1956 1964 1972 1980 1988 1996 2004 2012 2020 et al. 2016). However, adult gannets buffered their survival
to a considerable extent by feeding on alternative food,
such as saury Scomberesox saury and hake Merluccius sp.
Figure 2. Numbers of Cape Gannets breeding from 1956–2021
in (a) Namibia and South Africa and (b) South Africa west of Cape offal discarded by fisheries (Distiller et al. 2012, Crawford
Agulhas and Algoa Bay et al. 2014).
Other factors contributing to the decrease of Cape
Gannets have included unsustainable losses of fledglings
the past three generations that was projected to continue
(BirdLife International 2022). Sherley et al. (2019) to Cape Fur Seals around colonies (du Toit et al. 2004,
calculated generation length (G) for Cape Gannets as: Makhado et al. 2006); losses of adults to and disruption
G = A + 1/(1 − ϕ ), where A is age at first breeding and of breeding by Cape Fur Seals entering gannet colonies
a
ϕ is adult survival (Birdlife International 2000). Using A = (Crawford and Cooper 1996, Wolfaardt and Williams 2006,
a
4 years (Crawford 1999) and ϕ = 0.93, based on survival Sherley et al. 2019); losses of adults to disease (Crawford
a
estimates of Distiller et al. (2012) from 1990 to 2009, they et al. 1992, Williams and Ward 2002, Khomenko et al.
obtained G = 18.3 years, and 3G ~ 55 years. 2018); losses of birds at sea to oil spills (Crawford et al.
1983), and as by-catch in fisheries (Watkins et al. 2008,
Petersen et al. 2009a, 2009b); and reduced breeding
Important Bird and Biodiversity Areas and threats
success caused by heat waves or storms (Sherley et al.
2019) or human disturbance at colonies, especially during
Important Bird and Biodiversity Areas (IBAs) are identified
through the use of specific criteria. Criterion A1 is that former guano collections (Crawford et al. 1983).
‘the site is known or thought regularly to hold significant
numbers of a Globally Threatened species’ (BirdLife
International 2020). Guidelines for the application of IBA
criteria recommend that to meet A1 a site must support at
least 15 individuals (the equivalent of 5 Pairs/Reproductive
Units) of a species classified as EN, or 30 individuals (the
equivalent of 10 Pairs/Reproductive Units) of a species
classified as VU (BirdLife International 2020). All six extant
colonies hold more than these numbers of pairs and
therefore should be regarded as IBAs.
The major driver of the large decreases at Cape Gannet
colonies was scarcity of high-energy food (Crawford and
Shelton 1978, Crawford 2007, Crawford et al. 2007), which
negatively affected breeding participation (Crawford et al.
2019) and breeding success (Grémillet et al. 2008, Cury et
al. 2011), decreased nest attendance and body condition
of parents, and increased their foraging effort (Cohen et al. Cape Gannets at Lambert’s Bay (photo L Upfold)

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