Page 582 - Atlas Sea Birds Ver1

4.2 Harvest of seabird guano in the Benguela upwelling ecosystem

Authors: DB Tom, RJM Crawford, and AB Makhado

Abstract:
Accumulated deposits of seabird guano were harvested from islands in the Benguela ecosystem in the 19 and early 20 centuries. From
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the late 19 century, records exist of annual or periodic amounts of guano collected at islands in Namibia and South Africa. Much of this
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was deposited by African Penguins (Spheniscus demersus), Cape Gannets (Morus capensis) and Cape Cormorants (Phalacrocorax
capensis). These collections were phased out between 1975 and 1991, on account of decreasing yields and concern about detrimental
effects for dwindling seabird populations of harvesting guano, except at Ichaboe Island in Namibia where collections continued until 2016.
In northern Namibia, platforms at which cormorants could breed were constructed for the collection of their guano in the mid-20 century
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and continue to yield guano. This chapter collates information on guano harvests from 33 localities in Namibia and South Africa between
1896 and 2021.
Introduction:
The value of seabird guano as an agricultural fertiliser was recognised in the 1830s and led to a ‘guano-boom’ in southern Africa during
1843–1845, when accumulated deposits were collected from islands off Namibia and South Africa by companies operating from Cape
Town (Shaughnessy 1984). More than 330,000 tons were collected from islands between Saldanha Bay and Walvis Bay in this period (van
Sittert and Crawford 2003). In 1843, Ichaboe Island in Namibia was surrounded by 450 ships collecting guano there (Berruti 1989). This
led to the Cape Colony claiming Ichaboe Island in 1861 and a group of 11 other islands (Hollamsbird, Mercury, Seal, Penguin, Halifax,
Long, Possession, Albatross, Pomona, Plumpudding and Sinclair) off the coast of Namibia in 1866. The annexation was confirmed by
legislation passed in 1874 (Shaughnessy 1984). From 1851 concessions were issued for the harvesting of guano at the 12 islands off
Namibia and from 1859 at islands off South Africa. The leases for most islands off South Africa terminated in the early 1890s and for
those off Namibia in 1895. As leases expired, the collection of guano at islands was taken over by the government of the Cape Colony
(Shaughnessy 1984). In 1994, South Africa ceded the 12 islands off Namibia to that country.
Important producers of seabird guano in southern Africa were African Penguins (Spheniscus demersus), Cape Gannet (Morus capensis)
and cormorants (Phalacrocoracidae), especially the abundant Cape Cormorant (Phalacrocorax capensis) (Rand 1952, 1960, 1963a,b).
Cape Gannets usually build their nests from guano (Crawford and Cochrane 1990) so, after clearing guano from their breeding localities,
at times phosphatic sand removed from African Penguin (Spheniscus demersus) colonies at Dassen Island was mixed with guano and
spread over gannet breeding areas to facilitate their construction of nests (Ross and Randall 1990). Records of guano produced at
Ichaboe, Possession, Bird (Lambert’s Bay), Malgas and Bird (Algoa Bay) islands were corrected by subtracting quantities of phosphatic
sand taken to these islands. There were no records of phosphatic sand being taken to Hollamsbird and Mercury islands.
Quantities of guano harvested annually at 11 islands in Namibia and at 18 in South Africa were available from 1896 from records
maintained by the governments of these two countries. Some of the amounts were published in reports of the Government Guano Islands:
eight by CH Jackson between 1894 and 1904, two by J Spence in 1906 and 1907, 19 by WJ Zeederberg between 1908 and 1928, eight
by H Jackson between 1929 and 1936, four by JHC Hewitt between 1937 and 1940, and five by TL Kruger between 1947 and 1951. These
reports are listed on pp. 240–242 of Shelton et al. (1984). The values in these reports were not corrected through subtraction of phosphatic
sand taken to breeding areas of Cape Gannets and so may differ slightly from amounts indicated here.
Nineteenth century customs records of the Cape Colony and United Kingdom were used to estimate annual harvests of guano at
Namibian islands for the the period 1943–1895, which were thought to have been influenced by commercial and political as well as natural
factors, so that care needs to be taken when interpreting their fluctuations (van Sittert and Crawford 2003).
Farther north in Namibia, between 1923 and 1944 guano was collected from sandy islands in Sandwich Harbour, the annual harvests
ranging from 214–2,033 metric tons (Rand 1952, Cooper et al. 1982). From 1895–1903, c. 182,000 metric tons of guano were collected
at low islets in Cape Cross Lagoon, but it is uncertain whether the deposits were old or recent at the time of their collection (Rand 1952,
Cooper et al. 1982). The large amount collected compared to later harvests at other localities (Tables 1–3, Cooper et al. 1982) suggests
some guano had accumulated. At both Sandwich Harbour and Cape Cross Lagoon silting and other factors later caused the islands to join
the mainland and rendered them unsuitable for breeding and roosting by birds (Rand 1952).
Subsequently, four guano platforms, at which Cape Cormorants bred, were constructed between 1930 and 1971 at Bird Rock reef north
of Walvis Bay, Mile 4 Saltworks on the north of Swakopmund and in the central and northern portions of the Cape Cross Lagoon (Rand
1963b, Cooper et al. 1982). White-breasted Cormorants (P. lucidus) also bred at these four platforms and at a fifth built in the south of
Cape Cross Lagoon (Rand 1963b). However, no records exist of guano collected at the southern Cape Cross platform. Guano harvests
from the Namibian platforms from 1931–1978 were listed by Cooper et al. (1982).

Guano harvests (Crawford and Shelton 1978, Crawford et al. 2007). The
total recorded collection of guano at the platforms between
Guano harvests at the four platforms in northern Namibia 1931 and 2021 was c. 159,000 t (Table 1). However, this
from 1931 (the initial collection) to 2021 are shown in will underestimate the actual amount as values were not
Table 1. The overall yield at the platforms increased after available for the Cape Cross central platform in 1935,
1931 to an average of c. 3,000 metric t between 1971 and 1936, 1945 and 1950 (Cooper et al. 1982) and for that
1993, attaining a peak of 4,854 metric t in 1984, and then platform and the Swakopmund platform from 2007–2011
decreased. The increase can be attributed to ongoing and in 2014 and 2021. The number of Cape Cormorants
building and expansion of platforms until 1971 (Cooper at breeding at the platforms was significantly related to the
al. 1982) and the decrease to a collapse after the 1970s production of guano at them (Crawford et al. 2007).
of Namibian sardine (Sardinops sagax), a main prey item Guano harvests from 1896–2016 at 11 Namibian islands
of Cape Cormorants, and environmental perturbations located south of the platforms are listed in Table 2. The
that reduced the availability of food off northern Namibia total amount collected at the islands increased from almost

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