Conservation and Sustainable Use of the Biosphere - WBGU

106 E Diversity of landscapes and ecosystems
40m) and the long water residence time (approx 100
years), the lake is especially sensitive to eutrophication
(Vollenweider, 1968).
E 2.3.2
Formation of the tilapia species as a textbook
example of the theory of evolution
MacArthur and Wilson (1963) explained the number
of species on islands by establishing a balance
between the invasion of new species and the extinction
of existing species, with the resultant number of
species increasing with the size of the island. Barbour
and Brown (1974), Magnuson (1976) and Eckmann
(1995) were able to show, with a statistical analysis of
many lakes, that this regularity can also be applied to
lakes and the number of fish species does actually
increase with the size of the lakes studied. For Lake
Victoria this would thus mean a theoretical number
of around 100 fish species (Magnuson, 1976). However,
there are actually almost 400 cichlid species. As
a result of the molecular-biological examination of
their genetic material we can conclude that all tilapia
in Lake Victoria have descended from a single ancestral
form that migrated from Lake Tanganyika (Barel
et al, 1977; Meyer et al, 1990). These tilapias are a
textbook example of the process known as ‘adaptive
radiation’, comparable to the Darwin finches on the
Galapagos Islands. Adaptive radiation describes a
process in which new forms develop from an original
form after colonizing a habitat, by adapting to the different
biological conditions. In the case of tilapias
this process was favoured by low population sizes,
intensive brood care and the genetic isolation of individual
populations (Stiasny and Meyer, 1999). Since
it is probable that Lake Victoria almost completely
dried out around 12,000 years ago, it can be assumed
that this species diversity has developed over an
extremely short period of time for a speciation
process (Stiasny and Meyer, 1999). In contrast to this,
the large number of endemic species in Lake Baikal
in Siberia (around 2,000) is due to the great age of
this lake (approx 35 million years) (WBGU, 1998a).
E 2.3.3
The changing face of Lake Victoria’s ecosystem
The population in the catchment area of Lake Victoria
used to mainly live by subsistence farming and
animal husbandry; fishing played a subordinate role.
In 1962 the Nile perch from Lake Albert was
released at Entebbe; another release took place in
1963 (Lowe-McConnell et al, 1992).The Nile perch is
a fast-growing predatory fish that reaches a weight of
up to 200kg. One of the people who knew the east
African lakes best had warned against such stocking
measures (Fryer, 1960). The Nile perch really did
reproduce rapidly in Lake Victoria and caused the
extinction of many tilapia species. In the open water
93 per cent of all tilapia species were eliminated; on
the rocky coasts around 70 per cent, and in shallow
areas 30 per cent (Goldschmidt, 1997). Only species
that live in areas that cannot be sought out by the
Nile perch were protected (Kaufmann, 1992). Fears
initially expressed that the Nile perch population
could collapse after eliminating the tilapias, which
were their original source of food, have not been
realized. In particular, the following changes to the
food web in Lake Victoria arose (Goldschmidt et al,
1990):
• The Nile perch has replaced most of the fish-eating
tilapias as well as the catfish that feed on
tilapias (Bagus dokmae and Clarias gariepinus).
• The prawn Cardinia nilotica has replaced the previously
numerous, particle-eating cichlids and is
the most important food source for young Nile
perch. Furthermore, 30 per cent of the food of the
Nile perch is mosquito larvae of the genus
Chaoborus. During the day both species flee to
anoxic deep water layers, which the Nile perch
cannot reach because of its need for high levels of
oxygen (Branstrator and Mwebaza-Ndawula,
1998).
• Some of the ecological niches previously inhabited
by tilapias have not been occupied by other
species.This means that in today’s ecosystem there
is lack of species that feed on phytoplankton. As a
consequence, the biomass of the algal plankton
increased greatly. Tilapias that eat insect larva
were not replaced either, meaning that mosquito
larvae, above all, reproduced rapidly (Goldschmidt,
1997; Lehman et al, 1998).
E 2.3.4
Is the transformation of Lake Victoria a blessing
for the local population?
With its high fishery yields, the Nile perch has led to
a radical transformation of the economy around
Lake Victoria. Since 1980 the annual catch of Nile
perch has risen to 362,000 tonnes, 29 per cent of
which are caught in Kenya, 27 per cent in Uganda
and 44 per cent in Tanzania. The income of the three
riparian states from the export of Nile perch (marketed
as ‘Victoria perch’) are around US$140 million
per year. The Nile perch has become one of the most
important export products for Uganda and Tanzania.
The main buyers are Europe, Israel and Australia. In
the EU, imports of Nile perch rose form 4,000 to