Conservation and Sustainable Use of the Biosphere - WBGU

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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

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