A Biological Roller Coaster Ride CORE CASE STUDY^

in Lake Victoria

Sustaining Aquatic

Biodiversity

11

Lake Victoria, a large, shallow lake in East Africa (Figure 11-1),
has been in ecological trouble for more than 2 decades.
Until the early 1980s, the lake had 500 species of fish found
nowhere else. About 80% of them were small fish known as
cichlids (pronounced “SIK-lids”), which feed mostly on detritus,
algae, and zooplankton. Since
1980, some 200 of the cichlid spe-
cies have become extinct, and some
of those that remain are in trouble.
Several factors caused this dra-
matic loss of aquatic biodiversity.
First, there was a large increase in
the population of the Nile perch
(Figure 11-2). This large predatory
fish was deliberately introduced
into the lake during the 1950s and
1960s to stimulate exports of the
fish to several European countries,
despite warnings by biologists that
this huge fish with a big appetite
would reduce or eliminate many de-
fenseless native fish species. The population of this large and pro-
lific fish exploded, devoured the cichlids and by 1986 had wiped
out over 200 cichlid species.
Introducing the perch had other social and ecological effects.
The new mechanized fishing industry increased poverty and
malnutrition by putting most small-scale fishers and fish vendors
out of business. And because the oily flesh of the perch are pre-
served by use of a wood smoker, local forests were depleted for
firewood.
Another factor in loss of biodiversity in Lake Victoria was
frequent algal blooms. These blooms became more common in
the 1980s, due to nutrient runoff from surrounding farms and
deforested land, spills of untreated sewage, and declines in the
populations of algae-eating cichlids.
Also, the Nile perch population is decreasing because it
severely reduced its own food supply of smaller fish-
es—an example of one of the four scientific principles
of sustainability (see back cover) in action—and it also
shows signs of being overfished. This may allow a gradual in-
crease in the populations of some of the remaining cichlids.
This ecological story about the dynamics of large aquatic
systems illustrates that there are unintended consequences when
we intrude into a poorly understood ecosystem.

This chapter is devoted to helping us to understand the

threats to aquatic biodiversity and what we can do to help sus-

tain this vital part of the earth’s natural capital.

Courtesy of the African Angler

Figure 11-2 Natural capital degradation: the Nile perch

is a fine food fish that can weigh more than 91 kilograms

(200 pounds). However, this deliberately introduced fish

has played a key role in a major loss of biodiversity in East

Africa’s Lake Victoria (left).

Figure 11-1 Lake Victoria is

a large lake in East Africa.

LAKELAKE

VICTORIAVICTORIA

KENYAKENYA

ETHIOPIAETHIOPIA

TANZANIATANZANIA

UGANDAUGANDA

SUDANSUDAN

ZAIREZAIRE BARUNDIBARUNDI

RWANDARWANDA LAKE

VICTORIA

KENYA

ETHIOPIA

TANZANIA

UGANDA

SUDAN

ZAIRE BARUNDI

RWANDA

AFRICA