Conservation and Sustainable Use of the Biosphere - WBGU

The Indonesian shallow sea E 2.4
107
18,000 tonnes between 1990 and 1994 alone
(Megapesca, 1999). Initially, fishing was done from
small boats. The rapid development of Nile perch
fishing since 1990 was largely possible thanks to
improved catching methods.As a consequence of the
advancing centralization of the fisheries industry in
East Africa jobs are being lost and the local fishing
industry is becoming much poorer again (O’Riordan,
1997).Therefore, the question of future development
is raised from a socio-economic point of view. However,
the emerging ecological problems may be even
more serious in the long term:
• Overfishing: The future prospects for fishing on
Lake Victoria appear to be favourable only at first
glance. The slight drop in fishing yields since 1994
indicates that the populations are already being
overfished. The hauls from small boats fell from
50kg to 10kg per boat. The practice of using pesticides
to catch Nile perch, which has only recently
become known, could be an indication of catches
that are already falling dramatically (Chapter B).
In spite of the extremely high primary production,
fish production in Lake Victoria is comparatively
low.
• Ecosystem degradation and eutrophication: The
rapid eutrophication of Lake Victoria is another
uncertainty factor in the future of Nile perch fishing.
As a result of the introduction of the water
hyacinth (Eichhornia crassipes) in 1990, which
spread rapidly as a consequence of the lake’s high
nutrient content (Twongo, 1993), the fishing situation
has deteriorated further. The water hyacinth
shades the water below it, greatly impairing or
completely preventing the photosynthesis of the
phytoplankton due to a lack of light. As a consequence,
oxygen-demanding processes dominate
even near to the surface. With the high water temperatures,
the oxygen may be used up completely
within a very short time under these conditions. In
recent years there have therefore been several
incidents of mass fish deaths.
• Destabilization of the trophic structure: The
destruction of the endemic tilapia populations not
only resulted in the extinction of a unique,
extremely species-rich biological community. It
also meant that a complex and finely-balanced
food web was replaced by a simple food chain
dominated by just a few species. Due to the loss of
oxygen as a consequence of the heavy eutrophication
there is a growing restriction on the Nile
perch’s habitat.The tilapias compensated for their
low numbers of young with marked brood care
behaviour (K selection) and had a varied food
spectrum that protected resources. In contrast to
this, the food web is now dominated by the rapidly
reproducing Nile perch (r selection). Ecologists
fear that this food web, which is dominated by a
single keystone species, is unstable, especially
when the future existence of this single species is
under threat.
E 2.3.5
Loss of world natural heritage: destruction of
native fish populations in favour of a alien species
The introduction of the Nile perch into Lake Victoria
put the food security and economy of the riparian
states Kenya, Uganda and Tanzania onto a new footing
within less than 20 years. This was done at the
expense of the extinction of a significant proportion
of the tilapia species that lived there. A unique piece
of world natural heritage was destroyed in this way.
However, it is suspected that at least some of the
tilapia species that have disappeared would have
died out anyway as a consequence of the eutrophication
of Lake Victoria, regardless of the introduction
of the Nile perch. The destruction of the native fish
populations has caused a drastic change in the natural
structure, which is now dominated by a single
species. As a result of overfishing and the continuing
eutrophication of the lake and the associated loss of
oxygen, there may be a collapse or a major impairment
of the fishing industry.What started as a success
story at the expense of a unique piece of natural heritage
could prove to be Pyrrhic victory within a few
years.
E 2.4
The Indonesian shallow sea: destruction of an
ecosystem through overuse and mismanagement
Just like every evening, the elegant restaurants of
Hong Kong are overflowing. Rich business people
want to offer their trading partners from overseas
something really special today: the lips of the
Napoleon fish (Cheilinus undulatus). This luxury is
worth up to US$300 per guest to them, but first of all
the colourful fish, weighing up to 30kg, has to be
admired in the restaurant aquarium while it is still
alive. The Napoleon fish is the emperor of the coral
reef fish, and its exorbitant price is by no means least
due to the rapidly dwindling populations. Would the
guests from Chicago, Düsseldorf or Tokyo enjoy their
dinner quite as much if they knew how the fish is
caught? A few hours earlier the fish was anaesthetized
by a diver with a cyanide solution in one of
the extensive coral reefs off the coast of Indonesia.
Since the fish had retreated into one of the countless
cracks in the reef, the reef had first of all to be
destroyed with a crowbar. Only then could the anaes-