Temperature - European Investment Bank

WEST AFRICAN POWER POOL (WAPP) PÖYRY ENERGY LTD.
Mount Coffee HPP ESIA and RAP 2012-09-18
ESIA Report Page 68
11 AQUATIC FAUNA
11.1 Theoretical Considerations
The main effect of a dam and reservoir project on fish populations is often the fact that
the dam will constitute an obstacle to migration. Many species of fish carry out
migrations, and some of them depend on these migrations for reproduction.
A second effect on fish is the fact that a part of the river will change from rive to lake
conditions. While some species can adapt easily to this type of habitat, others cannot
and will therefore diminish in numbers or will disappear altogether from this area.
A third potential effect on fish can be caused by a change in river flow pattern d/s of the
dam, e.g. in cases where seasonally flooded areas serve as breeding grounds, and when
this flooding no longer takes place due to the regulating effect of the dam.
Finally, fish populations can be affected by the indiscriminate introduction of exotic
species (which is sometimes done as mitigation measure for a dam project, but very
often independently of that, e.g. by introducing the exotic rainbow trout, a good sport
fish, in European and Asian waters, often to the detriment of native species).
11.1.1 Changes in Fish Species in Man-made Lakes
From numerous observations on environmental modifications related to the creation of
dams (Lowe-McConnell, 1966; Bolon & Coche, 1974; Lauzane, 1988; Levêque and
Paugy, 1999; Pouomogne, 2000), the following need to be spelt out. If trees, shrubs and
grasses are flooded, this generates a massive organic fertilization, which in turn leads to
the development of phytoplankton, and periphyton on dead trees. Bottom water
becomes poorer in oxygen, and rocky substrate is covered with mud and dead leaves,
which provokes the disappearance of species depending on such substrates, namely
Mormyridae. Those species with larger feeding niches (Alestes spp, Schilbeidae,
Clupeidae, etc.) easily adapt themselves to the new condition. Microphages and
macrophytophages heavily multiply (Tilapias, Citharinus spp, Labeo spp) alongside
with their predators (Hemichromis, Clarias, Lates spp.). Some anadromous species as
Barbus spp, having the habit of moving up into smaller running water bodies to
reproduce diminish with the creation of the dam. It is however important to note that
typical anadromous (moving from the sea or big rivers to sources) or catadromous
(reproducing in fresh smaller water, and spending the most of the lifetime in the sea) are
scarce in tropical waters. Tropical species are mostly potamodromous (i.e. they spend
most of their lifetime in larger rivers where feeding resources are more abundant, but
move upstream into tributaries, where predation is relatively less, to breed).
In most tropical man-made reservoirs, it is observed that the main biomass consists of a
very small number of species, from 3 to 6, consisting of Cichlids and Clariids. In Volta
river (Ghana), populations of Mormyridae tremendously decreased following the
building of the dam. Year 1 following the closure of the dam showed the explosion of
tilapias (S. galileus) with a huge decrease of Labeo and Chrisichthys, and also Alestes
nurse: the latter species is more related to running water ecosystem. On the other hand,
the number of species following the closure of Kainji dam (Nigeria) increased from 28
to above 40, in relation to more feeding resources, wider space and capabilities of
escaping from predators. Fish species seem to easily adapt themselves to changing
feeding habitats or niches, but have more difficulties in adapting to new breeding