Geoscience Reference
In-Depth Information
Multiple human impacts also contribute to ecological change in rivers in poorer parts of
the world, of course. Madagascar, like many islands, has a great many 'endemic' species
(those found nowhere else) of all sorts, including fish, and its freshwater species are con-
sidered extremely vulnerable. Four of Madagascar's 64 endemic freshwater fish species are
feared extinct, and another 38 are endangered due to three main pressures: habitat degrad-
ation caused by deforestation, overfishing, and interactions with exotic species.
Madagascar's widespread deforestation has contributed to the degradation of aquatic hab-
itats in numerous ways. The loss of trees along river banks can result in changes in the
species found in the river because fewer trees means a decline in plant matter and insects
falling from them, items eaten by some fish. Fewer trees on river banks also results in less
shade. More sunlight reaching the river results in warmer water and the enhanced growth
of algae. A change in species can occur as fish that feed on falling food are edged out by
those able to feed on algae. Deforestation also typically results in more runoff and more
soil erosion. This sediment may cover spawning grounds, leading to lower reproduction
rates. More sediment can also clog the gills of fish, causing them greater stress which, in
combination with other pressures, can lead to their demise.
Overfishing of freshwater species is an intractable problem given the rising demand for fish
from a rapidly increasing human population in Madagascar and the great logistical diffi-
culties faced in enforcing any sort of environmental regulations. Exotic fish species intro-
duced to the island include both aquacultural and ornamental species, and their impact on
aquatic ecosystems has been profound. Some exotics have become naturalized, completely
replacing native fish in the central highlands of Madagascar and becoming widespread in
other parts of the island.
River regulation
Efforts to control the water level of rivers and the variability of river flows, to meet the
demands of society, date back to the earliest civilizations. Today, rivers are regulated for
many reasons, primarily to maintain an even flow for domestic, agricultural, and industri-
al needs, for hydroelectric power generation, for navigation, and to prevent flooding. The
major methods employed in river regulation are the construction of large dams (see below),
the building of run-of-river impoundments such as weirs and locks, and by channeliza-
tion, a term that covers a range of river engineering works including widening, deepening,
straightening, and the stabilization of banks.
