55UXOuRjI

Chapter 2: Freshwater Echosystems
31%
OF INDIGENOUS
FISH SPECIES ARE
THREATENED
...use of bulldozers
to construct
canal systems
compromises the
vital riparian zone
Fish threaten fish
The greatest threat to freshwater fish biodiversity, especially in the Cape fynbos
region, has been invasive alien fish species in terms of predation on and competition
with indigenous species. A large number of alien fish species were introduced to
South African waterways for angling purposes (such as black bass, trout, carp,
Mozambique tilapia, bluegill sunfish) and have proliferated.
Excessive water abstraction from naturally perennial rivers poses another severe
threat to our indigenous freshwater fish, especially during dry summers. Often,
levels of abstraction are so high that rivers stop flowing, resulting in habitat
depletion or water that is too warm to sustain river species.
Farming impacts freshwater
Rivers in farming areas are often canalised by bulldozing to regulate water supply
and stop flood damage to orchards and crops, which are frequently planted in
floodplains. This can affect the structure and functioning of a river.
The use of bulldozers to construct canal systems damages the river banks and beds,
and compromises the vital riparian vegetation zone, which is essential for river
bank stabilisation and nutrient cycling in the river, causing substantial erosion and
movement of sediment, which then fills pools with sand downstream. This has a
major impact on the indigenous rock catfishes that live in crevices between rocks.
A growing problem is also the excessive use of fertilisers and harmful pesticides
on crops, vineyards and orchards. Again, this problem is exacerbated when crops
and orchards are planted near or within river flood zones, because fertilisers and
pesticides then readily enter watercourses and rivers, which can have lethal effects
on aquatic life.
THE FIVE SOUTH AFRICAN THREATS TO FRESHWATER BIODIVERSITY
1 OVER-ABSTRACTION OF WATER
Removal of too much water,
especially during the dry months
of the year, exacerbated by
invasive alien plants.
2 POOR WATER QUALITY
Water quality problems
associated with non-point source
pollution from fertilisers, as well
as point-source pollution from
mining and failing wastewater
treatment works.
3 INVASIVE ALIENS
Impacts of invasive alien
vegetation in the riparian zone
and alien fish species.
4 DEVELOPMENT
Urban development in estuarine
functional zones.
5 HABITAT DESTRUCTION
Destruction of freshwater
habitats, especially from
bulldozing in the riparian zone.
© IMAGE: WWF-SA
Source: CSIR
Water Facts & Futures: Rethinking South Africa’s Water Future | Page
23