The Orange River is the longest watercourse in Southern Africa. Lying south of the Zambezi River, The Orange River rises in the Drakensberg Mountains and flows westward to discharge into the Atlantic Ocean. The river has a length of 2208 kilometres and drains 48 percent of the land area of South Africa and forms the national boundary between that country and Namibia. The total drainage area amounts to 896,368 square kilometres, and the discharge at the mouth is about 11.5 cubic kilometres per annum.
Excessive nutrient loading from overly intensive fertilizer usage in agricultural areas in the Vaal and middle reach Orange River is the major water quality issue in the basin. Headwaters areas of the basin support high endemism in flora and reptiles, while the middle reaches of the basin boast significant endemism in small mammals. Lower reaches of the basin support high endemism in both reptiles and small mammals.
The chief water quality concerns are within South Africa, and more specifically in the densely populated areas of Johannesburg, Pretoria and the Vaal Triangle. Exacerbating the issue are insufficiency and ageing of the wastewater treatment plants of that locale, and the fact that discharges from that high population density region is at a higher elevation than the principal dams along the Orange River; thus, inevitably polluted discharges from the densely populated area reaches these warm termperature reservoirs, which are then poised to generate elevated bacterial levels.
Within the Orange River system 21 different fish taxa have been recorded, most of which are benthopelagic. The three largest benthopelagic native species are: the 170 centimetre (cm) long North African Catfish (Clarias gariepinus), the 122 cm Flathead Grey Mullet (Mugil cephalus) and the endemic 92 cm Vaal-Orange Largemouth Yellowfish (Labeobarbus kimberleyensis). Other noteworthy native benthopelagics are the basin endemic 56 cm Orange River Mudfish (Labeo capensis), the 56 cm basin endemic Smallmouth Yellowfish (Labeobarbus aeneus), the 45 cm Redbreast Tilapia (Tilapia rendalli). L aeneus may be useful in algae control in the Orange basin, since this omnivorous bottom feeder consumes considerable algae in its diet. The 146 cm Wild Common Carp (Cyprinus carpio carpio) is the largest introduced benthopelagic alien species in the Orange River.
The largest fish species in the Orange River system is the 200 cm pelagic-neritic Leerfish (Lichia amia), which is a true aquatic apex predator, functioning at trophic level 4,5. Native demersal fish are the 40 cm South African Mullet (Liza richardsonii) and the near endemic 37 cm Rock Catfish (Austroglanis sclateri).
