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).
The largemouth yellowfish or Vaal-Orange largemouth yellowfish (Labeobarbus kimberleyensis) is a ray-finned fish species in the family Cyprinidae. This large freshwater barb is found in southern Africa.[2]
It has long been placed in Barbus, the "wastebin genus" for barbs, by default; however, the species is increasingly being restored to related yellowfish genus Labeobarbus which seems a much more appropriate placement. It is probably hexaploid like the other yellowfish. L. kimberleyensis shares mtDNA haplotypes with the sympatric smallmouth yellowfish (L. aeneus), but is morphologically distinct. This typically indicates either species that have recently diverged, or hybrid introgression, or morphs that are mistakenly considered distinct species. The latter does not seem likely in this case, as the two differ much in size alone, but the actual cause for the genetic similarity remains unstudied.[3]
The largemouth yellowfish occurs in the Orange and Vaal Rivers and their larger tributaries (e.g. the Riet River) in Lesotho, Namibia and South Africa. In the latter country, it is found in Eastern Cape Province, Free State, Gauteng, Mpumalanga, North-West Province and Northern Cape Province.[2]
L. kimberleyensis is predominantly found in deep pools (deeper than 2 metres/yards) of large rivers, as well as in the slow-moving water before weirs and river dams (e.g., Sterkfontein Dam). Abundant water weeds, overhanging riparian vegetation and other forms of plant cover seem to be essential for its well-being. They are predators of aquatic large invertebrates and small invertebrates; adults feed almost exclusively on fish. Spawning occurs in riffles during summer (around December/January); a large female can lay more than 60,000 eggs. They are slow-growing and long-lived and reach a total length of 30 cm (12 in) only after five years.[2]
Compared to some of its relatives, the stocks of the largemouth yellowfish are still relatively healthy and it is not considered a threatened species. It is listed as Near Threatened by the ICZN though, as many ecosystems in which it occurs are severely degraded, and if this does not change, it probably cannot maintain viable populations for long. In the lower Orange River, considerable numbers are still found. Damming may cut off local populations from spawning sites. The Vaal River is highly laden with pollutants from sewage outside the wet season, and fish kills have been reported due to this. As it is an apex predator, its population density cannot be high. It is popular with anglers and theoretically a valuable food species, but it is recommended to catch and release it until the water quality is improved – for one thing, catching them for food may deplete local stocks to the point of extinction, for another, as an apex predator it accumulates toxins and may not be safe to eat. Whether significant introgression with the smallmouth yellowfish (Labeobarbus aeneus) occurs and yields less viable hybrid offspring (which would also serve to decrease its stocks) needs to be determined.[2]
L. kimberleyensis is found in the Augrabies Falls and Richtersveld National Parks. Anglers are being educated about this flagship species and encouraged to practice catch and release, which is mandatory in Free State. The species has also been successfully transplanted to dams within its range that have nearby shallow-water regions for spawning.[2]
The largemouth yellowfish or Vaal-Orange largemouth yellowfish (Labeobarbus kimberleyensis) is a ray-finned fish species in the family Cyprinidae. This large freshwater barb is found in southern Africa.
It has long been placed in Barbus, the "wastebin genus" for barbs, by default; however, the species is increasingly being restored to related yellowfish genus Labeobarbus which seems a much more appropriate placement. It is probably hexaploid like the other yellowfish. L. kimberleyensis shares mtDNA haplotypes with the sympatric smallmouth yellowfish (L. aeneus), but is morphologically distinct. This typically indicates either species that have recently diverged, or hybrid introgression, or morphs that are mistakenly considered distinct species. The latter does not seem likely in this case, as the two differ much in size alone, but the actual cause for the genetic similarity remains unstudied.
