Fish on the Orange River

Lower Orange River Section

Because of the harsh aquatic environment, i.e. unpredictable erratic flows, droughts and episodic floods, the ORS (Orange River System) as a whole is relatively poor in indigenous freshwater fish species diversity. The fishes of the ORS have, over long periods of natural selection, adapted to a riverine environment and are mainly bottom feeders or predators. Because of these adaptations, they can benefit from the natural seasonal changes in environmental factors such as flow, temperature and turbidity. They generally spawn from the onset of spring through to autumn, when the river is in its annual high flow period, utilizing the flooded river banks and floodplains, conditions conducive to growth and survival of the young.

Fish Species

Presently, eight fish families are represented by 22 species in the ORS. Of these, three families, represented by five species are alien to Southern Africa, one species of an indigenous family is also alien and one species, indigenous to the subregion, of another family had been introduced to the ORS. The ORS thus only contains 15 indigenous freshwater fish species, representing five families – compare with KwaZulu-Natal’s ±85 species. Of the 15 indigenous fish species, six are endemic to the ORS with four of them Red Data listed. Of the remaining nine indigenous species, two are becoming threatened within the system, while the possibility exists that one could be an endemic subspecies. Freshwater fish species found between Augrabies Falls and the Orange River Mouth as below:

Longfin Eel (Anguilla mossambica)

It is the most numerous anguillid in southern Africa, with its distribution mainly restricted to the eastward flowing river systems draining into the Indian Ocean. Scientific records indicate that it is rare in the ORS, although anglers claim more frequent catches. Observations of eels (life history stage and size unknown) entering the Orange River mouth during the 1988 floods (about mid-February to April) had been reported. This eel can grow up to 1.2 metres long with the average weight of between 2-3 kilograms.

Carp (Cyprinus carpio)

It is an alien freshwater fish species, which had been introduced to the former Cape Province in 1896 and has, due to its ability to adapt to a wide spectrum of habitat conditions, the widest distribution range of all alien fish species in southern Africa. In the ORS, its distribution ranges from below the trout waters in the upper catchment to the Orange River Mouth. It follows varied diet, from vegetable matter to aquatic animals, and its habit of dredging its environment’s bottom mud, enables it to find food almost anywhere.

Straightfin Barb (Barbus paludinosus)

It is an extremely widespread tropical fish species in Africa, with its distribution in the ORS similar to that of B. trimaculatus and its numbers rather low. It prefers quiet to slow flowing, moderately vegetated bays, shores, backwaters, pools and impounded areas, the habitat type where its young is probably also bred and nursed. Occasional flood/high flow appears to be a vital component of its life history strategy, however, flow appears not to be this species’ major breeding stimulus.

Orange River Mudfish (Labeo capensis)

It is endemic to, distributed throughout the ORS, and the dominant large ORS fish species. However, its numbers decline towards the lower Orange River stretch between Aughrabies Falls and the Orange River Mouth. It is a detritivore appearing to be utilizing all aquatic habitat types in the Orange River System (ORS) from quiet, weedy backwaters to rapids. Its breeding is naturally triggered by local rains throughout summer, and spawning occurs in floodplains, main streams and rapids between mid-spring and late summer/ early autumn. Flow is a major stimulus for L. capensis’ breeding.

Largemouth Yellowfish (Barbus kimberleyensis)

It is endemic to, and widely distributed in the ORS, except for its absence from the Lesotho catchment. It is, however, not abundant in the regulated Orange River between Vanderkloof Dam and the Orange River Mouth. It is a predator and thus essentially a visual feeder which prefers clear, fast-flowing water with a sandy to gravel substrate. It takes approximately seven years to mature sexually, reaching sexual maturity at extreme minimum lengths of 35 (male) and 36 cm (female) and breeds in and below rapids during the first post-winter floods from early spring to summer. Under regulated conditions, temperature probably became its determinant breeding factor with low temperatures (hypolimnetic water) retarding breeding, initial growth rates and juvenile survival. Although predators generally occur at lower densities than prey species, this species’ extremely low numbers in the regulated Orange River to river regulation and increasing catchment utilization adversely affecting its reproduction, recruitment and dependence on visibility for feeding, and suggests that it is Red Data listed.

Banded Tilapia (Tilapia sparrmanii)

It has a natural distribution from the ORS northwards to the Kunene, Limpopo and Zambezi River Systems, and, from the Limpopo southwards, to the Tugela River in KwaZulu-Natal. It is widespread, but not abundant, in the lower Orange River. It is an omnivore, feeding on aquatic vegetation, zooplankton, small crustaceans, insect larvae, nymphs and worms. It prefers well vegetated, silted shores, pools and backwaters in slow flowing riverine areas. It is a substrate spawner, with monogamous habits, which breeds from spring to late autumn. Spawning and fertilization takes place in a hollow, and eggs and fry are guarded by both adults. Occasionally, eggs and fry are picked up in the mouth to be moved to other sites, but no mouth brooding occurs. This species seems to benefit from river regulation and catchment utilisation as river regulation and catchment utilisation stimulate the development of Phragmites reed beds, which, in turn provide shelter and enhance flow stabilisation and silt deposition, thus creating habitat for this fish species.

Sharptooth Catfish (Clarias gariepinus)

It has a wide distribution range in southern Africa, with the ORS its southern-most natural distribution and does not occur in large quantities under riverine conditions. It is an omnivorous scavenger, usually feeding at the bottom or in thickly weeded areas. Large members of this species are also known to be cannibalistic. Its habitat ranges from deep profundal to shallow littoral areas Equipped with suprabranchial organs (pseudo lungs), it can be found in water of low oxygen content and because of this air breathing capability, it can succumb easily to pesticide sprays. Oranjemund inhabitants reported large numbers of dead C. gariepinus, washed up on the Atlantic Ocean shores during and after the 1988-flood, indicating that this species could be vulnerable during extreme flood conditions.

Rock Catfish (Austroglanis sclateri)

It is endemic to Orange River System and widely distributed in the ORS in both warm and cold waters, but is, however, not common, even in its preferred habitat and is therefore Red Data. It is an omnivore, feeding primarily on aquatic insects, larvae and nymphs, with large specimens taking small fish. Predation (visual feeding) thus also forms a major part of its feeding habits. It appears to be highly specialised regarding its habitat requirements, consisting basically of a rock/sand/gravel substrate and ranging from bedrock with/without scattered rocks and sandy to gravel substrates, to rocky pools, and riffles, with the surrounding aquatic environment adhering to specific water quality standards. It is reported that its populations to show a “patchy” distribution downstream of Vanderkloof Dam, suggesting that river regulation and catchment utilization, together with the resulting constant turbidity levels, fragmented it populations in the regulated Orange River, restricting it to clearer slow flowing to stagnant water.

Southern Mouthbrooder (Pseudocrenilabrus philander)

It has a wide distribution range, occurring from the Vaal, Orange (below Orange-Vaal confluence) and Uvongo (KwaZulu-Natal) Rivers, northwards. It is a predator, with its young feeding on zooplankton while the adults feed on small fish, crustaceans, aquatic insect larvae and hovering insects at the surface. Its habitat ranges from rocky rapids, through rocky shores, sandy open water and backwaters, to flowing channels, with preference given to sheltered vegetated areas. It is a mouth brooder, which mainly breeds in slow flowing sheltered pools and backwaters during mid-spring to mid-autumn. This species has benefited from river regulation because of its commonness in the extensive reed beds in the lower Orange River.

Mozambique Tilapia (Oreochromis mossambicus)

It is widely distributed in the African east coast rivers, from Beira to as far south as the Brak River, south of Port Alfred (South Africa). It is the only known indigenous freshwater fish species introduced to the ORS, and specifically the lower Orange River section between Aughrabies Falls and the Orange River Mouth. This euryhaline mouth brooder was originally imported to the Hardap Dam (Fish River: Namibia), from where it made its way to the mentioned lower Orange River stretch. When last recorded, its distribution ranged between downstream of Goodhouse to the Orange River Mouth. It is being expected that this species will eventually extend its distribution range to include the whole of the Orange River section between Aughrabies Falls and the Orange River Mouth.

Namaqua Barb (Barbus hospes)

River Sardine (Mesobola brevanalis)

It has a wide, discontinuous distribution in southern Africa. In the ORS, it is restricted to Orange River stretch between Augrabies Falls and the Orange River Mouth where it is the most common and abundant fish species found in the open water habitats of the mainstream, quiet backwaters as well as flowing channels and rapids, preferring quiet open and/or backwater habitats. Its abundance and habitat preference suggest that it benefits from river regulation. Its external morphology suggests it to be an active open water feeder, which feeds on planktonic crustaceans, aquatic insects from the bottom or mid-water. Attracted to light at night.

Threespot Barb (Barbus trimaculatus)

It is a widespread tropical African freshwater fish species. In the Orange River System, its distribution includes the Vaal River catchment, as well as the Orange River below the Orange-Vaal confluence. Its habitat preference is rapid areas. Feeds on insects and other small organisms. Used as bait for the tigerfish.

Smallmouth Yellowfish (Barbus aeneus)

It is endemic to, and widely distributed throughout the ORS and is presently the most abundant large fish species in the Orange River. It is an opportunistic omnivore, which prefers clear, fast-flowing water and a sandy to gravel substrate. Diet includes water fleas, snails, plankton, small mussels, insects, small fish, algae and detritus.