Smallmouth yellowfish is a species of ray-finned fish in the genus Labeobarbus. Attaining a mass of around 9kg and a 500mm fork length, it is an opportunistic feeder eating a variety of food types ranging from plant material to aquatic insects, crabs, shrimps and small fish.
Juveniles often display black spots on its dorsal side that aides in hiding from predators. As the fish matures the black spots tend to go away and the fish’s general appearance becomes golden yellow with a lighter coloured dorsal side.
One distinct characteristic of the smallmouth yellowfish is that it has what is referred to as a sub-terminal or “inferior” mouth type. Fish have evolved to have different types of mouths depending on what their diet is and how they feed. Inferior mouth types generally denote that the fish is a bottom feeder and eats things such as crustaceans or shellfish. Further the smallmouth yellowfish has two pairs of barbels lateral of this sub-terminal mouth.
The smallmouth yellowfish is a resilient and adaptable species that is widespread across its natural distribution range. It has the ability to inhabit smaller streams owing to its smaller size compared to the largemouth yellowfish. Although endemic to the Orange-Vaal River System, interference by man such as inter-basin transfer schemes, accidental relocations as well as intentional stocking for recreational purposes has greatly increased distribution. The smallmouth yellowfish has successfully inhabited a number of rivers and dams outside its natural range including the Sabie, Olifants, Gouritz, Sundays, Fish, Great Kei and Limpopo Rivers. There are also large populations in several dams in the Orange-Vaal River catchment, notably Sterkfontein, Gariep and Vaal Dams.
As mentioned the smallmouth yellowfish has a sub-terminal mouth and therefore mainly an omnivorous bottom feeder. Diet includes algae, snails, zoo plankton, small mussels, insects, small fish and detritus.
Smallmouth yellowfish become sexually active at around 7 years of age and a fork length of 300mm. The fish migrates upstream or to other suitable gravel beds to spawn usually after the first substantial rainfall of the season through to late summer. The shallow fast water is highly oxygenated and a highly productive zone for algae, diatoms and aquatic insect larvae and nymphs. After fertilization, the larvae of smallmouth yellowfish typically hatch after 3 days.
Both the smallmouth yellowfish as well as the largemouth yellowfish has at some point been assigned IUCN Red List status. Whilst the smallmouth yellowfish has now been removed, the largemouth yellowfish is still listed as “near-threatened.”
The biggest threat undoubtedly to the preservation of the smallmouth yellowfish is pollution. Effluent from sewerage plants, acid mine water return flows as well as irresponsible farming activities all pose a serious threat.
Other threats include the population explosion of grass carp in the middle Vaal as well as an increase in largemouth bass in the river system. Along with carp these fish has now become widespread and common in parts of the Vaal river and tributaries and compete with smallmouth yellowfish for food. Illegal netting, as well as different provincial fishing laws further contributes to the conservation status.
Organisations such as The Yellowfish Working Group (www.fosaf.org.za) and SAVE (Save the Vaal River Environment) is constantly engaging stakeholders and government in not only the protection of a valuable and ecologically sensitive river but also to promote the long-term conservation needs of yellowfish.