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Rationale

The major decline in population size occurred during the introduction of Smallmouth Bass (Micropterus dolomieu) in to the Berg River System in the 1940s and 1950s. In addition, the system is experiencing growing anthropogenic impacts which will cause further population reductions. A subpopulation in the Eerste River System has gone extinct, with serious declines noted in the Boesmans subpopulation. This area occupied by the species is 0.078 km² (26 km x 3 m) calculated using riverlengths (Boesmans (3 km), Goedverwacht-Platkloof (4 km), Leeu (2 km), Krom (4 km), Hugo (2 km), Wemmers (8 km) and Upper Berg (3 km) populations) however based on the IUCN approach of a 2x2 km² grid overlay onto distributional records, the area of occupancy AOO is calculated as 90 km² which is used in this assessment and a extent of occurrence (EOO) of 2,382 km². Four locations are known, defined from the areas affected by alien invasive fish species (including Smallmouth Bass and Rainbow Trout (Oncorhynchus mykiss)) and barriers to these; namely Boesmans, Goedverwacht-Platkloof, Krom and the remaining range area. There is continuing decline in the habitat condition as a result of invasion and excessive water abstraction. The population is severely fragmented due to biological barriers to gene flow as a result of Smallmouth Bass and other alien fishes. In addition, in-stream dams also affect upstream migration opportunities. This species is assessed as Endangered B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v).

Distribution

Berg River Redfin (Pseudobarbus burgi) is endemic to the Berg River System and occurs in the Boesmans, Goedverwacht-Platkloof, Leeu, Krom, Hugos and Wemmers (Olifants and Drakenstein) tributaries as well as in the upper reaches of the Berg River (tributary type habitat) above the Berg River Dam (Bloomer and Impson 2000, Swartz 2005). The upper Berg River and Wemmers subpopulations are also the only subpopulations of this species that partially occur within protected areas (Hottentots Holland Nature Reserve complex and Limietberg Reserve complex). A recent new record for the Leeu River in the Grootwinterhoek Nature Reserve brings the subpopulations associated with nature reserves to three. The Eerste River system's population may have belonged to this taxon, but have gone extinct (Skelton 1987, Skelton 1988). Recently, based on a genetic study (Bloomer and Impson 2000), morphological evidence (Skelton 1988) and field observations, the Verlorenvlei subpopulation of this species has been described as a new species Pseudobarbus verloreni (Chakona et al. 2014) and is excluded from this assessment.

Habitat View more ecological information

Population trend

Trend

There are subpopulations in the Boesmans, Goedverwacht-Platkloof, Leeu, Krom, Hugos and Wemmers (Olifants and Drakenstein) tributaries as well as in the upper reaches of the Berg River (tributary type habitat) above the Berg River Dam. The strongest subpopulations are in the upper Berg River and the Olifants and Drakenstein tributaries of the Wemmers River. There is continuing decline in the Boesmans tributary because of excessive water abstraction high up in the river in the dry season (October to March). Subpopulations are severely fragmented due to a biological barrier to gene flow as a result of Smallmouth Bass and other alien fishes. Rainbow Trout (Oncorhynchus mykiss) is a significant threat to the Berg River Redfin in headwater streams. The subpopulations in the upper Berg River and those above Wemmershoek Dam (Olifants and Drakenstein Rivers) co-occur with Rainbow Trout which probably have localized impacts during low temperatures and high flows in winter. These tributaries however become marginal for trout species as a result of sub-optimal summer temperatures which enable the Berg River Redfin to co-occur with Rainbow Trout. Recruiting populations of Berg River Redfin are currently present at all known localities. In the case of the Hugos River, Rainbow Trout are established in the upper reaches of the river and may have been the reason for the range contraction observed for the Berg River Redfin which presently only occur in downstream sections of the river. However, several of the subpopulations are probably large enough to survive long-term without the need for immigration.

Threats

Alien predatory fishes are the main threat, especially Smallmouth Bass (Micropterus dolomieu) which has isolated populations in upper reaches of tributary streams. Rainbow Trout (Oncorhynchus mykiss) has an impact higher up in some of the tributary streams where they are able to survive due to cooler temperatures. The threat of Rainbow Trout is especially relevant in the tributaries above the Berg River Dam (Assegaaibos and upper Berg) and Wemmershoek Dam (Olifants and Drakenstein). Historical records for Berg River Redfin exist for the upper Hugos River but their absence was confirmed in 2012, possibly as a result of the presence of Rainbow Trout and the impacts of a trout farm on the banks of the upper Hugos. The relatively recent introductions of African Sharptooth Catfish (Clarias gariepinus) and their successful establishment in the main Berg River will have to be monitored to assess what impact this alien will have on populations, as African Sharptooth Catfish have been shown to be able to invade headwater tributaries (Ellender et al. 2015). Excessive water extraction, both for agricultural and residential use has severely impacted upon lower reaches of tributary streams. Impacts from intensive agriculture (water abstraction and agrichemical pollution) is likely to be negatively affecting the Krom River population as Berg River Redfin was detected only in the uppermost reaches of this river during a 2012 survey with the river being too small to provide habitat in most areas.

Conservation

All rivers where the species has viable populations have been listed as “Fish sanctuaries” in the Atlas of Freshwater Ecosystem Priority Areas in South Africa (Nel et al. 2011). Public awareness has been increased by creating signage of the freshwater fishes of the Berg Water Management Area for the general public. In the upper Leeu River, alien plant clearing has been implemented by the Twenty-Four Rivers Irrigation Board to improve the health of the riparian zone and increase dry season flows. Similarly, alien plant clearing has been implemented very successfully in the upper Berg River to improve surface flow for the Berg River Dam. Numbers of Berg River Redfin appear to be increasing in this part of the river in response to the rehabilitation. A research study has been initiated to investigate thermal tolerance of native fishes and Rainbow Trout to determine habitat use by these species in relation to temperature. This will provide an improved understanding of the occasional co-occurrence of Berg River Redfin with non-native Rainbow Trout, especially given the future threat of climate change.

References

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