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3 November 2022
Fish species in Nuwejaars River extremely vulnerable
There are only three native freshwater fish species left in the Nuwejaars River catchment and Agulhas Plain areas near the coastal towns of Arniston and Bredasdorp.
Four species found in the rivers in this Overberg region are alien. The fish of particular concern to researchers and conservationists is the Sharptooth Catfish (Clarias Gariepinus) which they fear could pose a disastrous threat to indigenous fish species in the area. 

This was revealed at a recent community engagement session where researchers at the Institute for Water Studies (IWS) at the University of the Western Cape (UWC) shared their research findings derived from the catchment area with representatives from Cape Nature, the Department of Water Affairs and community nature organisations in the Elim and the Bredasdorp region.
Professor Jenny Day

Professor Jenny Day, an extraordinary professor at IWS, is a freshwater ecologist with particular interest in the conservation and management of riverine and wetland invertebrates and the effects of water quality on inland waters. 

She said the three native species that occur are the Cape galaxias (Galaxias zebratus), the Cape kurper (Sandelia capensis) and the Heuningnes redfin (Pseudobarbus burchelli). 

Other alien fish species are the smallmouth bass, largemouth bass and bluegill sunfish, which also pose a considerable threat to the local fish species, all of which are small and vulnerable.

“The Sharptooth Catfish is a very large and voracious fish, able to wipe out entire populations of fish species in no time. It is even able to walk overland. 
An illustrated image of a Sharptooth Catfish

“I fear that fishing enthusiasts would make the mistake of releasing more of this breed into the rivers in the area, and unless communities are educated on the destructive nature of this fish and the dire consequences it may have for the environment, it would become impossible to manage,” she said.

Prof Day also presented research findings on ground and surface water on the Agulhas Plain. She quoted the results of UWC researchers Errol Malijane (surface and groundwater data), Nolitha Khungwayo (nutrients) and Adeola Abegunde (trace elements and pesticides).

She noted when it came to surface water quality, the major questions are usually how the properties and chemicals of the river water vary over time and what the water quality is like downstream.

Furthermore, there are questions about how water quality is affected by pollutants such as nutrients and fertilisers, pesticides and other agrichemicals.


Prof Day said that based on their data, the overall assessment of the Nuwejaars River is that it enjoys a good overall condition which is fairly typical of a mid-reaches stream in the South Western Cape. The only exception is physical damage in upstream reaches, which reduces habitat availability and suitability and decreased water quality downstream of the dairy farm. 

“The drought and rivers drying up were detrimental to a ‘normal’ invertebrate community, but fortunately for us, recovery after the recent drought was rapid.”

Prof Day said the results of these studies stem from data collected on Agulhas wetlands going back to the 1970s. This includes studies by UWC, the University of Cape Town (UCT) and independent studies on the Agulhas Plain rivers and wetlands faunal studies.


“We wanted to find out what was there, understand the habitat requirements and identify species and methods suitable for biomonitoring.

“We found about 15 species of frog on the Agulhas Plain; about 10 are common and occur widespread in SA, for example the Rattling Frog, (Semnodactylus wealii) and the Cape river frog (Amieta fuscigula).

“Rose’s rain frogs are unusually terrestrial for frogs; they can’t swim, spend a long period of dormancy in a self-made cocoon, lay eggs in jelly in a burrow and have no tadpole stage. Their young are born as young froglets.”

An example encountered was the very vulnerable Rose's Mountain Toad (Capensibufo rosei), endemic to the winter rainfall region, breeding in small temporary pools. 

Another endangered frog detected is the Cape Platanna, usually confined to ‘black’ fynbos waters.

The good news for researchers and conservationists is the new subpopulation of the critically endangered Micro Frog (Microbatrachella capensis) found in a hidden wetland in the Nuwejaars Wetlands Special Management Area (SMA).”
The Heuningnes River flowing into the Soetendalsvlei, on the Agulhas Plain area
Images supplied. Main image: One of the indigenous fish found - Cape galaxias (Galaxias zebratus) Source: iNaturalistNZ