Small impoundments: a necessary evil for fish conservation in a global biodiversity hotspot

Small impoundments represent one of the most widespread anthropogenic changes to the riverscapes in the Cape Fold Ecoregion (CFE) at the southern tip of Africa - a region which originally lacked natural standing water bodies. These impoundments were primarily built for agricultural and livestock farming and have proliferated with increasing demand for water. Consequently, there are few free-flowing streams in the CFE. Construction of small impoundments has been widely documented to have serious ecological and biodiversity impacts, including changes in hydrologic regimes and fragmentation of historically connected populations of stream-inhabiting fishes, with adverse effects on their genetic characteristics. Small impoundments have also created favourable habitats that have facilitated the proliferation and spread of non-native species. 

While these negative impacts cannot be denied, observations and preliminary results from a recent comprehensive survey of the Great Fish River system by researchers and students from the South African Institute for Aquatic Biodiversity and Rhodes University are showing that small impoundments can also benefit biodiversity. This survey was conducted in January 2017 as part of Delsy Sifundza’s MSc research (funded by the NRF-FBIP Karoo BioGaps Project) to map the distribution and determine the status of remnant populations of the Eastern Cape Rocky, Sandelia bainsii (Figure 1). The Eastern Cape Rocky is a highly threatened endemic stream fish which has been listed by the IUCN as Endangered since 1987 due to rapid decline in population sizes and severe decline in its historical distribution range.

A picture of the Eastern Cape Rocky, Sandelia bainsii, showing the species’ live colour pattern.

Our survey indicated that the Kat River is the remaining stronghold of the Eastern Cape Rocky in the Great Fish River system. The survival of this species in the upper section of the Kat River has been facilitated by the presence of several impoundments of various sizes (Figure 2) which have prevented the spread of non-native fishes, such as the sharptooth catfish, small-mouth yellowfish and banded tilapia which are now dominant in the mainstem Great Fish River. Sandelia bainsii was abundant at sites above the weirs that formed the upper limit of non-native fishes in the Kat River. In these river sections, S. bainsii occurred with three other native fishes of the Great Fish River system: Labeo umbratus (moggel), Enteromius anoplus (chubby head barb) and Glossogobius callidus (river goby). Some of these impoundments are therefore likely to have formed effective barriers that could have protected the genetic integrity of the original L. umbratus genetic lineage in the Great Fish River which is threatened by potential hybridisation with a genetically distinct lineage of this species that was introduced into the Great Fish through the Orange-Fish tunnel Inter-Basin Transfer. The Kat River therefore represents an important sanctuary of highly threatened endemic fishes of the eastern CFE and should be prioritised for protection. There is critical need for building awareness among the communities to prevent the spread non-native fishes in the Kat River catchment. There is also need for establishing collaboration between researchers, conservation authorities and the local farming communities and land owners to identify the weirs that are preventing the upstream migration of non-native fishes to ensure that they are effectively secured and protected from potential breach or flood damage.

 A. A major weir in the lower Kat River close to the confluence with the Great Fish River is an impassable barrier for fishes from the mainstem Great Fish; B. a moderately sized weir in the Upper Kat where the lower distribution limit of Sandelia bainsii was recorded during the survey in January 2017.  

Research Team

Miss Delsy Sifundza (MSc candidate, Department of Ichthyology and Fisheries Science, Rhodes University)

Mr Tadiwa Mutizwa (MSc candidate, Department of Ichthyology and Fisheries Science, Rhodes University)

Dr Albert Chakona (South African Institute for Aquatic Biodiversity)

Dr Wilbert Kadye (Department of Ichthyology and Fisheries Science, Rhodes University)