The Knersvlakte is typically known as an arid habitat housing plants that have adapted to extreme heat and water scarce conditions. With an annual rainfall above 100mm being the exception rather than the norm, it easily explains why most of the area’s rivers are non-perennial and that a number of river systems have droë (dry) or sout (salt) as part of its name. Expecting to find fish in the area would thus be unthinkable.
Two fieldtrips by CapeNature staff, earlier this year, yielded the unthinkable.
After some reports by locals pointing to the location of the existence of two populations of fish in the area, the Knersvlakte conservation team accompanied by Riaan van der Walt of CapeNature’s Porterville office investigated the claims by surveying the rivers in question.
Due to time constraints only perennial sections of the Salt River, and one of its tributaries, the Rooiberg River (non-perennial) seemed to have ideal habitats for fish populations which were identified and sampled. Shallow areas were checked using hand nets and where possible fyke nets were put out overnight. Some brave souls put their bodies on the line and snorkeled the deeper pools. With water temperatures dropping down to below 10°C Celsius, the lunch and fireplace provided by one of the adjacent land owners can only be described as lifesaving!
Sadly, the only fish found during the three-day survey was the invader species Mozambique tilapia ( Oreochromis mossambicus). It is believed that during elevated water level periods, the species may have moved up from the Olifants River into its tributaries, of which the Salt River is one. A second possibility is that a dam near one of these river systems may have been stocked with the species which subsequently escaped.
With the species tendency to invade almost all the water bodies that it inhabits, it was interesting to observe relatively low numbers despite an abundance of food in the form of mosquito larvae being available. Van der Walt explains “Mozambique tilapia is known to handle salinity levels higher than what we find here, hence factors such as temperature, dissolved oxygen, dry spells and even predation collectively could have contributed to keeping its numbers in check.” Said Van Der Walt “We’re also not ruling out finding indigenous species but with evidence of Mozambique tilapia we can’t be too optimistic.”
With most of the areas rivers remaining largely unexplored, it is envisaged that more surveys will be conducted in 2013.