Invasion of a headwater stream by non-native fishes in the Swartkops River system, South Africa

In South Africa, fishes in headwater streams are increasingly being threatened by non-native fish invasions. These invasions originate primarily from source populations in mainstream rivers and impoundments. In the Blindekloof stream, a Swartkops River headwater tributary, the non-native Micropterus salmoides was eradicated from the stream by conservation authorities in 1989. As a result of this eradication subsequent occurrences of non-native fishes are most likely a result of upstream invasions from the mainstream Swartkops River. The Blindekloof stream therefore provided a unique opportunity to investigate whether M. salmoides would reinvade after its removal in 1989 and if the more recently introduced Clarias gariepinus and Tilapia sparrmanii would invade the Blindekloof stream. During snorkel and electrofishing surveys four non-native species were recorded (M. salmoides, M. dolomieu, T sparrmanii, C. gariepinus). The large predators, M. salmoides, M. dolomieu and C. gariepinus were neither abundant nor widespread and T sparrmanii was the only non-native species that had successfully established in the Blindekloof stream. Where M. salmoides and M. dolomieu were observed, all other fish species were absent. By contrast, the native species were present at all sites where C. gariepinus and T sparrmanii were found. The Pseudobarbus afer population was predominantly limited to a 1.7 km section of the stream that was not invaded and is located above a waterfall. The limited distribution range of this Endangered species makes it particularly vulnerable to predation by non-native fishes. The results of this study indicate that these non-native fishes rapidly invaded the Blindekloof stream following their introduction into the mainstream.