Invasions by non-native fishes are considered a primary threat to native freshwater fishes, particularly in headwater streams which typically have low species diversity but a high degree of endemism. Intentional stocking and illegal introductions of the mainstream Swartkops River in the Eastern Cape of South Africa have resulted in the presence of five non-native fishes in the mainstream. These fish may invade the headwater streams where they may pose a significant threat to native fishes such as the endangered Eastern Cape redfin minnow Pseudobarbus afer.

In the Blindekloof stream, a Swartkops River headwater tributary, alien fishes were eradicated by nature conservation authorities in 1989. The Blindekloof stream is situated in the Groendal Wilderness Area and provided a unique opportunity to investigate whether alien fishes would reinvade this stream.

In a 2010 survey carried out by C·I·B core team member Olaf Weyl and others, pools in a 9 km stretch of the Blindekloof stream, between its confluence with the Swartkops River and its source, were surveyed by snorkelling and electrofishing. Of five non-native fishes in the mainstream four, largemouth bass, smallmouth bass, banded tilapia and sharptooth catfish, had invaded the Blindekloof stream. Of these, only banded tilapias were widespread and abundant while the larger catfish and bass were limited to large pools. In pools where largemouth and smallmouth bass occurred, no other fishes were present. Eastern Cape redfin minnows only occurred in non-invaded pools and were most abundant above a natural barrier which excluded non-native fishes.

The study showed that that this headwater stream was reinvaded which indicates that eradication of alien fishes in headwater streams is ineffective in the absence of physical barriers that would prevent reinvasion from downstream.

Read the paper:

Ellender B.R, O.L.F Weyl, E.R Swartz (2011) Invasion of a headwater stream by non-native fishes in the Swartkops River system, South Africa. African Zoology 46 (1): 39-46.