Fish distributions in the Rondegat River, Cape Floristic Region, South Africa, and the immediate impact of rotenone treatment in an invaded reach

Abstract

Alien fishes are considered the most serious threat to native headwater stream fishes in South Africa. A 4 km reach of the Rondegat River is the first section of a South African river to be rehabilitated through the attempted removal of alien fish by using the piscicide rotenone. The objectives of the current study were to establish the distribution and relative abundance of native and alien fish prior to treatment, and to assess the immediate impact of the treatment on the fish population. Forty-three sites were sampled using backpack electrofishing, snorkel transects and underwater video analysis. In the invaded lower reaches, native Labeobarbus capensis was detected only at very low densities, while three other native fish species were not detected. Alien fish were not detected above a barrier waterfall 5 km upstream of the river’s confluence with a reservoir. The fish density of 97 fish per 100 m² in non-invaded reaches was more than an order of magnitude higher than that of 7 fish per 100 m² in the invaded reach. A total of 470 Micropterus dolomieu and 139 L. capensis were removed from a 4 km treatment zone during the rotenone operation. No fish were detected in this area after the rotenone treatment.

Keywords: Austroglanis gilli, Barbus calidus, Clanwilliam Dam, Galaxias zebratus, invasion, Lepomis macrochirus, non-native, smallmouth bass, Tilapia sparrmanii

African Journal of Aquatic Science 2013, 38(2): 201–209