Preliminary evaluation of the impact of invasive smallmouth bass Micropterus dolomieu on native fish abundance in the Witte River, Cape Floristic Region, South Africa

The impact of invasive smallmouth bass, Micropterus dolomieu, on the abundance of two native fish species, Breede River redfin, Pseudobarbus burchelli, and Cape kurper, Sandelia capensis, was investigated in the Witte River tributary of the Breede River in South Africa’s Cape Floristic Region (CFR). Native fish density was estimated in five pools and three riffles both above and below a 2 m-high waterfall that acts as a barrier to M. dolomieu. Both native species were relatively abundant (mean ± S.E. fish/100 m2: 152.10 ± 19.16–10.33 ± 6.06) at sites above the waterfall where M. dolomieu was absent, but absent or scarce (0–0.53±0.38 fish/100m2) at sites below it with M. dolomieu. There were no significant differences in measured environmental variables between sites above and below the waterfall, and M. dolomieu is thus implicated as the main factor responsible for the differences in native fish densities. These findings highlight the critical role of waterfalls in excluding non-native fishes from native fish refugia in the CFR.Key words: non-native predatory fish, Pseudobarbus burchelli, Sandelia capensis, waterfall barrier