Fish populations in the Rondegat River, a mountain stream in the Olifants-Doring system in the Cape Floristic Region, South Africa were surveyed to assess the impact of predatory alien invasive smallmouth bass Micropterus dolomieu (Lacepède) on the indigenous fishes. This was the first such attempt to quantify the predatory impacts of M. dolomieu within this region. The Rondegat River is home to five species of indigenous fish and is partially invaded by M. dolomieu, which has penetrated the lower river up to a waterfall barrier. Seasonal surveys were conducted at five sites above, and five below, the waterfall. Physical habitat was measured at each site. Four of the five indigenous fish species were absent at bass-invaded sites. Labeobarbus capensis (Smith), while still present below the waterfall, appeared to have suffered a near-total loss of post-spawning recruits. Analyses of physical habitat quality failed to explain the loss of indigenous fish below the waterfall, although sedimentation may have increased the vulnerability of the catfish Austroglanis gilli (Barnard) to M. dolomieu predation by obliterating benthic cover. Consequently, predation by M. dolomieu was presumed to be the critical mechanism explaining the loss of indigenous fishes in the lower Rondegat River.
Keywords: Cape Floristic Region, diversity, impacts, indigenous fish, invasive fish, Micropterus dolomieu, predation
African Journal of Aquatic Science 2005, 30(2): 167–173