The relationship between the exotic predators Micropterus salmoides and Serranochromis robustus and native stream fishes in Zimbabwe

Samples of fish were collected from 42 stations in streams around Harare. Zimbabwe. The total abundance of fish was lower by 50% or more at stations where the exotic predators Micropterus salmoides and Serranochromis robustus were present compared with stations where they were absent. The greatest differences in density occurred among small Barbus species, whose populations were 86% lower (S. robustus alone) and 99% lower (M. salmoides alone). Populations of small rock and sand catlets were less dense when S. robustus. but not M. salmoides. was present probably because the latter did not enter the riffles where the catlets live. These predators apparently did not affect densities of certain other species in the system that were either cryptic, bottom-dwelling forms or too large. Low Barbus diversity and abundance in streams containing exotic predators is of concern because this group of fluvial fishes is threatened throughout southern Africa by dam-building, pollution, and other factors. (C) 2001 The Fisheries Society of the British Isles.