CASE STUDIES OF THE INVASION BY FOUR ALIEN FISH SPECIES (CYPRINUS CARPIO, MICROPTERUS SALMOIDES, OREOCHROMIS MACROCHIR AND O. MOSSAMBICUS) OF FRESHWATER ECOSYSTEMS IN SOUTHERN AFRICA

ABSTRACT

The history of the invasion of 42 catchment regions in southern Africa (excluding Mozambique) by an altricial species (Cyprinus carpio), a moderately precocial spies (Micropterus salmoides) and two highly precocial species (Oreochromis macrochir and O. mossambicus) is examined. The abundance of these species in different catchments is discussed in relation to the diversity of indigenous species. Results suggest that:

1. Depauperate catchments of the southern Cap are vulnerable to invasion irrespective of the life-history style of the alien species, compared to catchments (in Zimbabwe and north eastern Transvaal) with a higher diversity of indigenous species.

2. Precocial species are more successful than altricial species in establishing populations in natural waters in more diverse ecosystems.

3. Degradation and modification of the environment (particularly the building of impoundments) has played an important rob in increasing the susceptibility of freshwater ecosystems to invasion.