THE EFFECTS OF TROUT-FARM EFFLUENTS ON BENTHIC INVERTEBRATE COMMUNITY STRUCTURE IN RIVERS IN THE SOUTH-WESTERN CAPE, SOUTH AFRICA

Abstract

Summary The south-western Cape is currently responsible for 45% of the total annual trout production in South Africa and further expansion of this industry in the region is likely. A preliminary survey of seven trout farms situated on the upper reaches of rivers in the southwestern Cape was undertaken to determine whether there was a common trend in their effect on the rivers. Results indicated that the impact of the farms on the benthic invertebrate communities of the rivers ranged from mild to severe, based on the degree of change in the structure of the communities from upstream to downstream of the effluent outlets. Those farms situated on mountain streams had the greatest impact and those on the downstream foothill had a lesser impact. The reduced impact in the foothill zone was probably because these reaches were already disturbed by other catchment activities. Of the three farms that were situated on mountain streams and source areas, two used plastic portapools and the third earth dams. There was ...