Original Articles

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

DOI: 10.1080/10183469.1995.9631358
Author(s): C.A. BrownFreshwater Research Unit, Zoology Department,, J.M. KingFreshwater Research Unit, Zoology Department,

Abstract

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 a substantial increase in the number of oligochaetes downstream of both ‘portapool’ farms and yet, despite being situated in the same sensitive river zone, this did not occur downstream of the farm that used earth dams. The general impact of trout farm effluent on the mountain-stream and source zones was to eliminate or greatly reduce the number of Limnichidae, Helodidae, Plecoptera, Elmidae, Heptageniidae and Ephemerellidae, and, in the case of portapool farms, to replace these with Naididae, Lumbriculidae, Chironomidae and Planaria. Once-off chemical samples were also collected at each site and, acknowledging the limitations of the sampling strategy, results showed that the particulate fraction of the effluent was probably responsible for the recorded reaction of the biota.

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