M.Sc. / South Africa can be classified as a semi-arid country and the management of its precious water resources is essential. Environmental pollution, caused by the development of industry, technology and informal settlements, threaten the health status of many freshwater ecosystems. The health of all living organisms living in aquatic ecosystems is therefore subsequently affected by this decrease in water quality. The health of an ecosystem is thus often reflected by the health of its fauna. Fish are relatively sensitive to changes in their surrounding environment, including an increase in pollution. Fish health may as a result reflect, and give a good indication of the health status of the aquatic ecosystem in which the fish occurs. The initial toxic effects of the pollution may, however, only be evident on cellular or tissue level before significant changes can be identified in fish behaviour or external appearance. Histological analysis appears to be a very sensitive parameter and is crucial in determining cellular changes that may occur in target organs, including the liver. A histological investigation may therefore prove to be a cost-effective tool to determine the health of fish populations, hence reflecting the health of an entire aquatic ecosystem within a bio-monitoring process. Within the South African context, limited research has been conducted regarding the histology and histopathology of endemic fish species, and its value and effectiveness in aquatic monitoring. The aim of this study was to investigate and validate whether fish liver histology can be used as a monitoring tool, to indicate the health status of an aquatic ecosystem. In order to accomplish this, a preliminary histological study was done on the liver of Clarias gariepinus, a freshwater fish species endemic to the southern African region. Fish were collected in the Rietvlei Dam, Marais Dam and the Palala River, chosen as the three aquatic ecosystems for this study, due to their dissimilar pollution levels. Fish liver samples were fixed in 10% neutrally buffered formalin and prepared for light microscopy analysis using standard techniques for Haematoxylin and Eosin (H & E) staining. From the results obtained during this study, it was concluded that the liver histology of C. gariepinus did appear to be a sensitive monitoring tool of aquatic health. The histological investigation of the fish livers obtained in the Palala River, chosen as an unpolluted site, appeared to be in an overall healthier condition than the liver histology of fish obtained from the two relatively polluted sites, the Rietvlei Dam and the Marais Dam. This conclusion was obtained by means of an objective, quantitative analysis. The histological results in this study were quantified in terms of a histological index. An index value representing the specific histological characteristics of the liver was assigned to each individual specimen indicating either a healthy histological structure (index value of 0-2) or a possible pathological condition (index value of 3-6). According to the quantitative assessment, an average histological index value of 2.5 (indicating a relatively healthy histological condition) was calculated for the unpolluted site while a higher average index value of 3.2 (indicating a pathological condition) were calculated for both the relatively polluted sites. The fish liver histology did therefore reflect the health status of the aquatic ecosystem in which the fish specimen occurred. It was therefore concluded that the liver histology of C. gariepinus seemed to be a sensitive biomarker of environmental pollution, and hence an effective monitoring tool for estimating the health status of an aquatic ecosystem. / Dr. G.M. Pieterse