Water resources are frequently under inspection as pollution increases. Therefore an urgent need has arisen for sensitive bio-monitoring tools in toxicant impact assessment to indicate the effect of pollution on fish health in polluted aquatic ecosystems. Histopathological assessment of fish tissue allows for early warning signs of disease and detection of long term injury in cells, tissues or organs. Various biochemical and biological studies of fish have been used to assess the consequences of environmental toxicants on fish, but histology is able to enhance and add quality to the research carried out by identifying cellular alterations and quantifying the results. The aim of this study was to assess the degree of pollution in the Marais Dam (MD) and Rietvlei Dam (RVD) in the Rietvlei Nature Reserve (RNR) by determining the health status of the sharptooth catfish, Clarias gariepinus, using fish histology as a bio-monitoring tool.The MD and RVD situated within the Sesmyl Spruit serve to provide the area with water and are separated by a natural wetland. The dams were chosen for their elevated levels of toxicants. Four sampling surveys, two low flow and two high flow, were carried out over a period of two years. During each survey twenty fish were sampled per dam. Water and sediment were sampled for metal and endocrine disrupting chemical analysis. A standardized health assessment protocol employed to determine the effects of the toxicants included analysis of blood constituents, fish necropsy, calculation of condition factor and organosomatic indices, and a qualitative and quantitative histological assessment of three target organs: the gills, liver and gonads. These organs were removed and processed according to standard techniques for histological assessment. Histopathological alterations of each organ were identified (qualitative assessment) and subsequently quantified by means of a standard methodology of criteria to assess and compare fish health (quantitative assessment). The total index (Tot-I) values from the quantitative histological assessment were statistically analyzed using the ANOVA method. Potential endocrine disrupting chemicals and metals in the water of both dams were present at levels that could be toxic. The degree of pollution was more severe in MD than RVD. Examination of tissue from the gills, liver and gonads revealed marked histopathological alterations. The quantitative histological assessment indicated that fish collected from MD were in a significantly more severe histopathological condition than those of RVD. These results indicate that the wetland may still function as a natural filter. The qualitative and quantitative histological assessment objectively compared C. gariepinus specimens from MD and RVD and the effects of the toxicants on the health of the fish. / Dr. G.M. Pieterse