A histological assessment allows aquatic scientists to assess fish health in polluted aquatic ecosystems at tissue and cellular level. However, a firm knowledge and understanding of normal histological structure is essential to ensure accurate and objective results. Hence, the histologist must be able to distinguish between toxicant induced lesions and the range of histological characteristics considered to be normal for that specific species. However, limited histological reference material and data describing normal conditions are available for southern African fish species. The aim of this baseline study was to establish reference material, both qualitative and quantitative, for two southern African freshwater fish species used as indicator species in toxicity studies at the University of Johannesburg, Clarias gariepinus and Oreochromis mossambicus. The reference material includes descriptive histology of selected target organs, and the associated values and intervals for related quantitative health aspects including somatic indices, condition factor, blood parameters (haematocrit and plasma proteins) and quantitative histological results. Essentially, this project originated from a need to better understand the normal histology of C. gariepinus and O. mossambicus, as a range of histological characteristics were identified in control groups in previous toxicity studies which could not be confirmed to be associated with normal conditions. To be able to conduct a baseline study, with the purpose of establishing reference material, it was essential that the history of the specimens be known and documented including age, sexual maturity, nutritional status and physical and chemical water quality data. The project therefore involved the breeding of fish specimens under controlled conditions. A subsequent necropsy and qualitative and quantitative histological assessments were executed on five target organs (usually included in toxicity studies within the department): liver, gills, gonads, heart and kidney. Fish of both species were bred in an environmental room in reconstituted, reverse osmosis water and reared until sexually mature. Twenty specimens with an approximately 50:50% sex ratio were collected for both species. A necropsy was performed on each specimen and tissue samples of the selected target organs were processed using standard techniques, and prepared for the qualitative and quantitative histological assessments using light microscopy. The results The results showed that no macroscopic abnormalities were identified during the necropsy regarding external features or internal organs of the specimens within the sample groups, and all fish appeared to be in good health. The target organs were then examined microscopically, followed by a description of the normal histological structure (qualitative assessment). In addition, the target organs were assessed by means of a specific quantitative histological assessment protocol, which provides a standardised methodology of criteria to objectively assess fish health (quantitative assessment). Histological alterations identified during this assessment were quantified and subsequent reference index values could be calculated for each organ. / Dr. G.M. Pieterse

D.Sc. / Please refer to full text to view abstract