A histology-based fish health assessment of four commercially and ecologically important species from the Okavango Delta panhandle, Botswana

The health status of four commercially and ecologically important fish species from the Okavango Delta was assessed, using a histology-based health assessment protocol, to establish baseline data for future toxicity studies. Following the calculation of a necropsy-based health assessment index (HAI), the histological responses of the liver, gills and gonads were assessed and compared between Clarias gariepinus, C. ngamensis, Oreochromis andersonii and Serranochromis angusticeps. Population HAI results showed that O. andersonii was most affected in terms of necropsy-related alterations, followed by S. angusticeps, whilst identical low values were calculated for C. ngamensis and C. gariepinus. Quantitative histological results corresponded with the HAI results. The liver and gills of O. andersoniiwere most affected in terms of the percentage prevalence of the histological alterations identified. These were mostly associated with inflammation, and progressive and regressive changes. The histology of the liver and gills was also more affected in terms of the type and severity of the histological alterations identified, compared to that in the gonads of all four species. The majority of alterations were most likely caused by the prevalence of parasitic infections.