Ecotoxicity of Mercury Contaminated Sediment Collected from Mabubi River (Geita district, Tanzania) to the Early Life Stages of African Catfish (Clarias gariepinus)

The quality of Hg contaminated sediments in artisanal gold mining areas of Tanzania have to date only been assessed through bulk chemical analysis. However, measurement of contaminant levels alone has a limited ability to predict adverse effects on living resources. In this study we investigated the possible effects of Hg contaminated sediments from the river Mabubi which drains the artisanal gold mine on the hatching success, larval survival and growth of the African catfish (Clarias gariepinus). The tested species is a resident fish whose demersal behaviour keeps it in frequent contact with the sediment. Sediments collected downstream of the mine decreased survival and impaired growth of Clarias gariepinus but did not affect its hatching success. These effects were observed up to six kilometres downstream of the mining activity. The calculated 5 d- LC50 value (larval survival) was 1.75 μg/g Hg dw (95% CL of 0.72 – 2.53), the 5d-NOEC for hatchingwas >2.3 μg/g Hg dw and that for larval survival and growth was 0.23 μg/g Hg dw. In conclusion, chemistry and ecotoxicity results from this study suggest that sediments collected downstream of the mine adversely affect catfish and probably other fauna and as such present a considerable local environmental risk.