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Abstract
Concentrations of heavy metals (Cr, Co, Fe, Ni, Cu, Zn, As, Se, Pb and Hg) in muscle tissues of two fish species (Clarias gariepinus and Oreochromis niloticus) in an Ethiopian rift-valley lake (Hawassa) and a neighboring stream (Boicha) were determined. Target hazard quotients (THQ), hazard index (HI) and target cancer risk (TCR) were used as indices to evaluate potential human health risks from fish consumption. Average concentrations of metals decreased in the order Zn>Fe>Se>Cu>Hg>As>Ni>Cr>Co>Pb and Zn>Fe>Se>Cu>As>Hg>Cr>Ni>Pb>Co in muscle tissues of C. gariepinus and O. niloticus, respectively. Mean concentrations of Hg (0.34+0.04 mg kg-1) in muscle tissues of C. gariepinus from Lake Hawassa, and As (0.18+0.05 mg kg-1) as well as Hg (0.46+0.03 mg kg-1) in muscle tissues of C. gariepinus from Boicha stream were above the safety limits set by WHO/FAO. Likewise, mean concentrations of As (0.31+0.03 mg kg-1) and Hg (0.19+0.05 mg kg-1) in muscle tissues of O. niloticus from Lake Hawassa and Cr (0.19+0.03 mg kg-1), As (0.33 +0.04 mg kg-1), and Hg (0.34+0.09 mg kg-1) in O. niloticus from Boicha stream were also above safety limits. From the results of human health risk assessments it was concluded that Cr, Cu, Hg and As pose potential health risks due to consumption of the two fish species from both water bodies. Moreover, effects of all heavy metals put together may affect human health as indicated by the high HI. Effluents from industries are assumed to be the main sources of the heavy metals. Therefore, regular monitoring of the water bodies and policy interventions with respect to waste disposal are recommended to protect the health of the ecosystem and the public. 
 Keywords: Fish consumption, health risk, heavy metals, Lake Hawassa
Highlights
Extended use of metals in industries has led to increased release of harmful heavy metals into the environment
The maximum mean concentration of heavy metal obtained in muscle tissues of C. gariepinus was for Zn (22.79+1.19 mg kg-1) from Boicha stream and the minimum value was for Pb (0.05+0.002 mg kg-1) from
The maximum mean concentration in muscle tissues of O. niloticus was for Zn (22.64+4.45 mg kg-1) from Boicha stream and the minimum value was for Pb (0.01+0.001 mg kg-1) from
Summary
Extended use of metals in industries has led to increased release of harmful heavy metals into the environment. Were to determine the concentrations of heavy metals in muscle tissues of the two most common fish species: C. gariepinus and O. niloticus from Lake Hawassa and Boicha stream, and to estimate the human health risks from the consumption of these fish species.
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