Distribution of total mercury in surface sediments and African catfish (Clarias gariepinus) from Akaki River catchment and Aba Samuel Reservoir, downstream to the mega-city Addis Ababa, Ethiopia

Abstract

Abstract Total mercury (THg) levels were determined in sediment (n = 22) and muscle of Clarias gariepinus (n = 36) in dry and rainy seasons from Akaki River catchment and Aba Samuel Reservoir, Ethiopia in 2016-2017. The analyses of THg were performed using cold vapor-atomic absorption spectrometer (CV-AAS) after acid digestion technique. The relationship of THg levels to size of the fish has also been investigated. The THg levels in sediments range from 9.5–43.8 mg/kg dry wt in the dry and 20.6–175.4 mg/kg dry wt in the rainy seasons. This study indicated that the levels of THg in sediments varies spatially and seasonally. In both seasons, THg levels increased from upstream to downstream areas, revealing increased anthropogenic inputs of mercury pollution. It was also found that the THg concentrations in all the sediment samples were below the US EPA guideline value of 200 mg/kg dry wt. The THg levels in Clarias gariepinus were in the range from 360–924 mg/kg dry wt in the dry and 325–1233 mg/kg dry wt in the rainy seasons, and had positive relationship to the total length and body weight. The THg concentrations found in Clarias gariepinus were within the acceptable guidelines of the FAO/WHO. Therefore, the muscle of Clarias gariepinus from the study area is not hazardous to human health with respect to mercury. However, the average value of THg seems to be slightly higher than results reported from other Ethiopian freshwater bodies. Thus, monitoring of mercury species in sediment and different organs of fish are strongly recommended.