Heavy metal bioaccumulation and human health risk in Clarias Gariepinus and Oreochromos Niloticus from Kpata, lower River Niger, Nigeria

Abstract

Fish from waters contaminated with heavy metals have long been recognised as one of the routes through which humans acquire such metals from their environment, yet information on fish tissue metal burdens, accumulation factors and associated health risk along the River Niger in Nigeria remains scarce. Cadmium, lead, chromium, zinc, mercury, copper and arsenic were quantified in liver tissues of Clarias gariepinus and Oreochromis niloticus caught from the Kpata reach of the Lower River Niger at Lokoja during the dry season (January to March 2025) and the wet season (June to August 2025). Livers were subjected to wet acid digestion in a nitric and perchloric acid mixture before atomic absorption spectrophotometry. Bioaccumulation factors were computed relative to ambient water and sediment, and human health risk was evaluated through the estimated daily intake, target hazard quotient, hazard index and margin of exposure, using a per capita consumption rate of 8.26 grams per person per day. With the sole exception of mercury, all metals exceeded WHO and FAO permissible limits for fish in both species. Clarias gariepinus accumulated higher tissue burdens than Oreochromis niloticus, consistent with its benthic habit, and the bioaccumulation order for both species was Cr > Zn > Cd > Cu > As > Pb > Hg. Individual target hazard quotients were below unity, with chromium the highest (0.271 in C. gariepinus, 0.267 in O. niloticus), and the hazard indices (0.443 and 0.430 respectively) remained below one. The margin of exposure for chromium was nonetheless narrow (36.8 to 37.5), so that higher consumption could raise the chromium hazard quotient toward unity. Periodic tissue monitoring and species targeted consumption advisories are recommended to safeguard chronic consumers along this reach of the River Niger.