Heavy metal bioaccumulation and human health risk in clarias gariepinus and oreochromis niloticus from Kpata, lower River Niger, Nigeria

Abstract

Fish taken from waters contaminated with heavy metals are a recognised route by which those metals enter the human diet. This study assessed heavy metal bioaccumulation and the associated human health risk in Clarias gariepinus and Oreochromis niloticus from the Kpata reach of the Lower River Niger at Lokoja, Nigeria. Forty specimens of each species were collected in each of two hydrological periods, January to March 2025 representing the dry season and June to August 2025 representing the wet season. Liver tissue was digested and analysed for cadmium, lead, chromium, zinc, mercury, copper and arsenic. Bioaccumulation factors were computed relative to ambient water and sediment, and human health risk was evaluated through the estimated daily intake, the target hazard quotient, the hazard index and the margin of exposure. All seven metals were detected in both species in both seasons. Chromium recorded the highest liver concentration in both species, at 1.110 and 1.090 mg per kg in the dry season and 1.163 and 1.127 mg per kg in the wet season, followed by zinc and copper. Concentrations were significantly higher in C. gariepinus than in O. niloticus for most metals, which is consistent with the benthic and opportunistic feeding habit of the former. With the exception of mercury, concentrations were higher in the wet season than in the dry season. Bioaccumulation factors were below unity for every metal in both species relative to both water and sediment, indicating that liver concentrations did not exceed those of the surrounding matrices. Target hazard quotients were below unity for every metal, and the hazard index reached 0.443 for C. gariepinus and 0.430 for O. niloticus, both below the threshold of concern. Chromium contributed 61.2 and 62.1 percent of the respective hazard indices and is therefore the risk determining metal. The study concludes that consumption of fish from this reach does not at present pose a cumulative non carcinogenic risk at the intake rate modelled, but that chromium warrants continued monitoring.