Abstract The effect of intensive mining on the health status of fish living in the upper basins of the Ulindi and Elila rivers was investigated between June 2018 and March 2022 by comparing the reproductive, hepatic and gill systems of fish collected from sites upstream and downstream of the mining sites. Biometric indices and organ histopathology are biomarkers that were studied at two different trophic levels using two fish species Oreochromis niloticus and Labeobarbus pellegreni. Histological evaluation was performed on the liver, gonads and gills of both species. Water and sediment samples were analysed for total mercury (T-Hg) concentration by (atomic absorption spectrophotometry). Histological changes were assessed semi-quantitatively, and the results were graded according to the severity of the histological responses. The results showed that L. pellegrini accumulated more T-Hg than O. niloticus. The T-Hg level was higher in the organs of both species during the rainy season than during the dry season in the following order: Gonads > Liver > Gills and exceeded the WHO/FAO recommended limit (T-Hg = 1.0 mg/kg wet weight) only in the testes and gills of L. pellegrini collected at the downstream sites of the Ulindi and Elila rivers. This does not necessarily reflect the level in the carcass. Taken together, the results suggest that mercury accumulated downstream of gold mining operations is altering the health of fish populations and that L. pellegrini can be considered a sentinel species for monitoring this pollution. To protect fish health, it is recommended that certain mining practices such as the misuse of mercury be avoided throughout the Congo River basin. References barshi, M. M., Dantala, E. O., & Mada, S. B. (2017). Bioaccumulation of heavy metals in some tissues of croaker fish from oil spilled rivers of Niger Delta region, Nigeria. Asian Pacific Journal of Tropical Biomedicine, 7(6), 563‑568. https://doi.org/10.1016/j.apjtb.2017.05.008 Abdel-Baki, A. S., Dkhil, M. A., & Al-Quraishy, S. (2011). Bioaccumulation of some heavy metals in tilapia fish relevant to their concentration in water and sediment of Wadi Hanifah, Saudi Arabia. African Journal of Biotechnology, 10(13), 2541‑2547. https://doi.org/10.5897/AJB10.1772 Abdel-Khalek, A. A. (2015). Risk Assessment, Bioaccumulation of Metals and Histopathological Alterations in Nile tilapia (Oreochromis niloticus) Facing Degraded Aquatic Conditions. Bulletin of Environmental Contamination and Toxicology, 94(1), 77‑83. https://doi.org/10.1007/s00128-014-1400-9 Abiya, S., Odiyi, B., Ologundudu, F., & Akinnifesi, O. (2018). Assessment of Heavy Metal Pollution in a Gold Mining Site in Southwestern Nigeria. Journal of Genetics and Cell Biology, 1(2), 30‑35. Adams, D. H. (2010). Mercury in wahoo, Acanthocybium solandri, from offshore waters of the southeastern United States and the Bahamas. Marine Pollution Bulletin, 60(1), 148‑151. https://doi.org/10.1016/j.marpolbul.2009.09.031 Adeogun, A. O., Onibonoje, K., Ibor, O. R., Omiwole, R. A., Chukwuka, A. V., Ugwumba, A. O., Ugwumba, A. A. A., & Arukwe, A. (2016). Endocrine-disruptor molecular responses, occurrence of intersex and gonado-histopathological changes in tilapia species from a tropical freshwater dam (Awba Dam) in Ibadan, Nigeria. Aquatic Toxicology, 174, 10‑21. https://doi.org/10.1016/j.aquatox.2016.02.002 Agbohessi, P. T., Imorou Toko, I., Ouédraogo, A., Jauniaux, T., Mandiki, S. N. M., & Kestemont, P. (2015). Assessment of the health status of wild fish inhabiting a cotton basin heavily impacted by pesticides in Benin (West Africa). Science of The Total Environment, 506‑507, 567‑584. https://doi.org/10.1016/j.scitotenv.2014.11.047 Ahmadi, A., Moore, F., Keshavarzi, B., Shahimi, H., & Hooda, P. S. (2022). Bioaccumulation of selected trace elements in some aquatic organisms from the proximity of Qeshm Island ecosystems : Human health perspective. Marine Pollution Bulletin, 182, 113966. https://doi.org/10.1016/j.marpolbul.2022.113966 Alam, A., Chadha, N. K., Joshi, K. D., Chakraborty, S. K., Sawant, P. B., Kumar, T., Srivastava, K., Das, S. C. S., & Sharma, A. P. (2015). Food and feeding ecology of the non-native Nile Tilapia Oreochromis niloticus (Linnaeus, 1758) in the River Yamuna, India. Proceedings of the National Academy of Sciences, India Section B: Biological Sciences, 85(1), 167‑174. Allouko, J.-R., Kone, K., & Bony, K. Y. (2021). Etat biologique du sédiment de la lagune Aghien à partir de l’Indice Malacologique de qualité des systèmes Lacustres (IMOL)(Sud-est, Côte d’Ivoire). International Journal of Biological and Chemical Sciences, 15(6), 2380‑2387. Almin, M. R. (2011). Etude des mécanismes induits par de fortes températures stérilisantes chez un poisson tropical, le tilapia du Nil, Oreochromis niloticus. Université François Rabelais.241. https://doi.org/10.1016j.aquatox.2005.05.014.