Akewake Geremew (PhD)Marshet Adugna (PhD)Sumeya Bader2025-08-172025-08-172024-06https://etd.aau.edu.et/handle/123456789/6862Heavy metals pose a significant threat to aquatic ecosystems due to their persistence, toxicity, and bioaccumulation. Biomonitoring, using bioindicator organisms, assesses water quality and ecosystem health. Fish parasites have recently been proposed as alternative bioindicators of heavy metal pollution due to their higher accumulation levels than their hosts. This study aimed to assess the level of heavy metal accumulation in three commercially important fish species, their helminthic parasites as well as sediment and water samples in Koka Reservoir. A cross-sectional study was conducted with 337 fish samples bought during the pre-rainy season (May). The fish included African catfish (Clarias gariepinus), Nile tilapia (Oreochromis niloticus), and Common carp (Cyprinus carpio) with specimen number 105, 197, and 35, respectively. Sediment and water samples were collected from six sites. Three nematode parasites were identified in C. gariepinus (Contracaecum, Porrocaecum, and Camallanus sp.), while only one nematode (Contracaecum sp.) was found in O. niloticus. In the tissue of C. gariepinus, metals accumulated in the order Fe>Zn>Cu>Mn>Cr. Conversely, in O. niloticus and C. carpio, the order of metal accumulation was Fe>Zn>Mn>Cu>Cr. The metal concentration in parasites of C. gariepinus and O. niloticus followed the orders Fe>Zn>Cu>Mn>Cr and Fe>Mn>Zn>Cr>Cu, respectively. The metal concentration in sediments followed the order Fe>Mn>Zn>Cr>Cu. This study highlighted the prospect of C. gariepinus, O. niloticus and Contracaecum sp. as potential indicators of heavy metal pollution. By taking advantage of their accumulation abilities, we can enhance environmental monitoring and suggest intervention measures to reduce heavy metal pollution in water bodies.en-USClarias GariepinusCyprinus CarpioBioconcentrationBiomonitoringHealth RiskHeavy MetalsLake KokaNematodesOreochromis NiloticusSedimentThesis