الفهرس 
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Abstract
In the present study, 1086 O. niloticus of different life stages were obtained alive from Abbassa and Sahl El-Hussinia fish farms in Sharkia governorate to study the monogenetic trematodes infesting O. niloticus during the different seasons of the year. Fish and water samples were collected from the same localities for determination of pesticide residues. The results of this investigation revealed that, about 66.66% of examined O. niloticus were clinically infected with monogenea sp. The rate of monogenean sp. infestation was 70.41% among the examined O. niloticus collected from Abbassa fish farms and 62.40% in Sahl El Hussinia fish farms. Monogenetic trematodes showed higher prevalence during autumn season (92.54%) followed by spring, summer and winter (85.95, 65.38 and 58.54%, respectively). Monogenetic trematode showed the highest prevalence among the juvenile stage (88.26%) then pre adults and adults (87.52 and 56.12%). The fish infested with monogeneasis showed signs of respiratory distress, hyperemic gills in some infested fish or pale and anemic in others. There detectable level of organochlorine pesticide residues in water samples collected from Abbassa fish farms were gamma chlordane (0.074 ppb), α-BHC (0.011 ppb), pp-DDT (0.28 ppb) and hepta epoxide (0.087 ppb). While, water samples collected from Sahl El Hussinia fish farms had detectable residues of gamma chlordane (0.005 ppb), α-BHC (0.021 ppb), enderin (156.71 ppb), gamma-BHC (3.31 ppb), heptachlor (0.093 ppb), alderin (0.11 ppb), HCB (0.017 ppb), and pp-DDD (0.001 ppb). Chlorpyrifos was the only organophosphrous pesticides residue that detected in the water samples collected from Abbassa and Sahl-El Hussinia fish farms (0.008 and 0.0016 mg/l), respectively. Gamma chlordane residues (3.81 ppb) was the only detected pesticide residues in gills of O. niloticus collected from Abbassa fish farms. While gills of O. niloticus collected from Sahl-El Hussinia fish farms had residues of gamma chlordane (11.56 ppb), alderin (19.37 ppb) and HCB (15.87 ppb).