Impact of agricultural drainage on Nile tilapia ( Oreochromis niloticus ) and African catfish ( Clarias gariepinus ), Egypt: heavy metals accumulation and biomarker responses

The effects of agricultural drainage on Oreochromis niloticus and Clarias gariepinus in El-Wadi Drain, El-Fayoum, Egypt, were investigated in 2025, focusing on metal accumulation and associated hematological, oxidative stress, and genotoxic responses. Water quality in El-Wadi Drain was markedly poorer than that of the reference site, with pronounced increases in salinity, ammonia, nitrite, and nitrate, along with substantial metal elevations, particularly in copper (7.64-fold), iron (6.73-fold), zinc (5.30-fold), and lead (13.70-fold). Fish from El-Wadi Drain accumulated significantly higher concentrations of metal, with the greatest increase observed for copper in O. niloticus gills, representing an elevation greater than 20-fold relative to the reference site. The metal pollution index confirmed greater overall accumulation in C. gariepinus. Hematological alterations included reduced red blood cells (RBCs) (−24.89% and −42.33%), Hb (−17.38% and −20.26%), and Ht (−12.95% and −25.42%), alongside elevated WBCs (+19.13% and +152.66%) in O. niloticus and C. gariepinus respectively. Oxidative stress markers showed increased MDA in liver (+40.05% in O. niloticus; +34.66% in C. gariepinus) and gills (+40.50% and +38.29%), elevated liver GSH (+130.43% and +99.16%), and decreased liver catalase activity (CAT) (−33.21% and −27.71%). Damage to DNA rose sharply, with liver tail DNA increasing by +151.77% and +154.38%, and gill tail DNA by +159.67% and +122.48% in O. niloticus and C. gariepinus respectively. Metal concentrations in water correlated strongly and positively with their respective levels in tissues. Tissue metals were also positively correlated with Malondialdehyde (MDA), Glutathione (GSH), and DNA damage, but negatively correlated with CAT. These findings underscore the negative impact posed by agricultural drainage pollution on freshwater systems.