The potential impacts of different cyanide concentrations on the mortality, behavior and growth of the economic fishes: Tilapia zillii, Clarias lazera and Chrysichthys reuppelli were examined. The growth of T. zillii and C. lazera were significantly reduced at lower and higher cyanide concentrations while C. reuppelli showed insignificant growth depression at lower levels of cyanide. The application of sublethal dose 0.03 mg/L of KCN on T. zillii for 42 days resulted in: (1) Severe and diffuse multifocal lamellar aneurysms for 64 % of the fish gills. (2) Delayed or suppressed the ovarian growth (OSI) and oocyte maturation (diameter). The histological studies of the ovarian sections of cyanide-exposed T.
zillii revealed impaired ovarian development. The oocytes of 76 % female T. zillii failed to complete the secondary yolk deposition and the proportion of atretic oocytes per total number of oocytes was significantly higher compared to the control group. Collectively, the findings of this study suggest that the use of cyanide compounds by fishermen in Bahr Shebeen Canal as a fish poison might reduce the survival of fish by impairing growth, respiration and reproductive success. The oogenesis disorder and the decrease in number of the viable eggs can cause serious disturbance in the
natural reproduction and a drastic decline in fish population.