**2.1 Specimen collection**

Specimens of adult catfish *C. gariepinus* were collected from the River Nile at Assiut and then were transported to Fish Biology Laboratory of Zoology Department, Faculty of Science, Assiut University. The fish (500–1200 g) were fed on a commercial pellet diet (3% of body weight per day) and kept together in 100 l rectangular tanks containing tap water (conductivity 2000 ls/cm; pH 7.5; oxygen 88–95% saturation; temperature 27-28 ºC; photoperiod 12:12 light: dark). After 2 week acclimatization, fishes were used for the experimental setup.
