Materials and methods
Live Tilapia fish species (Oreochromis niloticus, O. aureus, Sarotherodon galilaeus and Tilapia zillii) were collected from El-Riah El-Tawfequi at Benha City, 50 km. north Cairo, between May and August 2000.
Morphologically healthy apparent fish measuring 14 - 16 cm in total length were used for this study (15
for each species).