The present study aimed to investigate the effect of partial or complete replacement of the fish meal (FM) by cotton seed meal (CSM) in diets of Nile tilapia, Orcochromis niloticus, to reduce feed costs. Therefore, five
experimental diets contained 0, 25, 50, 75 and 100% CSM as replacements of FM. The final body weight decreased with increasing the level of replacing FM by CSM. However, the replacement of 25% of FM by CSM had no significant
effect on the final BW, while the higher replacing levels (50, 75 or 100%) significantly (P<0.05) reduced the final BW of the Nile tilapia and the same trend was also observed for body length, weight gain, specific growth rate, feed
intake, feed conversion ratio and protein efficiency ratio.
At the experimental termination (90 days), the highest condition factor (K) value was recorded for fish fed on diet 2 (CSM 25%), followed in a descending order by those fed on the control diet, D1 (CSM 0%); D5 (CSM 100%); D3
(CSM 50%) and D4 (CSM 75%) respectively. Moreover among the other levels, the final K values of the Nile tilapia were not significantly affected by replacing levels of dietary FM by CSM.
Hematocrit and haemoglobin values (hematological parameters) were almost decreased proportionally with increasing the dietary CSM protein level. Comparedwith fish fed the control diet (CSM0), all replacing levels of FM by CSM
significantly (P<0.05) decreased He matocrit and haemoglobin values and the largest declines occurred for fish fed diets containing 50-100% CSM protein replacements. Values of serum transaminases (aspartate aminotransferase, AST and alanine aminotransferase, ALT) almost increased with each increase in CSM level as a substitute for FM in Nile tilapia diets. Results of proximate analysis of the whole fish body showed that as thedietary CSM level increased, protein content significantly (P<0.05) decreased while the opposite trend was observed for fat content where the increasing levels of CSM in the experimental diets significantly (P<0.05) increased fat content ofthe whole fish body. The inclusion of 25% CSM in the experimental diets had no significant effect on ash content while the other replacing levels (50, 75 and 100%) significantly reduced ash content of whole fish body.