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The present study was to evaluate the impact of feeding level and fish size on the growth factors,carcass composition and digestibility of rainbow trout. Six experimental treatments were assigned by the use of three feeding levels (maintenance, medium and satiation levels) and two fish sizes (43.02 ± 2.93 and 231.32± 6.74g) according to a 3×2 factorial design.  There were three replicates for each treatment and the experiment lasted for 59 days. The results showed that there is a significant interaction effect between fish size and feeding level for body weight gain, final body weight and specific growth (p<0.05). Body composition (dry matter, energy, moisture, fat and protein) was affected by feeding level and size of the fish (p<0.05). However, the interaction effect between fish size and feeding level for body composition was not significant (p>0.05).The digestibility measurements showed that fish size and feeding level  changed  significantly digestibility of protein, lipid, energy and dry matter in rainbow trout (p<0.05). Maximum growth at satiation feeding level can reduce feed conversion ratio and increase waste production at both fish sizes.

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Aims: The aim of this study was to investigate the effect of dietary lemon peel (Citrus limon) essential oil on growth, hematological parameters, some of serum biochemical parameters and liver enzymes in rainbow trout (Oncorhynchus mykiss) juveniles. Materials & Methods: For this purpose, 144 rainbow trouts with an average initial weight of 95.05 ± 0.41 g were randomly stocked in four treatments containing different levels of lemon peel essential oil ( 0 (control), 200, 400 and 600 mg lemon peel essential oil/kg diet) and fed for eight weeks. Findings: The results showed the highest growth factors in treatments of 400 and 600 mg lemon peel essential oil/kg diet (p<0.05). Inclusion of essential oil in diet increased hematological parameters as compared to the control except MCV, MCH, MCHC and increased serum biochemical parameters by contrast to the control except glucose, triglyseride and cortisol (p<0.05). liver enzymes of ALT and AST showed no significant differences among treatments containing essential oil and LDH exhibited  no significant differences among 400, 600 mg essential oil/kg diet and the control treatment (p>0.05), but ALP amount significantly increased in essential oil treatments in a comparison to the control (p<0.05). Conclusion: Overall, these results indicated using 400 mg essential oil in rainbow trout diet could improve growth, hematological and some serum biochemical parameters of rainbow trout without any negative effects on liver.