The present study was carried out to determine the effects of four rations on growth, chemical composition and digestibility of the Rainbow trout (Oncorhynchus mykiss Walbaum, 1792). Four test diets were formulated to have the same levels of protein (40%) and energy (gross energy: around 4800 cal/g), however, these test diets were composed of basic diet with different carbohydrate (0, 3, 12 and 18%) to lipid (0, 6, 15 and 18%) ratios (Diet 1: 0/18; Diet 2: 3/15; Diet 3: 12/6 and Diet 4: 18/0). Fifty fish weighing 99.0 ± 0.65g were placed in each of the four experimental tanks and were fed with one of the experimental diets for 13 weeks consisting of two trial periods (Periods I and II). Growth parameters and chemical composition of fish flesh were monitored. Effects of the diets on apparent digestibility coefficients (ADC) were also determined. At the end of the study, the fish growth was affected significantly (P<0.05) with carbohydrate and lipid levels in the diets. Higher growth, better feed conversion ratio and protein efficiency ratio were observed in experimental groups fed with Diets 1 and 3. There were no significant (P>0.05) changes in specific growth rate, although higher values were observed in fish fed with Diets 1 and 3. The experimental group fed with Diet 4 showed a lower (P<0.05) growth and feed conversion ratio. The hepatosomatic index (HSI) showed no significant (P>0.05) changes in experimental groups fed with the diets. The percentage of water, protein and ash of fish flesh did not show any (P>0.05) change. However, the muscle lipid content of fish significantly (P<0.05) decreased as carbohydrate level increased.  The ADC values were also affected (P<0.05) with carbohydrate and lipid levels in the diets. The ADC of lipid was lowest in the experimental group fed with Diet 1. The ADC values for dry matter, proteins and energy were lowest for fish fed on Diet 4, while for ash it was lowest for Diets 2. These results indicate that the best balance between the dietary lipid and carbohydrate levels was obtained on Diet 3, without lower growth.

Key words: Oncorhynchus mykiss, carbohydrate, lipid, growth, digestibility.