The aim of this study was to investigate effects of 5 levels of Phytoplankton Spirulina additive to commercial feed on body weight gain (BWG), feed conversation ratio (FCR) and feed intake in rainbow trout during first 50 days of feeding. Four hundred-fifty Rainbow trout mean body weight 30 ± 3 gr were randomly allocated into fifty 1.5×1.5 meter fiberglass tanks (30 fish in each tank). The tanks received aerated well water with a flow rate of 5–7 Lit. min -1. The temperature, pH and dissolved oxygen values of the water were 12.2 ± 0.1ºC, 6.9, and 10.2 ± 0.3 mg/Lit., respectively and monitored daily. Experimental diet was formulated according to nutrients requirements of rainbow trout using suggested standards. Spirulina was replaced with soybean meal of diets in five different levels, T1: kept for control and 0% Soybean meal with Spirulina, T2: 20% Soybean meal with Spirulina, T3: 40% Soybean meal with Spirulina, T4: 60% Soybean meal with Spirulina, T5: 80% Soybean meal with Spirulina which fed the experimental diets twice daily for 50 days. Data were processed in excel and statistical analysis was performed using SAS (ver/ 9.1). Our results indicated that fish which fed 2, 3, 4 and 5 diets had a significantly better FCR than group 1(P<0.01). Also, BWG in treatment 2, 3, 4 and 5 were significant different compare to the control group (P<0.01). Additionally, feed intake in treatments 2, 3, 4 and 5 were significantly higher than compare to control (P<0.01). According to our result Spirulina is substitute protein source for replacing with soybean meal in Rainbow trout breeding systems.
