A 63- day feeding trial was conducted to study the effect of fishmeal substitution levels with Chlorella vulgaris meal (25, 50, 75 and 100%) on the growth performance, nutrient digestibility, immune responses and the stress resistance of juvenile narrow clawed crayfish, Pontastacus leptodactylus (6.19 ± 0.13 g). The highest values of final weight (25.52 g), specific growth rate (2.24% body weight day−1), protein efficiency ratio (3.24), the protein productive value (54.67%), in vivo ADCOM (88.40%) and in vivo ADCCP (93.50%) and the lowest feed conversion ratio (1.49) were observed in the juvenile crayfish fed the diet containing 75% fishmeal substituted level with chlorella meal (p < 0.05). With an increment in the dietary inclusion level of chlorella meal, the activities of alkaline protease (r2 = 0.97), lipase (r2 = 0.98), amylase (r2 = 0.97), phenoloxidase (r2 = 0.98), superoxide dismutase (r2 = 0.99), lysozyme (r2 = 0.96) and nitric oxide synthase (r2 = 0.98) were promoted as a third-order polynomial regression model (p < 0.05). There was an increasing trend in Lactobacillus count (r2 = 0.99) and a decreasing trend in Escherichia coli count (r2 = 0.97) with an increase in the fishmeal substitution level with chlorella meal. Based on the broken line regression model, the dietary fishmeal substitution level (%) with chlorella meal for maximum growth (SGR) and weight gain values and minimum FCR value of crayfish were estimated to be 85.49%, 78.14% and 78.13%, respectively.

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