A 126-day feeding trial was conducted on the juvenile rainbow trout (60.5 ± 3.19 g) to assess the effect of cooked mango-seed kernels (CMK) in the diet on growth performance and biological indices, five levels (10, 20, 30, 40 and 50 g kg-1) of CMK together with a control diet were used (n=3) at 18 raceway systems at a density of 50 fish per tank. Specific growth rate (SGR) of fish fed the diets containing 50 g kg-1 CMK (2.16% day-1) increased significantly (p≤0.05) compared to all inclusion levels of CMK (mean: 1.62% day-1) and control diet (1.12% day-1). Feed conversion ratio (FCR) of fish fed the diets containing 50 g kg-1 CMK (1.01) decreased significantly (p≤0.05) compared to all inclusion levels of CMK (mean: 1.37) and control diet (1.91). With an increase of CMK in the diet from 10 to 50 g kg-1, protein efficiency ratio (PER) as one of the most important of nutritional efficiency indices increased significantly (p≤0.05) from 2.16 to 4.18 compared to control (1.28) and then it remained insignificantly constant. With an increase in CMK dose from 10 to 50 g kg-1, the levels (ng ml-1) of plasma triiodothyronine (T3) of fish fed the diets of CMK increased significantly (p≤0.05) from 3.7 to 4.2 ng ml-1 compared to control (1.9 ng ml-1). The results of present study showed it is possible to include 50 g kg-1 CMK as a supplement in the diet of juvenile rainbow trout.

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