Abstract
This study aimed to evaluate the effects of Bifidobacterium animalis and Bifidobacterium lactis on growth, length-weight relationship and some biological indices of rainbow trout, Oncorhynchus mykiss fry. An iso-nitrogenous (50% crude protein) diet was supplemented with three different concentrations of a mixture of equal quantities of the two strains of Bifidobacterium, namely T1 (1×107CFU/g), T2 (2×107CFU/g), T3 (3×107CFU/g) and a control (without bacteria). A total of 480 trout fries with an initial mean body weight of 0.53±0.19 g were randomly divided into four groups, each with 4 replicates. The fries were distributed into 16 fiberglass tanks (20 L total capacity) each filled with 15 L of water (water exchange rate of 2 L min-1 and permanent aeration) at a density of 2 fries per liter. During the 8-week feeding trial, the mean water temperature was 17.66±1.33°C and that of pH was 7.63±0.08. All fish groups demonstrated positive allometric growth. Trout fries fed with diet T1 recorded significantly higher (P<0.05) gonadosomatic index, specific growth rate and mean weight gain. The highest final mean length (14.12±4.57 cm) was recorded in fry fed with diet T2. The lowest concentration of Bifidobacterium, T1 induced the best growth and it enhanced the biological indices of O. mykiss.
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