Dietary administration of nucleotides to enhance growth, humoral immune responses, and disease resistance of the rainbow trout (Oncorhynchus mykiss) fingerlings.

A feeding study was conducted to determine the effect of dietary nucleotides (NT) on growth performance, immune responses and resistance of rainbow trout fingerlings to Streptococcus iniae. A basal diet supplemented with 0 (control), 0.5, 1, 1.5 and 2 g NT kg(-1) to formulate five experimental diets. Each diet was randomly allocated to triplicate groups of fish with initial average weight of approximately 23 g. After 8 weeks of feeding trial, fish fed diets with 1.5-2 g NT kg(-1) had the highest final weight and SGR, followed by groups fed the diets with 0.5-1 g NT kg(-1), and lowest in fish fed the NT-free control diet. The feed conversion ratio (FCR) was better when the fish were fed diet with 2 g NT kg(-1). The ACH(50) level and lysozyme activity in serum were found to be significantly (P < 0.05) greater in fish fed nucleotides at 1.5-2 g kg(-1). The fish fed with 2 g NT kg(-1) had higher IgM level. The challenge experiment showed that relative percent survival (RPS) increased significantly (P < 0.05) with increasing levels of nucleotides from 0.5 to 2 g kg(-1) and was lowest in the control group. We therefore recommend dietary nucleotide administration at 1.5-2 g kg(-1) to promote growth and enhance immunity and resistance against S. iniae.