The SI of leukocyte stimulated with PHA-P was higher in fish fed diets supplemented with ≥240 mg NT/kg than in fish fed diets supplemented with ≤120 mg NT/kg.After the feeding trial, 10fish were randomly selected fromeach aquarium and were challenged with Streptococcus iniae for 7 days, higher survival (>80%) were observed in fish fed diets supplemented with NT than fish fed the NT unsupplemented control diet (56.7%).