Effects of Dietary Phosphorus on Resistance of Channel Catfish toEdwardsiella ictaluriChallenge

Young-of-year channel catfish Ictalurus punctatus were fed an egg-white-based purified diet supplemented with serial concentrations of phosphorus from monosodium phosphate for 10 weeks; they were subsequently challenged by a 24-h immersion in a cell suspension of Edwardsiella ictaluri (105.4 cells/mL) at 25 ± 1°C. Fish responses, which included weight gain, serum phosphorus concentration, serum alkaline phosphatase activity, mortality of challenged fish, and antibody production, were compared with dietary phosphorus concentration by regression analysis. Significant (P < 0.05) quadratic responses occurred in weight gain, serum alkaline phosphatase activity, mortality of challenged fish, and antibody production. Serum phosphorus concentration showed a significant linear response. The break points in the quadratic response curves showed that 0.38, 0.40, and 0.42% dietary phosphorus amounts were required for maximum alkaline phosphatase activity, survival from E. ictaluri challenge, and weight gain, respectively. This study showed that dietary phosphorus concentration influences the resistance of channel catfish to E. ictaluri challenge and that the dietary requirement for maximum weight gain is sufficient for maximum resistance against E. ictaluri challenge in young fish.