EFFECTS OF DIETARY PHOSPHORUS ON RESISTANCE OF CHANNEL CATFISH TO EDWARDSIELLA-ICTALURI CHALLENGE

Young-of-year channel catfish Ictalurus punctatus were fed an egg-whit e-based purified diet supplemented with serial concentrations of phosp horus from monosodium phosphate for 10 weeks: they were subsequently c hallenged by a 24-h immersion in a cell suspension of Edwardsiella ict aluri (10(5.4) cells/ml) at 25 +/- 1 degrees C. Fish responses, which included weight gain, serum phosphorus concentration, serum alkaline p hosphatase activity, mortality of challenged fish, and antibody produc tion, were compared with dietary phosphorus concentration by regressio n analysis. Significant (P < 0.05) quadratic responses occurred in wei ght 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 respo nse curves showed that 0.38, 0.40, and 0.42% dietary phosphorus amount s were required for maximum alkaline phosphatase activity, survival fr om E. ictaluri challenge, and weight gain, respectively. This study sh owed that dietary phosphorus concentration influences the resistance o f channel catfish to E. ictaluri challenge and that the dietary requir ement for maximum weight gain is sufficient for maximum resistance aga inst E. ictaluri challenge in young fish.