RESPONSE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) GROWING FROM 50 TO 200 G TO SUPPLEMENTS OF DIBASIC SODIUM-PHOSPHATE IN A SEMIPURIFIED DIET

Effects of increasing dietary concentrations of phosphorus on growth, feed intake, feed conversion, composition of gain and concentration of inorganic phosphate in plasma were studied in rainbow trout. Twelve g roups of 20 trout initially weighing 53 +/- 0.6 g/fish were fed semipu rified diets containing 19.6 MJ digestible energy per kilogram of dry matter. Twelve levels of phosphorus ranging from 1.03 to 10.96 g/kg dr y matter were achieved by replacing inorganic binder with dibasic sodi um phosphate in 11 graded levels. Nonlinear responses of trout to incr easing dietary phosphorus concentration determined over 53 d were desc ribed using exponential functions. Feed intake, growth rate and feed c onversion ratio as well as plasma inorganic phosphate concentration in creased with increasing dietary phosphorus concentration. The concentr ations of phosphorus, calcium, magnesium and potassium in weight gain increased, whereas concentrations of lipids and energy in weight gain decreased with increasing dietary phosphorus concentration. Concentrat ions of protein and sodium in weight gain were unaffected. Different c oncentrations of dietary phosphorus were required for achieving 95% of the plateau value determined for desired traits. In growth rate and p hosphorus deposition, the required phosphorus concentrations were 3.7 and 5.6 g/kg dry matter, respectively. However, dietary phosphorus was utilized most efficiently (88%) at a dietary concentration of 2.5 g/k g dry matter. At the dietary phosphorus concentration that resulted in maximum phosphorus deposition (5.6 g/kg dry matter), phosphorus utili zation was about 60%. Supplemental phosphorus from dibasic sodium phos phate was completely available to trout which must be considered in fo rmulating recommendations. Based on this work, 0.25 g available phosph orus/MJ digestible energy is recommended for trout diets.