A new approach to estimating the minimum dietary requirement of phosphorus for large rainbow trout based on nonfecal excretions of phosphorus and nitrogen.

A new method was developed to estimate the minimum dietary requirement of phosphorus (P) for large fish for which conventional methods are not suitable. The method is based upon nonfecal (mainly urinary) excretion of inorganic P and total nitrogen from fish placed in a metabolic tank. In the first experiment, small and large rainbow trout (body wt 203 and 400 g, respectively) and, in the second experiment, P-sufficient, P-deficient and starved rainbow trout (different in diet history; body wt 349-390 g) were fed a constant amount (standard feeding rate) of semipurified diets with incremental P concentrations once daily at 15 degrees C. In all cases, there was no measurable excretion of P when dietary P concentration was low; however, beyond a specific dietary concentration, excretion of P increased rapidly. The point where the fish started to excrete P was assumed to be the minimum dietary requirement. By d 3 of consuming the experimental diets, the response of the fish to dietary P concentration stabilized, and excretion of P remained constant within dietary treatment groups for the subsequent sampling days (d 6, 9 and 12). The minimum dietary requirement of available P for fish having body wt of 203 and 400 g was estimated to be 6.62 and 5.54 g/kg dry diet, respectively, and that for P-sufficient, P-deficient and starved fish was estimated to be 4.06, 5.83 and 4.72 g/kg dry diet, respectively, when feed efficiency is 1.