RESPONSE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) GROWING FROM 50 TO 170 G TO SUPPLEMENTS OF EITHER L-ARGININE OR L-THREONINE IN A SEMIPURIFIED DIET

We studied the effects of increasing dietary concentrations of either L-arginine or L-threonine on growth, feed intake, feed conversion rati o and composition of gain in rainbow trout. Semipurified diets contain ing 20.1 MJ digestible energy/kg dry matter, with wheat gluten and cry stalline amino acids as sole sources of amino acids, were fed to rainb ow trout (initial mean body weight 47 +/- 0.7 g). In one series of 12 diets, arginine concentration ranged from 5.0 to 23.8 g/kg dry matter; in a second series of 12 diets, threonine concentration ranged from 3 .7 to 21.0 g/kg dry matter. Each diet was fed to a group of 20 fish. D uring the experiment of 51 feeding days, dry matter intake, weight gai n, feed conversion ratio, protein concentration of gain and total prot ein deposition followed exponential functions. For achieving 95% of th e potential maximum protein deposition, dietary concentrations of 11.6 g arginine and 10.4 g threonine/kg dry matter were required. Arginine and threonine were both utilized most efficiently at dietary concentr ations of similar to 6 g/kg dry matter. At low dietary concentrations of arginine, deposition of this amino acid exceeded the quantity fed. Recommended dietary concentrations of arginine and threonine will depe nd on the trait desired in the trout.