THE PROTEIN REQUIREMENT OF BROWN TROUT (SALMO-TRUTTA) FRY

The protein requirement of brown trout of INRA DC 87 strain was studie d by feeding diets containing graded levels of protein. These diets we re formulated to be isoenergetic on a digestible energy basis and cont ained protein from 38 to 65%. The fish were 1.15 g triploid fry reared in stream water. Triplicates of 300 fish per replicate were used for each diet. The fish were fed for 52 days a ration which was modified e very 2 weeks according to biomass, but the actual overall ration corre sponded to excess feeding. Proximate analyses were performed on whole body and epaxial muscle at the end of the trial. No significant growth improvement was obtained over 53% protein, while best feed efficiency was apparently observed with 57% protein, The lower the dietary prote in, the better the nitrogen utilization (estimated by protein efficien cy ratio or productive protein value). Body protein content was not re lated to dietary protein but low protein levels resulted in higher bod y lipid content. Essential free amino acid contents in the muscle were plotted against the dietary protein levels and the values of the prot ein level corresponding to the slope changes were compared with the es timations of the dietary requirement, One free amino acid (threonine) showed a slope changing point corresponding to the values of the requi rement determined with specific growth rate, while two other amino aci ds (arginine and valine) led to values lower than the estimated requir ement, Estimations of the absolute requirements in g protein per fish per day or g protein per 100 g fish per day were not accurate because of poor control of the actual feed intake. The optimal protein level o f brown trout determined under these conditions seems to exceed that o f the salmonids of the genus Oncorhynchus, but it could be similar to that of Salmo salar.