The protein requirement of brown trout ( Salmo trutta) fry

The protein requirement of brown trout of INRA DC 87 strain was studied by feeding diets containing graded levels of protein. These diets were formulated to be isoenergetic on a digestible energy basis and contained 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 every 2 weeks according to biomass, but the actual overall ration corresponded 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 protein, the better the nitrogen utilization (estimated by protein efficiency ratio or productive protein value). Body protein content was not related to dietary protein but low protein levels resulted in higher body lipid content. Essential free amino acid contents in the muscle were plotted against the dietary protein levels and the values of the protein level corresponding to the slope changes were compared with the estimations of the dietary requirement. One free amino acid (threonine) showed a slope changing point corresponding to the values of the requirement determined with specific growth rate, while two other amino acids (arginine and valine) led to values lower than the estimated requirement. 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 of brown trout determined under these conditions seems to exceed that of the salmonids of the genus Oncorhynchus, but it could be similar to that of Salmo salar.