Effects of fish size and diet adaptation on growth performances and nitrogen utilization of rainbow trout (<i>Oncorhynchus mykiss</i> W.) juveniles given diets based on free and/or protein-bound amino acids

The quality of dietary protein is an important factor influencing fish growth. It is usually assessed by amino acid (AA) composition, protein digestibility and protein utilization efficiency. Here it was investigated with rainbow trout (Oncorhynchus mykiss W.) juveniles 1) if the molecular form of the ingested nitrogen (free (F) AA, peptides or proteins) may also affect the dietary protein quality; 2) if this possible influence may be affected by juvenile size and adaptation to the diet; and 3) what is the optimum synthetic FAA to protein ratio. Two experiments were carried out at 15 °C (3 tanks/treatment) in which 1050 small juveniles (0.70 g) and 450 large juveniles (2.85 g) were randomly assigned to fifteen 15 l-tanks (70 fish/tank) and to fifteen 45 l-tanks (30 fish/tank), respectively. In both experiments, fish were fed twice a day to satiation for 9, 17 and 25 feeding days (3 periods) five isoenergetic and isonitrogenous (412 g crude protein/kg dry matter (DM)) diets containing graded levels of coated FAA, replacing 0, 25, 50, 75 or 100% (D0–D100) of the cod muscle meal, an intact protein. Compared to D0, growth rate (DGC, % per day) and feed intake (g DM/kg0.75 per day) were significantly reduced only for small juveniles and large juveniles fed diets containing at least 50 and 75% of FAA, respectively. Protein deposition (g/kg0.75 per day) was not significantly reduced for juveniles fed diets D25–D50, but well for juveniles fed diets D75–D100. The decrease of growth rate and protein deposition at FAA inclusion rate below 75% was largely explained by a reduction of voluntary feed intake while nitrogen retention efficiency was significantly reduced only for diet D100. The maximum FAA level that still ensured reasonable growth (85% of maximum) was 65%, at both juvenile sizes, after an adaptation period of 17 days. Larger juveniles tolerated higher FAA dietary levels than smaller juveniles, indicating ontogeny-related changes in FAA utilization efficiencies. Finally fish accustomed to FAA-rich diets were able to tolerate higher FAA dietary levels but the maximum FAA inclusion level was modulated by fish size. In conclusion, here we show that the dietary protein quality is dependent of the molecular form of the ingested nitrogen in rainbow trout juveniles and that this quality is modulated by the ratio of synthetic FAA to protein (or FAA inclusion rate), the adaptation of the fish to the diet and the fish size