Effects of changes in dietary DP/DE ratio during the growing phase on growth performance and carcass composition of rainbow trout (Oncorhynchus mykiss, Walbaum)

Abstract

Rainbow trout (800 fish) (average weight: 96.5 ± 0.7 g) were fed to satiation two isolipidic extruded diets differing in crude protein content. Diet A = 36.8 and diet B = 42.9% of dry matter. A feeding trial was conducted for 84 days according to the following experimental design: treatment AA: diet A used for the entire trial: treatment BB: diet B used for the entire trial; treatment AB: diet A used during the first part of the trial and diet B during the second; treatment BA: diet B used during the first part of the trial and diet A during the second. The first part lasted 41 and the second 43 days. Apparent digestibility of dry matter, N-free extract, gross energy and phosphorus (P) were higher in diet B (79.81 vs 84.45; 51.34 vs 71.14; 82.53 vs 87.12 and 62.72 vs 70.45%, respectively for diet A and B) (P < 0.05). Fish fed diet B for the entire trial showed the highest final weight and specific growth rate and the lowest FCR (+ 26.1%; + 19.4% and −21.4% in comparison with treatment AA, respectively) (P < 0.05) while treatments AB and BA had similar and intermediate values between treatments BB and AA. After 41 d, fat and P body contents were significantly modified by dietary treatment (EE: 10.9 vs 12.4%; P: 0.46 vs 0.43 both as% WW respectively for diet A and B) (P < 0.05) but not at the end of the trial. There were no statistical differences in gross energy intake (GEI, KJ) per kg of body weight (b.w.) between treatments in the first part of the trial (average of two diets: 413.9 ± 5.73 KJ). In the second period, GEI was lower in treatments BA (321.4 KJ) and BB (332.4 KJ) in comparison with treatment AB (363.4 KJ) and AA (350.9 KJ) (P < 0.05). However, when results were expressed per unit of weight gain, values were similar in the two periods and were influenced only by diets. In fish fed diet B during the entire experimental period, retained energy increased compared to diet A. The amount of nitrogen and phosphorus released into the environment per kg of fish growth were lower in fish fed diet B for the entire trial compared to diet A and intermediate for treatments AB and BA.