Growth, nitrogen and energy utilization of juveniles from four salmonid species: diet, species and size effects

The effect of dietary digestible protein/digestible energy (DP/DE) ratio on growth, feed efficiency (FE), digestibility, nitrogen (N) and energy utilization, and body composition of juveniles from four salmonid species reared in freshwater was investigated in a series of trials. Another objective of this study was to investigate how the FE, and N and energy utilization changed as fish grew. Four diets were formulated to be isoenergetic (DE=20 MJ/kg) but contain different DP/DE ratios, 24, 22, 20, and 18 g/MJ, achieved through reduction of DP level (53% to 39%) and increase of lipid level (19% to 26%). Diets were hand-fed to near-satiety to triplicate groups of lake trout (initial body weight (IBW), 47 g) and Atlantic salmon (IBW, 25 g) for 280 days at 13 degreesC and to rainbow trout (IBW, 47 g) and chinook salmon (113W, 24 g) for 84 and 140 days, respectively, at 12 degreesC. Within species, weight gain was not affected by DP/DE (P < 0.05). However, a significant decrease in FE was observed with decreasing DP/DE for all species, except chinook salmon. Digestible N retention efficiency (NRE) increased linearly (P < 0.05) with decreasing DP/DE ratio while there was no diet effect on digestible energy retention efficiency (ERE) for all species. The effect of diet on FE, N and energy utilization as fish grew was investigated for lake trout and Atlantic salmon. As lake trout and Atlantic salmon grew, significant linear decreases in FE, NRE and ERE, irrespective of diet, were observed. The decrease in NRE and ERE as fish grew correlated with decreasing dressed carcass yields, and significant increases in body dry matter and lipid contents. More research is needed in order to gain further insight into species and size effects on feed and nutrient utilization. (C) 2004 Elsevier B.V. All rights reserved.