The thermal dependence of the protein‐sparing effect in rainbow trout (Oncorhynchus mykiss, Walbaum 1792)

The authors performed an instantaneous bioenergetic study with rainbow trout (Oncorhynchus mykiss) of 206.3 g ± 2.9 g in a group respirometer of nine 250 l tanks at five different water temperatures (12, 14, 16, 18, 20°C) to determine the optimal thermal condition for a maximal visualization of the protein‐sparing effect. Twelve fish per tank were tested at a stocking density of 9.94 kg m−3 ± 0.14 kg m−3 and fed three low‐protein/high‐energy diets with constant crude protein content of c. 35% and three different energy contents (17.35, 18.76, 20.50 MJ kg−1) once daily at a ration of 1.3% body weight (n = 3). Energy levels were increased by adding gelatinized wheat starch as a carbohydrate source and fish oil, canola oil and palmitin as lipid sources. Three different dietary digestible protein/digestible energy ratios (DP/DE: 20.38, 19.08, 18.09 mg kJ−1) were achieved by replacing bentonite as a non‐nutritive filler with carbohydrates and lipids. Oxygen consumption and ammonia excretion were assessed to obtain the potentially retainable energy (RE) and ammonia quotient (AQ) as benchmarks for potential growth and protein‐sparing effect. The results showed the lowest relative metabolic combustion of protein at 16.9°C ± 0.1°C. The authors determined this temperature to set the optimal thermal condition for the induction of a maximum protein‐sparing effect in juvenile rainbow trout. Increasing the DP/DE ratio significantly altered the magnitude of the relative metabolic protein use but had no effect on its interactions with temperature. The authors were able to reduce average metabolic fuel use of protein across diets from 16.2% ± 2.3% at 12°C to 8.0% ± 1.2% at 16°C. This study found no relevant significant differences of RE with the environmental temperature.