Effects of different protein and carbohydrate levels on growth performance and feed utilisation of brook trout, Salvelinus fontinalis (Mitchill, 1814), at two temperatures

A 12-week feeding trial was conducted to determine the optimum dietary protein requirement of brook trout, Salvelinus fontinalis, at 15 and 19°C. Twelve iso-energetic (22 MJ · kg^-1) and iso-lipidic (23%) diets (36-58% protein at 2% increments) were prepared. Fish (29.45 ± 3.25 g · fish^-1) were fed 2% of body weight per day, divided into two equal rations. The specific growth rate (SGR, % · day^-1), feed efficiency ratio (FER), productive protein value (PPV), productive lipid value (PLV) and productive energy value (PEV), apparent digestibility of diet (AD_DM) and protein (AD_CP) were significantly higher at optimum temperature (15°C). Increasing PPV with increasing dietary carbohydrate and with decreasing dietary protein content was due to the protein-sparing effect of carbohydrates. A piecewise regression (broken line) model between the SGR and digestible dietary protein level revealed that the digestible dietary protein requirement of brook trout was 44 and 40% at 15 and 19°C, respectively. When PPV (digestible protein retention basis) was modelled with a broken line, the digestible protein requirement of brook trout was 39 and 35% at 15 and 19°C, respectively. A reduction in dietary protein content balanced by increased gelatinised carbohydrate might be useful for improving the protein utilization efficiency for growth at 15 and 19°C; however, the growth and feed efficiency was lower at the elevated temperature.