Temperature affects the growth and nutrient utilisation of fish. Diet formulations to the specific protein and energy requirements of brook trout, Salvelinus fontinalis, particularly at higher temperatures will be required for sustainable production. The optimum temperature for brook trout growth is 15¬¨‚àûC, however, in Australia, summer water temperature is often elevated to about 19¬¨‚àûC. Considering this, protein and energy requirements of brook trout were determined at 15¬¨‚àûC and 19¬¨‚àûC. Using a dose-response model, the digestible dietary protein requirement of brook trout for optimum growth rate was 44% and 40% at 15¬¨‚àûC and 19¬¨‚àûC, and for optimum protein efficiency was 39% and 35% at 15¬¨‚àûC and 19¬¨‚àûC, respectively. Using factorial modelling, the maintenance requirements for digestible protein were 0.11 gDP¬¨‚àëkg-0.70¬¨‚àëd-1 (15¬¨‚àûC) and 0.22 gDP¬¨‚àëkg- 0.70¬¨‚àëd-1 (19¬¨‚àûC) and for energy were 29.87kJDE¬¨‚àëkg-0.80¬¨‚àëd-1 (15¬¨‚àûC) and 36.66 kJDE¬¨‚àëkg-0.80¬¨‚àëd- 1 (19¬¨‚àûC). Specific growth rate, feed utilisation indices and apparent digestibility of nutrients were significantly higher at 15¬¨‚àûC. Higher levels of gelatinised carbohydrate increased protein efficiency and although liver glycogen storage was increased, it did not cause any pathological symptoms in the liver or intestine. The activity of glycolytic enzyme (PK) in the liver was increased with increasing levels of gelatinised carbohydrate and with higher temperature. Liver lipogenic enzyme (G6PDH) activity was neither affected by temperature nor dietary gelatinised carbohydrate level. Brook trout used gelatinised maize starch effectively for energy at 26%, and at least 13% dietary gelatinised carbohydrate should be added to brook trout feeds to reduce protein catabolism (GDH activity). The effect of replacing energy from lipid with carbohydrate on growth performance, nutrient utilisation and digestibility of brook trout was evaluated at 15¬¨‚àûC and 19¬¨‚àûC. Energy source had no effect on growth, protein utilisation and feed utilisation. At both temperatures, 26% carbohydrate improved the apparent digestibility of dry matter (ADDM), gross energy (ADGE) and energy from carbohydrate (ADCHO-E). Higher levels of gelatinised carbohydrate increased the activity of ˜í¬±-amylase; however, at 19¬¨‚àûC activity was lower than at 15¬¨‚àûC. There is great potential for high levels of gelatinised maize starch as an alternative energy source in brook trout diets to replace protein and lipid without compromising the growth or the function of liver and intestine. While growth was better at 15¬¨‚àûC, carbohydrate effectively met the increased energy requirements at 19¬¨‚àûC. This study has defined digestible protein and energy requirements for brook trout which can be applied to commercial feeds for production under optimum and challenging summer conditions.