Effect of dietary protein and energy levels on rainbow trout (Oncorhynchus mykiss) reared in brakish water

A completely randomized design, with 3×3 factorial arrangement, was conducted to test various ratios of dietary protein to energy (P/E) for rainbow trout, reared in brackish water. Three crude protein levels (35, 40 and 45%) and three energy levels (370, 400 and 430 Kcal/100g) at each protein level were utilized. Semi-purified diet containing fish meal, casein and gelatin as the sources of protein and dextrin, starch and oil as the sources of energy was used. Each experimental diets was fed to triplicate groups of 20 fish with average weight of 81.5g, in nine 2000 lit flow through fiberglass tanks, each partitioned into three sectors. Average water temperature, dissolved oxygen, pH and EC were between 15±20C, 6.5-8.1 mg/L, 7.7-8.6 and 25400 µm/s, respectively. Fishes were fed for 84 days at a rate between 1.6-2% of body weight per day, in three equal amounts, adjusted bi- weekly. At each protein level, weight gain (WG%), average daily growth (ADG%), protein efficiency ratio (PER), apparent net protein utilization (ANPU%), specific growth rate (SGR) and condition factor (CF) increased, but feed conversion ratio (FCR) decreased with increasing energy level from 370 to 430 Kcal/100g. The highest growth performance was obtained by fishes fed by 35% protein, 430 Kcal/100g energy diet with P/E ratio of 81.4 mg protein/Kcal energy. Carcass fat and moisture were affected by dietary protein and energy levels, whereas, carcass protein and ash levels were similar between dietary treatments.