Effects of feeding levels on growth performance, feed utilization, body composition, energy and protein maintenance requirement of fingerling, rainbow trout, Oncorhynchus mykiss (Walbaum 1792)

A 10-week feeding experiment was conducted to evaluate the effects of feeding level, energy and protein maintenance requirement of rainbow trout, Oncorhynchus mykiss, fingerling (5.65±0.45cm; 1.42 ±0.25g) by feeding casein-gelatin based purified diet (45% CP; 3.67 kcal  g-1 GE) at six feeding levels 2-7% of BW/day in two equal meals, at 0800 and 1700 h, in triplicate, with 20 fish per trough fitted with continuous water flow-through system water volume 60L. Maximum live weight gain, best-feed conversion ratio (FCR), best specific growth rate (SGR) and highest protein efficiency ratio (PER) were reported with fish fed 4-5% BW day-1. However, quadratic regression analysis for weight gain, FCR, PER, protein retention efficiency (PRE) and energy retention efficiency (ERE) data indicated the break-points occurred at 5.33, 4.50, 4.48, 4.63 and 4.74% BW day-1, respectively. Body composition also produced significant (p<0.05) differences with respect to each feeding level, maximum protein, lowest moisture and intermediate fat contents were reported at 4-5% feeding levels. Protein and energy retention values also produced significant (p<0.05) difference among each feeding level with maximum values were reported at 5% feeding level. Based on the above results, it is recommended that feeding in the range of 4.6 to 5.3% BW day-1, corresponding to 2.07-2.39g protein and 16.88-19.45 kcal energy g100g-1 of the diet day-1 is optimum for the growth and efficient feed utilization of rainbow trout, while 2-3% feeding levels (0.90-1.35g protein and 7.34-11.01 kcal energy) suggests that these amounts approximate the maintenance requirement of fish.