Dietary l-tryptophan and tank colour effects on growth performance of rainbow trout (Oncorhynchus mykiss) juveniles reared in a recirculating water system

The aim of the present study was to evaluate the possible anti-stressful effect of dietary tryptophan supplementation on growth of juvenile rainbow trout (Oncorhynchus mykiss) reared in different background colour using recirculating water system. Therefore, rainbow trout (4.7 g ± 0.02) were reared for 11 weeks in black, light blue and white tanks and fed either a commercial diet (CD) or the same diet supplemented with tryptophan (2 g 100 g−1 diet). Rearing in black tanks led to reduced final weight and total length, lower food consumption, food conversion ratio and body protein, while no differences were observed between fish reared in light blue or white tanks. Feeding the fish tryptophan supplemented diet resulted in depressed growth, increased food consumption and food conversion ratio, decreased body protein and increased body lipid, reduced liver total lipids and a marked increase in hepatosomatic index (least in fish reared in white tanks). It is concluded that rearing on a black background was stressful for rainbow trout juveniles, while the dietary level of tryptophan used failed as a stress-releasing factor and probably evoked an amino acid imbalance.