Full-fat soya for rainbow trout (Oncorhynchus mykiss) in freshwater: Effects on performance, composition and flesh fatty acid profile in absence of hind-gut enteritis

The objective of this study was to evaluate changes in trout performance, quality and flesh fatty acid profile when supplying a substantial proportion of dietary protein using full-fat soya. Six feeds were formulated with extracted soya (7.5%) and full-fat soya (0-25%) representing 7.5-32.5% of the formulation. The feeds were fed to duplicated tanks of rainbow trout (initial weight 110 g) for 8 weeks at 11 °C. Survival and growth rate were not significantly affected, however, both feed intake and FCR increased linearly in response to increased dietary soya level (P = 0.021 and 0.002, respectively). With the exception of carcass yield, which decreased linearly in response to increased soya level (P = 0.039), other basic quality attributes, e.g. fillet proximate composition and colour, were not significantly affected by the soya content of the feeds. The relative percentages of most flesh fatty acids responded linearly to changes in those derived from the soya lipids though notably, the relative percentages of 22:6n-3 and total n-3 fatty acids were not significantly affected (P = 0.788 and 0.095, respectively). There was no histological evidence of soya-induced hind-gut enteritis. Thus, a combination of extracted and full-fat soya provided between 18% and 36% of the crude protein without adverse effects on growth, survival or basic quality attributes. This was achieved in the absence of hind-gut enteritis without compromising the healthy eating image of farmed trout.