The utilization of plant-based ingredients in aquaculture feeds has been beneficial in augmenting the production of aquaculture by decreasing the dependency on marine components. However, these feeds now account for almost 80% of the scope 3 greenhouse gas emissions in commercial salmon farming. Furthermore, as consumers increasingly recognize the significance of sustainability, there is a demand to explore more environmentally friendly and sustainable alternatives that can minimize greenhouse gas emissions and broaden the array of ingredients utilized in feed formulation. Single-cell proteins are a promising substitute for plant-based ingredients, although their nutritional efficacy and safety in diets for salmonid fish need to be thoroughly scrutinized. Consequently, this study aims to evaluate the effects of replacing up to 20% of plant-based ingredients with torula yeast in commercially relevant diets for Atlantic salmon.
A dose-response experimental design was implemented to formulate five diets that contained increasing levels of torula yeast (0%, 5%, 10%, 15%, and 20%), and fed to quadruplicate groups of post-smolt Atlantic salmon over a 12-week period to evaluate their effects on performance. The fish, with an initial weight of 92.4 ± 7.3g, reached a final weight of 435.1 ± 52.7g. The diets’ impact on the fish was assessed through various growth performance indicators, nutrient utilization, blood chemistry, histology, gene expression in the liver and spleen, and metabolomic responses in the liver.
The research findings indicate that there were no significant differences between the experimental groups in terms of specific growth rate, thermal growth coefficient , or feed conversion ratio. However, there were notable changes observed in the plasma metabolites of fish fed with diets containing yeast. These changes included a reduction in t otal b ile a cids and indications of increased a lkaline p hosphatase levels, which could be related to alterations in liver metabolism. Moreover, there was an increase in c alcium levels in the plasma of fish in the experimental diets compared to the control group. The initial histological evaluation of the anterior and distal intestines of the fish revealed some variations among the groups . The fish fed with 10 and 20% yeast exhibited a reduction in intestinal fold length in the anterior intestine. In contrast, the submucosa width was reduced in the diets containing 10, 15, and 20% inclusion of torula yeast. In the distal intestine, there was no difference in villus length between the dietary treatments, but the submucosa was reduced at 15 and 20% of yeast inclusion. Furthermore, a reduction in the width of the lamina propria was evident from 15% yeast inclusion. The gene expression analysis of the liver and spleen did not indicate any adverse effect of the dietary treatments, nor was there any evidence that the fish were stressed. However, several genes related to carbohydrate metabolism were altered in the liver, which will be further investigated with metabolomics.