Effect of Candida utilis on growth and intestinal health of Atlantic salmon (Salmo salar) parr

In this study, Candida utilis was included as alternative protein source in diets for Atlantic salmon (Salmo salar) parr and the effects on growth performance and distal intestinal (DI) health were assessed. The potential of C. utilis to counteract possible adverse effects of a high soybean meal (SBM) diet was additionally assessed by using graded levels of C. utilis in combination with a 40% SBM diet. Six experimental diets were formulated: Fishmeal control (FM); FM with 20% C. utilis (FM2OCU); SBM-based diet containing 40% SBM (SBM); three SBM-based test diets, where wheat gluten and starch were substituted with increasing levels of C. utilis of 5%, 10% and 20% (SBM5CU, SBM10CU and SMB20CU, respectively). A total of 2700 Atlantic salmon parr with an average weight of 4.4 g were distributed into 18 tanks and the diets were fed in triplicates. The experiment lasted 28 days and sampling for histology and gene expression analysis was done at the end of the experiment. The fish grew to an average weight of 14.8 g in 28 days. Fish fed FM2OCU obtained a significantly higher specific growth rate (SGR) of 4.59 than the other dietary groups. While fish fed FM and FM2OCU displayed normal morphology of the DI, fish fed SBM-based diets showed mild histological changes in the DI that can be related to classical SBM induced enteritis (SBMIE). These changes included reduced height of simple folds and decreased presence of supranuclear vacuoles. The severity of these changes was not altered with increasing levels of C. utilis in the SBM diet. The expression of four of the five selected genes analyzed in the DI were altered by 40% dietary SBM inclusion. Aquaporin 8ab demonstrated the clearest changes among dietary groups, showing down-regulation independent of dietary inclusion of C. utilis. The strong down-regulation of aquaporin has been observed before and may be an indicator for intestinal barrier dysfunction. In conclusion, adding C. utilis to diets for Atlantic salmon parr gave high growth performance without any obvious adverse effects on intestinal health, but were unable to counteract the mild histology changes seen in DI of the SBM fed fish.