Torula yeast can reduce the dependence on plant proteins in Atlantic salmon feeds: Effects on growth, nutrient utilization, mucosal health, and spleen gene expression

Plant-based proteins have played a crucial role in the growth of aquaculture by enabling the production of large feed volumes while decreasing dependence on marine resources. Still, more protein sources are needed to support continued expansion and flexibility in feed formulations. This study evaluated torula yeast as a substitute for soy protein concentrate in diets for post-smolt Atlantic salmon ( Salmo salar L.). Five diets containing 0 %, 5 %, 10 %, 15 %, and 20 % torula yeast were fed to Atlantic salmon in quadruplicate for 12 weeks. Growth performance, digestibility, chemical composition, and efficiency ratios were assessed to determine nutrient utilization. Histology of the intestine, skin, and kidney was performed on fish fed the diets with 0 %, 10 %, and 20 % yeast. For these groups, transcriptomics was also used to examine spleen gene expression. Replacing soy protein with torula yeast did not impair growth, protein- or lipid digestibility, fish chemical composition, or protein efficiency. Intestinal fold height decreased in the anterior intestine of fish fed the diets with yeast, while the distal intestine was unaffected. At 20 % yeast inclusion, lamina propria width increased in the distal intestine, and mucous cell density declined in both intestinal segments, though mucous cell size and barrier status remained unchanged. Skin showed thicker stratum spongiosum and higher mucous production at 20 %, suggesting that yeast cell wall components may stimulate skin immunity. Kidney histology revealed an increased abundance of melanomacrophages at the highest yeast level. Spleen gene expression showed no stress or systemic immune activation. Overall, the results of the present study demonstrate that torula yeast protein has the potential to reduce the reliance on plant-based proteins in commercial feeds for Atlantic salmon. However, a 20 % inclusion level may cause subtle physiological alterations in the intestine and kidney that could indicate inflammation. Further research is needed to determine if these changes affect the long-term health and robustness of the fish. • Torula yeast can replace 20 % of plant-based proteins in diets for Atlantic salmon • Dietary protein digestibility was unaffected by torula yeast inclusion • Torula yeast altered intestinal and skin mucosal barrier status • Diet with 20 % torula yeast displayed increased prevalence of kidney melanomacrophages • Spleen gene expression indicated absence of stress and immune response