Yeast-based feed additives have emerged as promising functional feed additives (FFAs) to promote sustainable aquaculture development through enhanced gut health and immune modulation in fish. The present study evaluated the impact of autolysed brewer’s yeast (ABY) and soluble dried yeast extract (SDYE) in improving the intestinal and skin histology, immune response, and intestinal microbiome of Atlantic salmon parr (Salmo salar) over a 9-week feeding trial. Three experimental diets were produced: a control diet, a diet supplemented with ABY at 2.5 g/kg, and a diet supplemented with SDYE at 2.5 g/kg. These diets were administered to triplicate tanks of Atlantic salmon. The yeast-supplemented diets, especially ABY, improved intestinal mucosal fold length, lamina propria width, microvilli density, and intestinal goblet cell counts, as well as skin goblet cell counts. The yeast additives had no detrimental effects on the fish haematology, with no significant differences in haemoglobin concentration, red blood cell counts, and white blood cell counts among the treatment groups. Gene expression analysis revealed upregulation of il-1β and muc-2 in fish fed the ABY diet, indicating enhanced immune function and potentially mucosal protection. Intestinal microbiota analysis revealed Firmicutes as the most dominant phylum in all groups, followed by Actinobacteriota. Distinct bacterial community shifts were observed between the treatment groups, with a significant increase in the relative abundance of taxa such as Staphylococcus in yeast-supplemented diets and a significant decrease in Streptococcus and Weissella. Collectively, these findings suggest that the yeast additives, especially ABY, enhance gut health and immune function without compromising growth performance.

Soybean meal (SBM) is commonly used in aquafeeds due to its wide availability, reasonable protein content, and cost‐effectiveness. However, high SBM inclusion levels in the diets of carnivorous fish, such as Atlantic salmon ( Salmo salar ), can cause soybean meal‐induced enteritis (SBMIE), resulting in compromised gut health, reduced nutrient absorption, and impaired growth. An 8‐week study was conducted to evaluate the potential of brewers’ yeast‐derived functional feed additives (FFAs), specifically yeast cell wall β ‐glucans (P β G [purified β ‐glucan]) and yeast cytosolic extracts (YEs), to mitigate the adverse effects of SBMIE in Atlantic salmon parr. Fish were fed diets containing 30% SBM (30‐SBM) with either 0.02% β ‐glucan (30‐SBM+P β G) or YE at 1% (30‐SBM+YE1) and 2.5% (30‐SBM+YE2.5) inclusion levels and compared against a control diet without SBM (0‐SBM). The study assessed growth performance, haematological parameters, distal intestinal morphology, and the distal intestinal gene expression levels of enteritis biomarkers ( casp3b , pcna , and hsp70 ). The results showed that P β G and 1% YE supplementation significantly reduced the severity of SBMIE, with improvements in intestinal morphology, including reduced intraepithelial leukocytes (IELs) levels and goblet cell hyperplasia. Intestinal gene expression levels of casp3b and pcna were significantly downregulated in the P β G and YE fed fish relative to the 30‐SBM fed fish, indicating reduced apoptosis and more controlled cell proliferation. However, the effects of 2.5% YE supplementation were less pronounced, indicating a dose‐dependent response. These findings demonstrate that both P β G and YE from 100% Saccharomyces cerevisiae can alleviate SBMIE in juvenile Atlantic salmon by supporting gut health and modulating cellular recovery processes.