Evaluation of Yeast Meal ( Saccharomyces cerevisiae ) as a Fishmeal Substitute in High Plant Protein Diets for Hefang Crucian Carp ( Carassius auratus cuvieri ♀ × Carassius auratus red var ♂): Growth, Health, and Gene Expression

Aquaculture omnivorous and herbivorous fish frequently use plant proteins to make up for the lack of fish meal, but there are issues with inadequate intake and nutritional imbalance. One possible remedy is the synergistic impact of complex proteins. In this study, a 74‐day feeding trial was conducted to evaluate dietary yeast ( Saccharomyces cerevisiae ) protein (YP) as a fishmeal (FM) substitute in Hefang crucian carp ( Carassius auratus cuvieri ♀ × Carassius auratus red var ♂; initial weight: 40.49 ± 0.13 g). Five low‐FM, high‐plant protein diets (27% crude protein, 5% crude fat) were formulated with graded YP levels of 0%, 1%, 2%, 3%, and 4%, designated as groups YP0, YP1, YP2, YP3, and YP4, respectively. YP replaced FM on an equal‐weight basis, with FM decreasing from 4% in YP0% to 0% in YP4. The results showed that YP supplementation did not adversely affect survival rate, total antioxidant capacity, or muscle quality. Notably, the YP3 group (3% YP) achieved the highest final body weight ( p < 0.05), weight gain rate ( p < 0.05), and specific growth rate. This growth promotion was associated with the synergistic upregulation of hepatic growth hormone/insulin‐like growth factor (GH/IGF) axis‐related genes (IGF‐I and TOR), along with increased intestinal expression of peptide and amino acid transporters (PepT1, ASCT2, LAT2) in the YP3 group. Moreover, the YP3 diet neither increased deamination of excess amino acids nor elevated total nitrogen emissions, and no significant differences were observed in plasma catalase, superoxide dismutase, or lysozyme activities ( p > 0.05). In conclusion, dietary supplementation with 3% YP (replacing 3% FM) promotes optimal growth and health in Hefang crucian carp without compromising physiological status or environmental footprint.