Partially replacing plant protein sources with torula yeast in rainbow trout (Oncorhynchus mykiss) feed increases growth and factors related to immune status

In this study, the impact of adding 10% or 20% torula yeast (PB TY10 and PB TY20) to a fishmeal-free diet on digestibility and growth of juvenile rainbow trout was assessed. The yeast diets were compared with a plant-only diet (PB) and a diet containing 20% fishmeal (FM). The apparent digestibility of dry matter, protein, energy, and ash of yeast in trout (~120 g) were 78.6%, 93.8%, 84.8%, and 48.7%, respectively. All PB diets had higher protein digestibility and PB TY20 had higher ash digestibility than the FM diet. To assess growth, trout (~44 g) were divided into triplicate tanks (30 per tank) and fed one of the four experimental diets for 12 weeks. The PB diet resulted in lower growth and daily feed intake (p < 0.05), but adding yeast significantly improved them. Final weight of PB TY10 did not differ significantly from that of FM. Expression of oclna and cldn15 in the proximal intestine increased in trout fed yeast (p < 0.05). Expression of tlr2, mcsfr, and tnfa, as well as the plasma lysozyme concentration, increased with PB TY20 (p < 0.05). In conclusion, torula yeast has the potential to improve the robustness and growth of rainbow trout in the context of fishmeal-free diets.