Paecilomyces variotii Mycoprotein in Diets for Juvenile Rainbow Trout: Effects on Growth Performance, Intestinal...

Aquaculture continues to search for new protein sources that can promote fish growth and health, as well as reduce competition between food and feed. Thus, the current study evaluated the dietary inclusion of a Paecilomyces variotii-based microbial ingredient on technical pellet quality, growth performance and intestinal health of juvenile rainbow trout (Oncorhynchus mykiss). Five experimental diets were formulated to include 0% (control), 5%, 10%, 20%, and 30% of P. variotii. Fish (initial body weight: 43 ± 10 g) were fed with the experimental diets for 9 weeks in triplicate tanks per treatment. Increasing inclusion levels of P. variotii significantly influenced feed pellet physical characteristics, with reduced pellet durability but an increased water stability index. Growth performance did not differ significantly between the control group and fish fed diets containing up to 20% P. variotii. However, fish fed the 30% P. variotii exhibited reduced feed intake and growth, suggesting an upper limit for its dietary inclusion. Moreover, the transepithelial potential (TEP) in the proximal intestine was significantly lower in fish fed the 30% P. variotii diet compared with the control, indicating compromised epithelial integrity at higher inclusion levels. Although the villi length was significantly higher at a 30% inclusion level, no significant difference was observed in the goblet cell area among treatments. Overall, under the present diet formulation and processing conditions, the inclusion of P. variotii appears feasible at inclusion levels up to 20% (without adverse effects on growth performance or intestinal health). This supports its potential as a sustainable feed ingredient, yet further studies with longer feeding periods and different developmental stages of fish are needed to fully assess its applicability in commercial aquaculture settings.