
A groundbreaking study signals a shift in aquaculture practices, demonstrating that natural feed additives can pave the way for a more resilient and eco-conscious industry. The study has revealed the powerful impact of combining Saccharomyces cerevisiae (a type of yeast) and thymol, an aromatic compound derived from thyme, in revolutionizing trout farming. This natural approach has been shown to enhance fish growth, boost immunity, and strengthen antioxidant defenses, offering a sustainable alternative to traditional chemical-based solutions in aquaculture.
The research, published in Animals, investigated the effects of these natural additives on rainbow trout (Oncorhynchus mykiss) over a 60-day trial. The fish were divided into groups and fed diets supplemented with yeast, thymol, or a combination of both. The results were compelling: yeast significantly improved growth performance, while thymol enhanced immune and antioxidant responses. Together, they created a synergistic effect, leading to healthier fish and more efficient production practices.
Stronger Growth and Feed Efficiency
Rainbow trout fed diets containing yeast achieved remarkable growth improvements. Weight gain in these fish reached up to 348% of their initial weight, compared to 296% in the control group. Specific growth rates (SGR) also increased from 2.29% per day in the control group to 2.50% per day in fish fed diets with yeast and thymol.
Additionally, the feed conversion ratio (FCR), a key metric for feed efficiency, dropped significantly from 1.04 in the control group to as low as 0.89 in supplemented diets. These results highlight yeast's potential as a cost-effective solution for improving aquaculture productivity.
Enhanced Antioxidant Defense

Thymol demonstrated its role as a powerful antioxidant by reducing oxidative stress and strengthening the fish’s natural defenses. Levels of superoxide dismutase (SOD), an essential antioxidant enzyme, increased from 16.1 U/mg protein (control) to 27.4 U/mg protein in fish fed diets containing thymol and yeast.
Similarly, catalase (CAT) activity rose from 111 U/mg protein to 164 U/mg protein, while markers of oxidative stress, such as malondialdehyde (MDA), decreased by over 40%. These improvements suggest thymol’s effectiveness in protecting fish against environmental and physiological stressors.
The study also revealed significant improvements in immune parameters. Plasma lysozyme activity, an indicator of immune strength, increased from 19.0 U/mL (control) to 26.7 U/mL in fish fed the combined diet. Skin mucus alkaline phosphatase (ALP), a crucial component of mucosal immunity, rose from 1.27 U/mg protein to 1.96 U/mg protein, while bactericidal activity against pathogens like Aeromonas hydrophila and Yersinia ruckeri was notably enhanced. These findings underline the role of yeast and thymol in strengthening the fish’s defenses against disease.
The study further highlighted the combined effects of yeast and thymol on gut immune function. Gene expression analysis showed significant upregulation of immune-related genes such as TNF-alpha, beta-defensins, and heat shock protein-70 (HSP-70) in fish fed the combined diet. Notably, diets with 250 mg/kg thymol and yeast achieved similar benefits to those with higher thymol doses, showcasing the efficiency of the combination.
A Sustainable Solution for Aquaculture
As the aquaculture industry faces challenges such as disease outbreaks and environmental concerns, this study offers a natural, eco-friendly solution. By incorporating yeast and thymol into trout diets, fish farmers can reduce their reliance on synthetic additives and antibiotics, improving production efficiency while addressing sustainability goals.
The researchers emphasized the broader implications of these findings, particularly for global aquaculture. With rainbow trout production exceeding 950,000 tons annually, adopting functional feeds like these could lead to healthier fish, reduced ecological footprints, and increased consumer confidence in sustainable seafood.