The effects of feeding with zinc enriched yeast (Saccharomyces cerevisiae) on growth and carcass quality of rainbow trout(Oncorhynchus mykiss)

<p style="text-align: left;"><strong>Background and Objective:</strong> In alignment with the United Nations Sustainable Development Goals, food security and global nutrition are recognized as fundamental priorities in development agendas. Within this framework, fish plays a pivotal role as a primary source of animal protein, critically contributing to the nutritional health of the world&rsquo;s growing population. Rainbow trout (<em>Oncorhynchus mykiss</em>) is one of the most important cultured species in global aquaculture, widely utilized in the industry due to its high growth rate, ability to consume artificial diets, and adaptability to various rearing conditions. Nevertheless, aquaculture systems face several challenges, including environmental stressors, immunosuppression, and the indiscriminate use of antibiotics, that limit their efficiency and sustainability. In recent years, the use of natural feed additives, particularly probiotics such as the yeast <em>Saccharomyces cerevisiae</em>, has gained attention as an environmentally compatible and sustainable strategy. This yeast acts not only as a biological growth promoter but also enhances overall fish health by stimulating the immune system, improving nutrient absorption, and modulating the intestinal microbiota. Concurrently, zinc (Zn), as an essential trace element, plays a vital role in fish growth, reproduction, immune function, and metabolism; its deficiency can lead to reduced growth, impaired immunity, and structural abnormalities. Based on these premises, this study was designed to evaluate the effects of dietary supplementation with <em>S. cerevisiae</em> enriched with zinc on growth performance, carcass quality, and biochemical zinc levels in rainbow trout.</p> <p style="text-align: left;"><strong>Materials and Methods:</strong> This experimental study was conducted in autumn 2022 (Iranian calendar year 1401) at the Artemia and Aquaculture Research Institute of Urmia University. The yeast <em>S. cerevisiae</em> (PTCC 5269) was enriched with zinc oxide using the standard method described by Stehlik-Tomas et al. (2004). The successful enrichment was confirmed via lyophilization followed by acid digestion. A total of 720 rainbow trout fingerlings with an initial average weight of 29 &plusmn; 5 g were randomly assigned to four experimental groups, comprising a control group and three treatment groups receiving <em>S. cerevisiae</em> enriched with zinc at concentrations of 1&times;10⁶, 1&times;10⁷ and 1&times;10⁸ CFU/g, each with three replicates. The experimental period lasted 75 days: during the first 60 days, fish were fed diets supplemented with zinc-enriched yeast, and during the final 15 days, all groups received only the basal diet. Growth indices, including final weight, specific growth rate (SGR), weight gain percentage, and condition factor (CF), were calculated. Carcass quality was assessed by determining the percentages of muscle and liver protein and fat using the AOAC (2008) methodology. Zinc concentrations in serum, muscle, and liver tissues were measured on days 0 and 60 using atomic absorption spectrophotometry. Data were statistically analyzed using SPSS software (version 19) with one-way ANOVA and Tukey&rsquo;s post hoc test at a significance level of 5%.</p> <p style="text-align: left;"><strong>Findings: </strong>Results indicated that fish fed the 1&times;10⁷ CFU/g zinc-enriched yeast concentration exhibited the highest final weight (106.67 &plusmn; 8.4 g) and final length (20.45 &plusmn; 2.28 cm), which were significantly greater (<em>p</em>&lt;0.05) than those of the control group. Furthermore, all growth indices, including growth rate, specific growth rate, and condition factor, were significantly higher in all treatment groups compared to the control. Regarding carcass quality, muscle fat content in the control group (4.59%) was significantly lower than in the supplemented groups, whereas liver fat content in the control group (1.86%) was significantly higher than in the other groups. Muscle protein was slightly higher in the 1&times;10⁷ CFU/g group (18.87%), although this difference was not statistically significant. Zinc concentration in the supplemented diets showed a direct relationship with yeast concentration; the highest zinc levels were observed in the 1&times;10⁸ CFU/g group: serum (314&plusmn; 8 &micro;g/mL), liver (56.33 &plusmn; 0.6 &micro;g/g), and muscle (19.56 &plusmn; 0.2 &micro;g/g). Moreover, serum zinc levels were significantly higher than those in liver and muscle tissues.</p> <p style="text-align: left;"><strong>Conclusion:</strong> The findings of this study demonstrate that dietary supplementation of rainbow trout with <em>S. cerevisiae</em> enriched with zinc significantly improves growth performance and enhances carcass quality by reducing liver fat (an indicator of metabolic health) and increasing muscle fat (a marker of higher nutritional value). Given the absence of statistically significant differences in growth indices between the 1&times;10⁷ and 1&times;10⁸ CFU/g groups, the concentration of 1&times;10⁷ CFU/g is recommended as the optimal and cost-effective level. This approach not only reduces reliance on antibiotics but also represents a sustainable strategy for improving aquaculture productivity and final product quality in the aquaculture industry.</p>