Yeast and Derivatives in Aquaculture Nutrition and Feeds: A Reassessment of Key Functional Attributes

Yeast (Saccharomyces cerevisiae) and its various fractions are assessed as sustainable functional feed additives that address both nutritional requirements and health challenges in intensive aquaculture systems. This review is based upon existing knowledge of yeast-derived products, critically evaluating their applications within aquafeeds. Specifically, it explores the use of whole yeast, yeast cell walls, autolysates, hydrolysates, and individual bioactive compounds. The main emphasis is placed on the yeast products containing β-glucans, mannanoligosaccharides (MOS), nucleotides, and other bioactive compounds, which provide significant advantages for enhancing innate and adaptive immunity, thus modulating gene expression, and raising antioxidant responses across diverse aquaculture species. We state the optimal inclusion levels from the scientific literature, which may range from as low as 0.1% for whole yeast products and about 0.4% for specific fractions, though species-specific responses vary considerably. Furthermore, this review integrates some of these areas to present a cohesive overview of different mechanisms of action, species-specific applications, physiological impacts on the gut, and related organ and systemic health. We also note specific haematological parameters and disease resistance across fish and crustacean species that may be affected by yeast products. By integrating the mechanistic insights with production-oriented application, this review fills the knowledge gap by establishing a framework for the powerful nature of yeast’s multiple roles in aquaculture nutrition. It serves to scope the potential future of commercial technologies leading to novel yeasts as functional feed additives. Although the literature suggests potential benefits for growth, immunity, antioxidant status, and gut health, the magnitude of response is highly variable amongst investigations.