Nutritional and functional feed additives in fishes: A comprehensive review

The gastrointestinal tract of fish plays a vital role not only in nutrient digestion and absorption but also in maintaining immune function and overall physiological stability. In aquaculture, stressors such as intensive farming, environmental fluctuations, and imbalanced diets can compromise gut health, resulting in altered microbiota, weakened immunity, and poor feed conversion. This review explores the application of diverse feed additives aimed at preserving and enhancing gut health in fish species. Feed additives such as probiotics, prebiotics, synbiotics, postbiotics, phytogenic, organic acids, enzymes, functional lipids, and immunostimulants have been increasingly adopted as functional ingredients in aquafeeds due to their ability to support intestinal morphology, modulate microbial communities, stimulate enzymatic activity, and activate immune responses. Their use contributes to improved growth rates, better nutrient utilization, and increased resistance to pathogens, while also offering alternatives to antibiotics; a critical step toward combating antimicrobial resistance in aquaculture systems. The review also highlights the importance of understanding species-specific responses to these additives, as their efficacy depends on factors such as fish species, life stage, additive dosage, formulation, and environmental conditions. While many studies have demonstrated positive outcomes, there remains a need for deeper insights into the mechanisms of action, long-term effects, and additive interactions within the host microbiome. Advanced tools like metagenomics and metabolomics can help bridge these gaps. Additionally, developing novel bio-based and nanotechnology-enhanced additives holds promise for precision nutrition and sustainable production.