Microencapsulated Functional Additives in Commercial Diets: Effects on Growth, Health, and Intestinal Pro-Inflammatory Gene Expression in Juvenile Rainbow Trout (Oncorhynchus mykiss)

Functional feed additives can improve growth, health, and fillet quality in farmed fish, but their stability during feed processing is limited. This study evaluated the effects of dietary microencapsulated rosemary essential oil (REO), astaxanthin (AX), and butyric acid (BA) on growth, oxidative response, and fillet quality in juvenile rainbow trout (7.9 ± 0.3 g; 90 days). Growth parameters, intestinal and liver health, oxidative status, and fillet pH and color, together with proximate composition and fatty acid profile, were measured. All supplemented groups exhibited higher growth performance compared to controls. Histology and stress/inflammatory markers indicated no adverse effects on tissue health. Regarding fillet quality, REO maintained protein content, AX enhanced lipid content and color stability, and BA promoted a leaner fillet with higher protein deposition. Fillet pH remained more stable in REO and BA groups after frozen storage, suggesting improved product stability. Fatty acid profiles were moderately altered, with REO increasing C18:1n7 and AX enhancing polyunsaturated fatty acids, whereas BA had minimal effects. Plasma antioxidant enzyme activities were lower in supplemented fish, indicating improved redox balance. Overall, microencapsulation preserved the bioactive properties of REO, AX, and BA, supporting their practical use as targeted functional feed additives in rainbow trout aquaculture.