Effects of Dietary Dill Essential Oil on Growth Performance, Digestive Enzymes, Antioxidant/Immunological Parameters and Disease Resistance in Rainbow Trout, <i>Oncorhynchus mykiss</i>

This study investigated the effects of dietary dill essential oil (DEO) at concentrations of 0.5% (DEO0.5), 1% (DEO1), 2% (DEO2) and 3% (DEO3) on rainbow trout juveniles. A total of 300 healthy rainbow trout (20.70 ± 0.19 g), were fed for 60 days across five treatment groups: control, DEO0.5, DEO1, DEO2 and DEO3. The results revealed that the DEO1 and DEO2 treatments significantly improved the growth performance, intestinal enzymes activities, and serum levels of proteins, lysozyme (LYZ), alternative complement (ACH50), total immunoglobulin (total Ig) and antioxidant defence, compared with those in the control group (p < 0.05). Notably, the levels of mucosal LYZ, total Ig, ACH50 and protease were significantly elevated in the DEO1 and DEO2 treatment groups (p < 0.05). Furthermore, the serum levels of lipids, cytosolic enzymes, glucose and cortisol were significantly lower in the DEO1, DEO2 and DEO3 treatment groups (p < 0.05). The DEO2 treatment group presented the lowest serum levels of stress indicators (cortisol and glucose) and cytosolic enzymes. Additionally, the survival in the DEO treatment groups 14 days after challenge with Aeromonas hydrophila (A. hydrophila) was significantly higher than that in the control group (p < 0.05), with the DEO2 treatment group showing the highest survivorship. Overall, this study indicates that incorporating DEO at concentrations of 1% and 2% into the diet can substantially enhance growth performance, digestive efficiency, biochemical profiles, immune-antioxidant activity, and resistance to A. hydrophila in rainbow trout juveniles.