Carnosic acid-riched rosemary (Rosmarinus officinalis L.) extract improves largemouth bass (Micropterus salmoides)’s disease-resistance and regulates lipid metabolism

Rosemary extract (RE) had strong antioxidant activity which was used as an ideal substitute for antibiotic. However, few studies were explored its effects on fish species. Therefore, present study was conducted to determine the effects of RE on disease-resistance and nutrient metabolism in largemouth bass. Fish with initial body weight 9.07 g ± 0.01 g were fed with diets supplemented with gradient RE levels (0 %, 0.3 %, 0.6 % and 0.9 %) for 8 weeks. Following 8 weeks trial, Edwardsiella tarda challenge test were conducted to explore the ability of disease-resistance. Data on Edwardsiella tarda challenge test showed that all experimental groups exhibited higher survival rate than control group. RE supplementation significantly decreased the Ca2+ concentration and apoptosis rate and increased the mitochondrial membrane potential in blood cells. Meanwhile, the plasma biochemical indexes of antioxidant, immune response and key regulators in antioxidant, immune response presented an increasing pattern after RE supplementation. The growth trial results showed that visceral adipose index (VAI) and abdominal fat weight (AFW) showed a decreasing trend with the increase of RE addition levels, which further decreased growth performance. Further analyzed the lipid metabolism, we found that dietary RE addition markedly inhibited growth hormone-insulin like growth factor (GH-IGF), target of rapamycin (TOR) signalings and activated amino acid response (AAR) signaling to enhance lipid catabolism. Beyond that, the increased antioxidant capacity in turn reduces muscle cooking loss, hardness and increased muscle cohesiveness, springiness, chewiness. In conclusion, based on immunity, antioxidant capacity, disease-resistance and muscle quality, dietary 0.3–0.6 % RE addition was suggested in aquafeeds. •RE could activate lipid catabolism which was a stimulant to solve the problem of abnormal lipid deposition.•RE was an ideal substitute for antibiotic, for its strong antioxidant activity.•The increased antioxidant capacity in turn regulated muscle quality.•Dietary 0.3–0.6 % rosemary extract addition was suggested in aquafeeds.