Dietary supplementation with prebiotics, probiotics and synbiotics enhances innate immunity, antioxidant capacity, antimicrobial peptide expression and disease resistance in olive flounder (Paralichthys olivaceus) fed a low-fish meal diet.

This study evaluated the influence of dietary supplementation with prebiotics, probiotics and synbiotics on growth performance, immune response, antioxidant capacity, antimicrobial peptides (AMPs) expression and disease resistance in juvenile olive flounder (Paralichthys olivaceus) fed a low-fish meal (LFM, 45% fish meal) diet. Experimental diets were formulated by supplementing an LFM diet (Con) with 0.6% mannan oligosaccharides (Mos), Lactiplantibacillus plantarum (LP), Bacillus subtilis (BS), B. licheniformis (BL) and their combination (Syn; 0.15% Mos, 0.15% LP, 0.15% BS and 0.15% BL). Fish (initial body weight: 65.9 ± 0.09g) were randomly distributed into triplicate groups and fed the experimental diets for 11 weeks. After the feeding trial, growth performance, feed utilization efficiency, survival, condition factor and somatic indices showed no significant variation among dietary groups. Similarly, immune parameters and hematological indices remained comparable across all treatments. Aspartate aminotransferase and alanine aminotransferase levels were lower, whereas antioxidant capacity was higher in all supplemented groups. After the Edwardsiella piscicida challenge, cumulative survival, immune parameters and antioxidant capacity were significantly higher in probiotics and Syn groups than in Con group. The expression of AMPs (hepcidin, β-defensin, LEAP-2, NK-lysin and pleurocidin) was significantly upregulated in fish fed LP, BS, BL and Syn diets, whereas prebiotic supplementation showed a minimal effect on AMPs transcription. Therefore, these results demonstrate that dietary probiotics and synbiotics effectively enhance immunity, antioxidant defense and disease resistance of P. olivaceus when fed LFM diets without compromising growth performance. These findings support the use of such additives as sustainable alternatives to chemotherapeutic agents in intensive P. olivaceus aquaculture.