Condensed tannins from black wattle as a promising nutritional additive for Nile tilapia: Growth, immune and antioxidant responses, and gut morphology

The present study aimed to investigate the potential of condensed tannins (CT), derived from black wattle (Acacia mearnsii) tree bark, as a nutritional additive for Nile tilapia (Oreochromis niloticus) juveniles. CT are known for their antioxidant and immune-modulating properties, and their inclusion in aquafeeds may enhance fish health and performance. In this study, six diets were formulated: one control diet (Con) with no additives, and five experimental diets supplemented with CT extract at concentrations of 150, 250, 500, 750, and 1000 mg/kg (CT150, CT250, CT500, CT750, CT1000, respectively). The selection of these doses was based on previous studies indicating that lower concentrations (below 1000 mg/kg of diet) of tannins may offer benefits, while higher concentrations could exhibit antinutritional effects. After a 90-day feeding trial, fish fed the CT150 diet exhibited significantly higher growth compared to the control group. Additionally, fish in the CT150 group showed higher plasma lysozyme activity, while myeloperoxidase activity and hemolytic activity of the complement system were significantly higher in all tannin-fed groups compared to the control. Notably, CT150-fed fish demonstrated liver antioxidant responses comparable to or better than the control group, with no significant increase in lipid peroxidation, suggesting antioxidant protection. Histological analysis revealed a significant increase in intestinal villi density in the CT150 and CT500 groups compared to the control. While all groups showed an increase in absorption surface area (ASA), this increase was statistically significant only in the CT250, CT500, and CT750 groups when compared to the control. Furthermore, the CT150 diet led to the highest survival rate (80 %) following a bacterial challenge. These findings suggest that dietary supplementation with 150 mg/kg of CT from black wattle bark provides the most beneficial effects on growth, immune response, and survival in Nile tilapia juveniles.