The Inclusion of Supercritical Fluid Extract of Ulva lactuca in the Diet of Common Carp (Cyprinus carpio) Enhances Physiological Resilience and Antioxidant Status During Therapeutic-Dose Antibiotic Administration.

The widespread use of antibiotics in aquaculture has raised concerns regarding adverse physiological effects on fish, creating a need for dietary strategies that enhance fish resilience during antibiotic administration and support animal welfare under therapeutic treatment conditions. This study evaluated, for the first time, a phytochemically characterized supercritical fluid extract of Ulva lactuca (Ul-SFE), rich in flavan-3-ols (epigallocatechin, epicatechin gallate) and other phenolics, as a dietary supplement in common carp (Cyprinus carpio). Juvenile carp (n = 360; 20.20 ± 0.13 g) received a control or Ul-SFE-supplemented diet (50 mg kg-1) for 10 weeks, followed by 10 days of florfenicol or oxytetracycline exposure at therapeutic doses. Growth, plasma biochemistry, oxidative stress, and innate immunity were assessed. Ul-SFE significantly improved FCR, SGR, and PER during growth. Under antibiotic challenge, both florfenicol and oxytetracycline affected growth performance and biochemical status, though with different predominant patterns: florfenicol exposure was mainly characterized by reduced growth performance, while oxytetracycline exposure was mainly characterized by biochemical disturbance-reduced total protein and globulin, elevated glucose, ALT, and lipid peroxidation, and impaired antioxidant/immune status. Ul-SFE consistently attenuated these effects, reducing malondialdehyde by 24-30% and increasing antioxidant capacity by 5-7%; its protective effect on lysozyme was strongly diet-dependent (Diet × Antibiotic, p < 0.001), ranging from a 20% increase without antibiotics to negligible under oxytetracycline. These findings show Ul-SFE improves feed utilization and confers partial, treatment-dependent resilience against antibiotic-induced stress, supporting its potential as a functional feed additive in aquaculture.