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.