Short-term induced swimming activity enhanced innate immune parameters and antioxidant status of European eel (Anguilla anguilla)

The swimming activity, although an essential trait in the life cycle of fish, is still poorly understood in farmed fish. The current study aimed to investigate the impact of short-term induced swimming on the immune and antioxidant defence systems in European eel (Anguilla anguilla). Sixteen male yellow European eels (total length: 39.9 ± 0.7 cm; body weight: 108.8 ± 6.1 g) were individually placed in swimming flumes and divided into two groups: i) no swimming (n = 8); and ii) induced-swimming (n = 8) at 0.3 body lengths (BL)·s⁻¹ for 7 h. Swimming resulted in a 2-fold lower cortisol concentration in plasma, whereas plasma glucose, lactate, and several immune-related parameters did not present variations between groups. Interestingly, swimming led to higher lysozyme, peroxidase, and protease activities in skin mucus, whereas bactericidal activity did not show differences among groups. Additionally, the gene expression of interleukin 1 beta showed an up-regulation in the skin of fish with induced swimming, while no differences were observed in the head-kidney or gills. Furthermore, modulation of the antioxidant status was observed in the liver and posterior skeletal muscle after induced swimming. Fish subjected to swimming showed lower lipid peroxidation and higher reduced glutathione levels, increasing the reduced/oxidized glutathione ratio. However, no variations in the antioxidant status were observed between groups in the anterior skeletal muscle. This study showed modulation of immune and oxidative stress markers in European eels upon short-term induced swimming compared to non-swimming fish.