Swimming activity is a fundamental trait in the life cycle of most fish species. Adequate exercise has been shown to promote growth through muscle development, improve feed efficiency and reduce stress in some species. Although some studies have paved the way for a possible link between swimming activity and improved immunity and antioxidant status, the effects of induced swimming appear to vary greatly between species. In this study, several immune and oxidative indicators were analyzed in plasma, skin mucus, gills and skeletal (red and white) muscle of European seabass (Dicentrarchus labrax) after induction of different swimming conditions. Thirty-two specimens (total length: 11.5 ± 0.1 cm; body weight: 15.5 ± 0.6 g) were induced to swim for 6 h in individual swimming flumes at low (L, 0.8 BL (body-lengths) s-1); high (H, 2.2 BL s-1) and oscillating (O, 0.8–2.2 BL s-1) velocities. A non-exercised group was used as a control group (C). The swimming velocity applied for fish in the H group was near 70% of the critical swimming speed described for European seabass, close to the optimal swimming speed defined for this species.
The fish swimming in the H condition had a higher white blood cells (WBC) count and increased plasma cortisol concentrations compared to the C and L groups. No differences were found in the innate immune parameters in plasma and skin mucus among the experimental groups. Gene expression analyses revealed an up-regulation of inflammatory cytokines (tumor necrosis factor alpha, tnfα and interleukin 1 beta, il1β) in gills from H group. In addition, the activities of antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT) were increased in the red muscle of fish swimming under the H condition, as were the lipid peroxidation (LPO) levels. Nevertheless, fish swimming in the L group showed a higher ratio of reduced/oxidized glutathione (GSH/GSSG) in the red muscle. Interestingly, the GSH/GSSG ratio was increased in white muscle under all swimming conditions compared to the control. These results suggest a possible inflammatory response in the gills of fish swimming in the H group, as inflammatory cytokines are up-regulated, and plasma cortisol and WBC concentrations are increased. The higher activities of SOD and CAT as well as LPO levels also indicate a susceptibility to oxidative stress in the red muscles of fish in the H group. Nevertheless, the increased GSH/GSSG ratio in white muscle under all swimming conditions indicates an improvement in antioxidant status associated with swimming activity in this tissue. These results emphasize the need for appropriate swimming conditions for farmed European seabass, one of the most commonly farmed species in Southern Europe, which could prove to be an innovative, non-invasive method to promote fish welfare.
This research was supported by national funds through FCT – Foundation for Science and Technology (Portugal) within the scope of UIDB/04423/2020 and UIDP/04423/2020. C.E-S was supported by the Blue Young Talent PhD program (BYT-PhD) grant (UI/BD/150911/2021) funded by FCT.