Induced swimming reduced stress and modulated immune response and antioxidant status in juvenile rainbow trout (Oncorhynchus mykiss)

Promoting swimming in aquaculture has been proposed to enhance fish welfare. This study investigated the effects of 6 h of swimming on stress, immune, metabolic, and antioxidant status of juvenile rainbow trout (Oncorhynchus mykiss). Fish (n = 8) were individually subjected to one of four experimental conditions: minimal water flow (control, < 0.1 body-lengths (BL)·s⁻¹); low-speed (L, 0.8 BL·s⁻¹); high-speed (H, 2.3 BL·s⁻¹); and oscillating speeds (O, 0.8/2.3 BL·s⁻¹). Plasma cortisol levels were lower in swimming groups (L, H, O) than that in the control group, indicating reduced stress. Lysozyme and peroxidase activities increased in skin mucus of fish from L and H groups, reflecting enhanced mucosal immunity. Expression of cytokines (tnfα, il1β), inflammation modulators (nfκb1), and immune-related (lyz, ctsd) genes in head-kidney, gills, and heart did not vary among groups. In red muscle, increased citrate synthase activity across swimming conditions reflected enhanced aerobic metabolism, while cytochrome c oxidase activity was higher in fish from H and O groups, indicating elevated mitochondrial oxidative capacity. Lactate dehydrogenase activity was higher in red muscle in the O group, reflecting greater reliance on anaerobic metabolism under oscillating swimming. Increased GSH/GSSG ratio in red and white muscle of L and H groups indicated improved antioxidant status. Nonetheless, higher lipid peroxidation levels in the liver suggest increased oxidative stress in the O group, indicating that oscillating swimming may impose metabolic and oxidative challenges. Results show that short-term induced steady swimming under L and H conditions could be used beforehand when applying procedures that decrease welfare in farmed rainbow trout, such as handling or transport, aiming to reduce stress, improving immune and antioxidant responses.