Performance of Atlantic salmon reared under three different regimes of continuous aerobic exercise during the freshwater phase

Freshwater rearing conditions influence the growth and seawater adaptation of Atlantic salmon, and swimming exercise may enhance these adaptive processes. This study examined growth and physiological responses of Atlantic salmon subjected to continuous swimming at three speeds: low (0.5 body length per second, BL/s) and moderate (1.0 and 1.5 BL/s) for 11 weeks in freshwater, followed by transfer to brackish water. Fish trained at 1.0 and 1.5 BL/s demonstrated significantly higher specific growth rates and plasma insulin-like growth factor 1 levels after 5 weeks in brackish water, suggesting that moderate exercise enhances growth. Additionally, the 1.5 BL/s group showed a higher frequency of small-diameter white muscle fibers, suggesting hyperplastic growth. Although the expression of growth-related genes was not affected by swimming speed, moderate exercise groups had significantly lower plasma triglycerides and cholesterol levels, suggesting a shift of energy allocation towards growth. At the end of the freshwater phase, distinct energy allocation strategies were evident: the low-speed swimming group had higher hepatosomatic index and plasma inorganic phosphate levels, whereas the 1.5 BL/s group showed higher muscle adenylate energy charge, indicating enhanced muscle energy status. Fish in moderate swimming groups also had lower cortisol, creatinine (significantly different between 0.5 and 1.5 BL/s), and lactate levels (significantly different between 0.5 and 1.0 BL/s), suggesting an improved stress profile. Swimming exercise did not affect smoltification markers, including NKA activity or plasma sodium and chloride concentration. Overall, moderate swimming (1.0–1.5 BL/s) improved growth in Atlantic salmon, highlighting potential applications for aquaculture. • Fish swimming at 1.0 and 1.5 BL/s showed higher growth and plasma Igf1 levels. • The 1.5 BL/s group showed more small-diameter white muscle fibers, indicating hyperplasia. • Moderate swimming (1.0–1.5 BL/s) improved the stress profile of Atlantic salmon. • Distinct energy allocation strategies were observed between 0.5 and 1.5 BL/s groups. • Swimming exercise did not affect smoltification markers.