Consequences of long-term circular swimming on growth, muscle physiology, and lateral symmetry in Chinook salmon

Research involving exercise to optimise salmonid production performance has mostly focused on short exercise-training periods (< five months) with small size classes (< 500 g). There is urgency to understand the influence of sustained swimming on larger individuals and how to adequately scale exercise regimes for the larger smolt sizes that are now being transferred to sea. There is also a considerable knowledge gap in our understanding of using circular tanks to exercise salmonids and the impact that continuous circular swimming may have on muscle symmetry and physiology. This study aims to determine if exercise regimes optimised for small post-smolt salmonids can enhance production performance in larger post-smolts, while identifying possible muscle asymmetry resulting from exercise-training salmon in circular tanks. Post-smolt Chinook salmon were reared under low (LFR) or moderate (MFR) flow regimes for six months, to a larger size (1056 ± 11 g and 374 ± 1 mm) than typically studied in research. Several aspects of whole-animal performance were measured and matched with physical and biochemical responses on lower biological levels of organisation (i.e., tissue, cellular and molecular). Physical and biochemical responses of the skeletal muscle were further assessed with respect to the left and right fillets. Production performance (individual growth, tank level feed intake, growth and efficiency) was significantly better in Chinook salmon reared under LFR compared to MFR. Chemical composition (lipid and protein content) and muscle morphometrics did not differ between the flow regimes, even though genes involved in lipid metabolism and muscle repair and maintenance were upregulated in Chinook salmon reared under MFR. Lateral investigations of white muscle showed the right fillet (outward facing fillet) to have higher lipid content in the lateral and visceral regions, lower protein content across the entire fillet, and denser white muscle fibres. Genes involved in lipid metabolism and muscle development and contraction were also upregulate in the white muscle of the right fillet. This study shows that optimal exercise regimes for small post-smolt salmon are not necessarily suited to larger post-smolt salmon, and that exercise regimes for larger post-smolt salmonids may need to be optimised separately. Additionally, the results from this study suggest that continuous circular swimming may stimulate muscle blocks unevenly leading to muscle asymmetry and reveals a new set of questions associated with exercising salmonids in circular tanks. Further exploration is warranted to determine the relevance for commercial settings when larger rearing tanks are used. • Optimal flow regimes for smaller salmon did not scale with larger salmon. • Immune regulating genes play a role in muscle growth. • Continuous circular swimming unevenly activates white muscle blocks. • Continuous circular swimming leads to lateral asymmetry of the white muscle.