Physiological effects of water flow induced swimming exercise in seabream Sparus aurata
A longer on-land rearing period of Gilthead seabream Sparus aurata before transfer to sea-cages would allow the farmer to benefit from exercise-enhanced growth and resilience as induced by increasing water flow in the tanks. In this study, the physiological effects of flow-conditioning were investigated by subjecting large groups of experimental fish to minimal flow or to flow regimes inducing swimming exercise at 1 or 2 Body Length (BL) s-1 for a period of 8 months (Feb.-Oct.) in 1,500 l RAS tanks. Fish size after eight months of flow conditioning was 92 ± 27 g body weight (BW) for fish under minimal flow; 106 ± 24 g BW (+15%) at 1 BL s-1, and 125 ± 27 g BW (+36%) at 2 BL s-1. Although flow enhanced growth linearly with swimming speed, the number of malformed fish was significantly higher for swimming at 2 BL s-1 indicating that the mechanical load imposed by exercise was too high for a developing juvenile. Fish representing the three treatment groups were then used for: (1) a stress challenge netting test and plasma cortisol measurement (baseline, peaking and recovery levels); (2) blood plasma measurements of glucose, triglycerides, lactate, cholesterol, growth hormone (GH) and insulin-like growth factor I (IGF-I), and heart and muscle gene expression of the GH and IGF-I receptors and the muscle transcriptome by deep RNA sequencing (RNAseq). Flow conditioning at 1 BL s-1 provided optimal conditions for growth and stress coping without energetic depletion and morphological deformation. The absence of important differences in plasma GH and IGF-I, and expression levels of the receptors in heart and white skeletal muscle, indicated that other factors may be involved in growth enhancement. RNAseq of the white skeletal muscle showed that transcription regulators play an important role. Also expression of immune genes is strongly up-regulated. Whereas muscle of fish conditioned at 1 BL s-1 shows up-regulated expression of structural genes such as troponins and myosins, muscle of fish conditioned at 2 BL s-1 shows up-regulated expression of skeletal genes instead which may reflect the mechanism behind morphological deformations. Overall design: Deep-sequencing transcriptome analysis of muscle samples derived from sea bream (sparus aurata) specimens. The control group (group 1, n=3) was exposed to 8-months minimal water flow, group 2 (n=3) was exposed to an 8-months swimming exercise at 1 body length per second and group 3 (n=3) was exposed to an 8-months swimming exercise at two body lengths per second.