Long-term stress induced cortisol downregulation, growth reduction and cardiac remodeling in Atlantic salmon.

Stress and elevated plasma cortisol in salmonids have been linked with pathological remodeling of the heart and deterioration of fitness and welfare. However, these associations were based on biomarkers that fail to provide a retrospective view of stress. This study is the first whereby the association of long-term stress, using scale cortisol as chronic stress biomarker, with cardiac morphology and growth performance of wild Atlantic salmon (Salmo salar) is made. Growth, heart morphology, plasma and scale cortisol levels, and expression of genes involved in cortisol regulation of the hypothalamic-pituitary-interrenal axis of undisturbed fish (CONTROL) were compared with fish exposed daily to stress (STRESS) for 8 weeks. Though scale cortisol levels showed a time-dependent accumulation in both groups, plasma and scale cortisol levels of STRESS fish were 29.1% and 25.0% lower than CONTROL, respectively. These results correlated with the overall upregulation of stress-axis genes involved in the systemic negative feedback of cortisol, and local via 11β-hydroxysteroid dehydrogenases, glucocorticoid-, and mineralocorticoid receptors in the STRESS treatment at hypothalamus and pituitary level. These lower cortisol levels were, however, counterintuitive to the growth performance as STRESS fish grew 33.7% slower than CONTROL which likely influenced the 8.4% increase in relative ventricle mass in STRESS. Though compact myocardium area between the treatments was comparable, these parameters showed significant linear correlations with scale cortisol levels indicating the involvement of chronic stress in cardiac remodeling. These findings underscore the importance of scale cortisol as biomarker when associating chronic stress with long-term processes including cardiac remodeling.