Physiological performance and cardiac morphology of Atlantic salmon reared under slow and fast growth conditions

Early rearing environment affects performance later in life. In Atlantic salmon ( Salmo salar ) aquaculture intensive smolt production has been linked to deviating cardiac morphology and increased mortality risks following stressful events during the marine production phase. To investigate the effects of early growth environment on later life-stages, two smolt groups were produced; a fast-growing group reared at 13 °C under continuous light and a slow-growing group reared at 6 °C under a natural photoperiod. The two groups were smoltified and transferred to 9 °C seawater at the same time and at similar sizes, although the slow smolts were ≈ 1000 day degrees older. Respirometry and swim tunnel experiments were performed to assess physiological performances along with morphological analyses of the hearts. We hypothesized that the slower growth trajectory would allow for the development of heart morphology more resembling that of wild salmon and that this should translate into improved physiological performance. Fast-growing smolt had more misaligned and enlarged bulbi as well as asymmetric ventricles compared to slow-growing smolt. However, contrary to our hypothesis, we did not find clear evidence for impaired physiological performance in fast-growing fish. That is, neither standard nor maximum metabolic rates, absolute critical swimming speed, stress recovery, or haematological parameters at fatigue differed between treatments. Mortality risks associated with deviating cardiac morphology first tend to occur in larger sized fish than investigated here. We therefore conclude that while early rearing environment clearly modulates cardiac morphology, recently seawater adapted Atlantic salmon do not yet show signs of compromised functionality associated with cardiac morphological differences at the whole-animal level. Future research should aim to incorporate larger sized fish in physiological experiments for a more appropriate representation of the latter production phase in Atlantic salmon aquaculture and its associated fish welfare problems. • Slow and fast growing salmon were made using different temperature and light regimes. • Rearing environment affected heart morphology in recently seawater adapted salmon. • Swimming capacity, metabolic rate, stress recovery, and blood values were unaffected. • Lack of physiological differences is likely due to the life-stage and sizes assessed. • Studies need to include larger fish to represent issues in latter production phases.