<p>The aim of the study was to investigate the effect of different tank designs and water exchange rates on growth, stress, skin and gill health in juvenile lumpfish in order to validate if different rearing regimes could increase welfare and robustness of farmed juvenile lumpfish. Experimental trials were conducted to investigate how lumpfish growth, condition factor, skin and gill mucus activity, cortisol and serotonin activity was affected by rearing in two systems (tank (T), raceway (R)) and two water exchange rates (high (T-H): 1350 L h⁻¹; (R-H): 780 L h⁻¹ and low (T-L): 450 L h⁻¹; (R-L): 260 L h⁻¹) during a 66&#xa0;days laboratory trial. Furthermore, potential carryover effects on performance at sea were investigated by studying growth, condition and welfare of the same fish in small scale sea pens A combination of tanks and low water exchange rates (T-L) produced significantly smaller lumpfish, with significantly lower specific growth rate, and lower condition factor. Fish reared in raceways had significantly higher baseline plasma cortisol levels and elevated values of brainstem serotonergic activity compared to fish reared in tanks. There was an indication of lower defence activity in skin mucus cells in raceways at high water exchange, suggesting that raceways at high water exchange may have more favourable environmental conditions. The rearing conditions in the different tank designs at the hatchery stage did not seem to affect fish welfare during the subsequent stage in sea cages. Based on the significantly smaller size, slower growth rate and a condition factor below the threshold which represents acceptable fish welfare in lumpfish, rearing of juvenile lumpfish in tanks at the low water exchange rate (150% exchange of tank water per hour) is considered sub-optimal compared to exchange rates of 450% in tanks and 200% or 600% in raceways.</p>