The importance of exercise: Increased water velocity improves growth of Atlantic salmon in closed cages

There is increasing concern about Norwegian salmon farming and the possible environmental impacts from sea lice, escaped fish and release of toxic chemicals and organic emissions to the coastal waters. Closed containment systems (CCS) have the potential to eliminate the problems with sea lice and to reduce escapes and emissions. When closing the cages, water volumes and velocity are regulated and the identification of optimal current velocities for growth and fish welfare from sea transfer to harvest size becomes necessary. This study describes two trials with LOW (0.10-0.27 BL/s) and MODERATE (0.36-0.63 BL/s) water velocity on performance of post-smolt Atlantic salmon in CCS. In trial 1 (168 days, 10.9 degrees C, fish size: 884-3007 g and 41.5-59.0 cm), round weight increased with 219 g (p = .012) and condition factor with 0.11 (p = .016) in the MODERATE group compared with LOW group. The MODERATE group obtained specific growth rate (SGR) of 0.76 and thermal growth coefficient (TGC) of 2.75, compared to 0.72 and 2.56 in the LOW group. MODERATE water velocity was also associated with higher relative heart size (RHS) (p = .016), higher liver index (HSI) (p = .005), increased fillet yield (p <= .001) and lower levels of cathepsin activity in muscle tissue. In trial 2 (46 days, 7.1 degrees C, fish size: 327-482 g and 29.9-33.7 cm), round weight increased with 52 g (p = .019) and condition factor with 0.05 (p = .009) in the MODERATE group compared with LOW group. The MODERATE group obtained SGR of 0.77 and TGC of 2.68, compared to SGR of 0.60 and TGC of 2.02 in the LOW group. No significant difference was observed in white muscle cell hyperplasia, measured as the proportion of small (< 20 mu m diameter) muscle fibres (p = .145). Both trials showed only minor differences in slaughter yield, fillet quality (protein, fat, water) and mortality. The present study shows that moderate water velocity (0.36-0.63 BL/s) is favourable for growth rates for Atlantic salmon during the entire on-growing period in CCS. Effects on a broader range of metabolic variables and welfare indicators were also documented.