Abstract
The effects of hatchery rearing density (conventional or one third of conventional density) and feeding regime (high or reduced dietary fat levels) on burst‐swim performance and oxygen transport capacity were studied in hatchery‐reared Atlantic salmon Salmo salar , using wild fish as a reference group. There was no effect of rearing density or food regime on swimming performance in parr and smolts. The maximum swimming speed of wild parr was significantly higher than that of hatchery‐reared conspecifics, while no such difference remained at the smolt stage. In smolts, relative ventricle mass was higher in wild S. salar compared with hatchery‐reared fish. Moreover, wild S. salar had lower maximum oxygen consumption following a burst‐swim challenge than hatchery fish. There were no effects of hatchery treatment on maximum oxygen consumption or relative ventricle mass . Haemoglobin and haematocrit levels, however, were lower in low‐density fish than in fish reared at conventional density. Furthermore, dorsal‐fin damage, an indicator of aggression, was similar in low‐density reared and wild fish and lower than in S. salar reared at conventional density. Together, these results suggest that reduced rearing density is more important than reduced dietary fat levels in producing an S. salar smolt suitable for supplementary release.