Physiological and growth responses of juvenile Atlantic salmon (Salmo salar) transferred to seawater during different stages of smolt development

Developing a better understanding of the importance of transfer time/smolt window is critical for farmers of salmonids to maximise growth performance and minimise the poor growth/mortality that can be associated with SW transfer. This study examined physiological and growth responses of juvenile Atlantic salmon (Salmo salar) transferred to seawater (SW) after different lengths of exposure to a long-day photoperiod (LD 24:0). Four treatment groups were acclimated in experimental FW tanks for 8 weeks on a short-day photoperiod (LD 8:16), and then transferred to SW after exposure to LD 24:0 for a further 2 weeks (ETA – 196 degree days), 4 weeks (ETB – 392 degree days), 6 weeks (ETC – 588 degree days) and 8 weeks (ETD – 784 degree days). Fish from the ETA and ETB treatment groups exhibited reduced growth and condition factor after transfer to SW compared to ETC and ETD groups. Condition factor was significantly lower in fish from the ETA group after SW transfer compared to in FW, and to ETB, ETC and ETD treatment groups. ETA treatment had significantly higher pinhead numbers (CF < 0.8) compared to ETC in SW. Mortality was significantly related to treatment group, with higher mortality in ETA treatment (13%), followed by ETB (5.2%); ETC (1.4%) and ETD (0%). Pinheads were significantly varied in physiological profile to non-pinhead population and showed minimal growth over the 16-week experiment. Overall, pinheads were significantly lower in mass, fork length, condition factor and cholesterol levels than non-pinheads. Gill Na+/K+-ATPase activity was not significantly different between pinheads and non-pinheads in any treatment group. Osmolality was highly variable in ETA pinheads, but mean levels were not significantly different between pinheads and non-pinheads. This study demonstrated that at 14 °C, six weeks of long-day photoperiod is necessary for juvenile Atlantic salmon (~120 g) to minimise negative effects on growth, mortality and the occurrence of pinheading in the transferred population.