Effects of short-day treatment on long-term growth performance and maturation of farmed Arctic charr Salvelinus alpinus reared in brackish water

The effects of a 6 week short-day photoperiod followed by continuous light, applied during the juvenile phase of Arctic charr Salvelinus alpinus in fresh water on smoltification and on the long-term growth and maturity following transfer to brackish water (BW) (constant salinity of either 17 and 27 or increasing salinity in steps from 17 to 27) were investigated. Prior to salinity transfer, the juveniles were either reared at continuous light (C group) or reared for 6 weeks on a short day (8L:16D, S group) followed by continuous light (24L:0D). Increased salinity had negative effect on growth, with female fish reared at 17 salinity weighing 19 and 27% more than the salinity-step group (17-27) and the 27 salinity group, respectively. The stepwise acclimation to salinity had limited advantage in terms of growth rate. Short photoperiod for 6 weeks (November to January) followed by continuous light improved growth, but not seawater (SW) tolerance. Gill Na(+) , K(+) -ATPase activity and plasma Na(+) levels changed with time, indicating some variation in osmoregulatory capacity during the experimental period. Overall, there appear to be interactive effects on maturation from applying short-day photoperiod followed by rearing at higher salinities. Plasma leptin varied with time and may be linked to stress caused by the observed variations in osmoregulatory ability. It is concluded that changes in growth rates observed in this study are mainly related to rearing salinity with higher growth rates at lower salinities. Short-day photoperiod has some growth-inducing effects but did not improve SW tolerance. Farmers of S. alpinus using BW for land-based rearing should keep salinity at moderate and stable levels according to these results to obtain best growth.