Effects of daylength and winter fasting on growth and smolting of one-year-old Saimaa landlocked salmon (Salmo salar m. sebago Girard) under fish farm conditions

The effects of extended autumn daylength and winter fasting on growth and smolting were studied in one-year-old Saimaa landlocked salmon. Underyearling juveniles were either exposed to extended light LD 20:4 regime (LD group) during August-October or reared under routine fish-farm illumination (ND croup). After the photoperiod manipulation, LID fish were larger and a greater number of them belonged to upper size classes in comparison with ND fish. Upper size class fish from both photoperiod groups were selected for further experiments in which the fish were either fed or fasted during winter. Contrary to the LD fish, the ND fish increased their length and weight during January-March by which they achieved the size of the LID fish. There was a clear size difference between the ND fed and fasted groups during March-April, but the fasted fish compensated for their growth Suppression soon after the feeding was started again. The activity of gill Na+,K+-ATPase was lower in the ND fasted group during January-March as compared with that of the ND fed group suggesting that smolting may be switched off by poor nutritional conditions. The different gill Na+,K+-ATPase profiles between the ND and the LD fish may indicate disturbed endogenous smolting cycle of the LD fish. However, despite these differences, all groups, independently of the treatment, completed their smolting at the same time in June as indicated by elevated gill Na+,K+-ATPase activity, enhanced salinity tolerance, and hypoosmoregulatory ability. Lower levels of plasma sodium, chloride and magnesium in freshwater observed in all Groups during spring-early summer coincident with other changes typical for smolting indicate gradually increasing maladaptation of the fish to freshwater. These results as well as an observed increase in potassium concentration in freshwater during spring-summer suggest that downstream migration is more like a passive than active process.