No abstract

Groups of Atlantic salmon (Salmo salar) yearling smolts were reared in duplicate tanks supplied with freshwater or seawater, and subjected to di¡erent feeding frequencies, 100% (fed every day), 50% (fed every other day), 25% (fed every forth day) and 0% (starved), from 26 May to 26 July. After 8 weeks, all the groups were re-fed in excess for 6 weeks. Fish were maintained on their respective a priori salinity treatments during the 6-week follow-up period. Starvation for a period of 8 weeks in freshwater resulted in a loss of hypo-osmoregulatory ability when smolts were challenged with seawater and unfed smolts maintained in freshwater were unable to adapt to seawater in mid-July. Ration levels in£uenced the growth rate and body size signi¢cantly. The overall growth rate was higher in freshwater than at corresponding rations in seawater. Partial compensatory growth was observed in the 0 and 25% groups following re-feeding. Branchial Na 1 ,K 1 -ATPase (NKA) activity decreased rapidly in unfed smolts in freshwater and was the lowest in the starved group, whereas an initial increase was observed in those groups reared in seawater. After re-feeding NKA activity di¡erences decreased between the former feeding groups. Our results suggest that nutritional factors and/or energy levels are critical for the maintenance of hydro-mineral balance of salmon smolts.