Can Atlantic salmon smolt twice? Endocrine and biochemical changes during smolting

Smolting is characterized by morphological and physiological changes, some of which are reversible if fish remain in freshwater. Whether fish that smo lt in the first year will repeat physiological changes associated with smol ting a second time is not known. To assess whether Atlantic salmon (Salmo s alar) can smolt more than once, we sampled hatchery-reared Atlantic salmon at monthly intervals for 2 years beginning the fall after hatching. Fish sh owed differences in rate of growth and were easily differentiated by size i nto upper mode (UM) and lower mode (LM) by the first fall. In the first spr ing, gill Na+,K+-ATPase activity of UM and LM fish increased six- and three -fold, respectively. Plasma growth hormone levels in spring were significan tly elevated in UM fish but not in LM. Plasma cortisol levels changed littl e and gill corticosteroid receptor concentration did not differ between the groups. During the summer, gill Na+,K+-ATPase activity declined in both gr oups and remained low until the next spring. The second spring, growth horm one levels did not increase significantly and cortisol levels increased. Th e increase in gill Na+,K+-ATPase activity and cortisol and high saltwater t olerance indicate that UM Atlantic salmon can smolt in two consecutive year s.