Transepithelial ion exchange in smolting atlantic salmon (Salmo Salar L.)

Whole-body (but predominantly gill) Na(+) exchange, gill Na(+)/K(+)/ATPase activity and seawater tolerance were examined in juvenile Atlantic salmon during the smolting period. Transepithelial net Na(+) gain decreased steadily from late February showing a net loss in April and early May, returning to approximate equilibrium in mid-May. This seasonal net loss of Na(+) to the environment occurred slightly after maximal gill epithelial Na(+)/K(+)/ATPase activity and preceded maximal seawater tolerance. The results are discussed in relation to changes in gill permeability and salt intake via the diet.