Body Fluid Regulation in Smolting Atlantic Salmon

Abstract

Parr–smolt metamorphosis in yearling Atlantic salmon was accompanied by variations in plasma and tissue electrolyte levels, and in tissue water distribution. Plasma chloride declined sharply with the onset of smoltification, but recovered to values somewhat higher than those seen in parr with the attainment of the silvery parr stage. Some decrease, with no evidence of a secondary increase, was observed in tissue chloride concentrations. Changes in chloride space values suggested that parr–smolt metamorphosis is also characterized by a shift of tissue fluids from the extracellular to the cellular phase. These variations in water–electrolyte balance are thought to reflect a premigratory activation and secondary suppression of the extrarenal salt excretion systems employed for osmotic and ionic regulation in sea water. Their occurrence in fish estimated upon the basis of morphological characters to be large parr suggests that the metamorphic process may occur earlier in the life cycle of this species than is generally recognized.