Contrasts in osmoregulatory capacity of two Arctic charr, Salvelinus alpinus (L.), strains from northern Norway

The osmoregulatory capacities of two north Norwegian strains of Arctic charr, Salvelinus alpinus L., have been compared. Groups of fish were either held in fresh water from late-February to September, or were exposed to full-strength sea water (33–34‰) during the period (mid-May–late-June) at which anadromous charr would usually be at sea. The charr (4+ years old, 400 g initial body weight) used in the experiment were all hatchery-reared fish, either of an anadromous strain originating from the Hammerfest area (70°N), or of a landlocked strain originating from the Skjomen area (68°N). The growth of the two strains of charr was similar for fish held in fresh water. There were temporal changes in gill Na +/K +-ATPase activity and chloride cell numbers that seemed to be typical of a parr-smolt transformation, but charr of the landlocked Skjomen strain were not able to osmoregulate effectively and survive in sea water. Fish of the anadromous Hammerfest strain displayed increased gill Na +/K +-ATPase activity following transfer from fresh water to sea water in May. Charr of this strain were able to regulate plasma osmolality and plasma ion concentrations, and grew at similar rates to the fish maintained in fresh water. The results show that there are distinct differences in osmoregulatory capacity between fish of the anadromous Hammerfest strain and charr of the landlocked Skjomen strain.