If there is no salt water, then Arctic char wouldn't need physiological adaptations to survive their summer migrations out into the salt water. It's called osmoregulation. The Arctic char like other anadromous salmonids drink sea water and excrete the chloide ions via active transportation across the epithelium of their gills. The cells in the gill epithelium are rich in mitochondria, because this process requires energy to pump the chloride out against a concentration gradient. They would die if they couldn't do this. That or remain in the rivers and lakes year round, which we know they do not.
And as for the salinity of the Arctic sea water, it's not as salty as other areas of the worlds oceans, but it's far from fresh water. 30 ppt is pretty damn salty.