Plasma ionic regulation and gill Na+/K+‐ATPase changes during rapid transfer to sea water of yearling rainbow trout, Salmo gairdneri: time course and seasonal variation

Danish rainbow trout, Salmo gairdneri Richardson, (40–65 g) were transferred to 28%o sea water at intervals throughout the early spring and summer. Gill Na+/K+‐ATPase of fish kept in fresh water surged distinctly during May. Simultaneously, a body silvering occurred and plasma concentrations of Cl−, Na+ and total thyroxine (T4) decreased. The seawater transfer‐induced adaptive response in plasma electrolytes comprised a biphasic change, i.e., an adjustive peak phase and a regulatory phase lasting for 2 days and 1 week after transfer, respectively. Further, gill Na+/K+‐ATPase activity increased to a new level after an initial lag phase of 2–3 days, but electrolyte regulation was mostly initiated prior to the adaptive change in ATPase activity. In spite of increasing pre‐transfer freshwater Na+/K+‐ATPase activity during the spring, the electrolyte peak level, the degree of muscle dehydration and the mortality of fish transferred to sea water increased from April to July. The apparent uncoupling of freshwater Na+/K+‐ATPase activity and plasma electrolyte regulation in sea water is discussed in relation to smelting and prediction of hypo‐osmoregulatory performance.