A study of the effects of temperature and salinity on ionoregulation in the alewife, Alosa pseudoharengus, revealed that concentrations of sodium, potassium, and calcium in plasma and muscle were similar in fish adapted to fresh water and those adapted to sea water. The non-stressed alewife is apparently an excellent ionoregulator in both environments.
Acute exposure to cold caused a shift in plasma concentrations of sodium and calcium toward environmental concentrations, i.e., these ions decreased in fresh water and increased in sea water. An ionoregulatory failure due to cold is suggested. High temperatures had little effect on plasma electrolyte levels in fresh water whereas in sea water the concentrations of sodium, potassium, and calcium were elevated. Mortalities of the alewife in the Great Lakes may be caused by osmoregulatory failure induced by acute exposure to cold, but are probably not induced by exposure to elevated temperatures. Fish thermally acclimated in the laboratory or in Lake Michigan had essentially normal concentrations of blood electrolytes.
In muscle, sodium was reduced during exposure to acute cold in both fresh water and sea water. The effect of warm water on ion levels in muscle was, in general opposite that of cold. These data suggest shifts of ions or water between cellular and extracellular fluids of muscle. Such shifts might affect the function of excitable tissue. Muscle tremors and loss of equilibrium observed in dying alewives during summer mortalities might be brought about when sudden temperature drops (e.g., as a result of upwellings which are common occurrences in the Great Lakes) induce disturbances in cellular or extracellular concentrations of ions.
In the laboratory, alewife mortalities caused by increasing or decreasing temperatures were about equal in fresh water and sea water. These findings suggest that salinity does not modify the capacity of alewives to tolerate acute temperature stress.