Loss of seawater preference in chum salmon (Oncorhynchus keta) fry retained in fresh water after migration season

Abstract Salinity preference of chum salmon fry (Oncorhynchus keta) was measured by the use of a preference tank. In April, when they normally migrate down to the sea, freshwater chum fry weighing about 0.6 g preferred seawater diluted 1:2 with fresh water (11 parts per thousand in salinity) and seawater over fresh water. Chum fry acclimated to the diluted seawater for 1 day preferred seawater over fresh water. In May, however, freshwater fry preferred only diluted seawater. Furthermore, the preference for seawater of freshwater fry, and of fry that had become acclimated to diluted seawater, decreased significantly. Plasma levels of sodium ions in freshwater fish were 138–142 mmol/L in April and May. When the fish were transfered to diluted seawater in April and May, the levels of sodium ions increased gradually over 6 hours to the level in seawater‐acclimated fish, namely 153–155 mmol/L. Plasma levels of sodium ions in fry acclimated to diluted seawater did not change after exposure to seawater. In freshwater fry that were exposed to seawater, the maximum concentration in plasma of sodium ions was 165 mmol/L in April and 177 mmol/L in May. The preference of fry acclimated to seawater or to diluted seawater was also examined in April and May. More than 70% of fry that had acclimated to seawater showed a preference for diluted seawater and no preference for fresh water was noted in either the fry acclimated to seawater or to diluted seawater. Thus, for chum fry, the preference for seawater, as well as their adaptability to seawater, is maximal in April, during the migration period. No preference for fresh water was observed at any stage.