PHYSIOLOGICAL-RESPONSE OF AMERICAN PLAICE (HIPPOGLOSSOIDES-PLATESSOIDES) EXPOSED TO LOW-SALINITY

American plaice (Hippoglossoides platessoides) maintained under natural conditions of temperature (0-10 degrees C), salinity (26-28 g . L(-1)) and photoperiod (48 degrees N) were transferred in four consecutive seasons directly to seawater tanks at different salinities (28 (control), 21, 14, and 7 g . L(-1)). During each season, a major response to salinity was observed at 7 g . L(-1) and a minor response at 14 g . L(-1). After 42 d at the lowest salinity (7 g . L(-1)), plasma sodium concentrations had dropped by 28% below those of the controls, muscle water content had increased by 3%, and plasma potassium had decreased by 13%. In winter and summer, fish in the lowest salinity also had elevated plasma cortisol concentrations compared with controls. No mortality occurred at 7 g . L(-1) following three 96-h survival tests conducted in October, December, and February, whereas fish transferred to fresh water died within 24 h. These results suggest that American plaice is moderately tolerant to changes in salinity and that it should not be held for aquaculture purposes in estuarine conditions at a salinity below 14 g . L(-1).