Effects of temperature and salinity on osmoregulation and growth of Atlantic salmon (Salmo salar L.) smolts in seawater

One of the main developmental events of the smoltification process of Atlantic salmon (Salmo salar L.) is the pre-adaptation to an increase in salinity. Seawater acclimation involves a series of physiological changes which are critical for subsequent performance. The aim of this study was to monitor some important physiological mechanisms involved in seawater adaptation under different salinity (28 and 34 parts per thousand) and temperature (4 and 8 degrees C) regimes. An increase in plasma chloride levels and a decrease in muscle water was observed in all groups after 24 h of seawater exposure. Salinity did not affect plasma chloride levels nor tissue moisture, and no interactions between temperature and salinity were found. Temperature affected plasma chloride levels significantly after 12 h of seawater exposure, with the 4 degrees C groups having lower levels than the 8 degrees C groups. Between days 1 and 14, muscle water was observed to increase and then stabilise in the 8 degrees C groups, while the low temperature groups required a further 14 days until tissue moisture was at levels similar to the freshwater group. After an initial reduction, both groups at 8 degrees C showed elevated and stable gill Na+,K+-ATPase activities compared with the low temperature groups, which showed a long-term decrease. Salinity did not affect gill Na+,K+-ATPase activity and no interactions between temperature and salinity were found. During the first 2 months of seawater exposure, the growth pattern was affected by temperature only, while higher growth rate in brackish water at low temperature (4 degrees C) in the period between days 64 and 90 indicates that a reduction in salinity may improve long-term growth in the sea. (C) 1998 Elsevier Science B.V. All rights reserved.