Atlantic salmon smolts are more responsive to an acute handling and confinement stress than parr

Atlantic salmon pan; and smelts reared under a natural temperature and photoperiod regime were subjected to an acute handling and confinement stress in early May. Smelts had a mean plasma cortisol concentration of 10 ng/ml before stress and 242 ng/ml 3 h after initiation of stress which returned to pre-stress levels within 8 h. Parr: had a plasma cortisol concentration of 4 ng/ml prior to stress which increased to 11 ng/ml 3 h after initiation of stress and returned to pre-stress levels within 8 h. Plasma glucose was significantly higher in pan and smelts 3 h after initiation of stress; in parr, plasma glucose returned to pre-stress levels within 8 h, but not until 48 h in smelts. Plasma chloride concentration in smelts decreased from 139 to 124 mM 3 h after initiation of stress but returned to pre-stress levels within 24 h; plasma chloride in parr was not altered by stress. Plasma thyroxine of parr and smelts peaked at 3 h after initiation of stress and returned to pre-stress levels within 8 h, but smelts had 72% higher levels at 3 h. Pre-smelts (February) and smelts (May) reared under constant temperature (8-10 degrees C) were also subjected to a handling and confinement stress. Although peak levels of plasma cortisol 3 h after initiation of stress were twice as high in smelts, other physiological and endocrine responses were not substantially different between pre-smelts and smelts. The results demonstrate that Atlantic salmon smelts are more responsive to stress than pan: and that developmental differences are more important than seasonal changes. (C) 1998 Elsevier Science B.V. All rights reserved.