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 phot operiod regime were subjected to an acute handling and confinement stress i n early May. Smelts had a mean plasma cortisol concentration of 10 ng/ml be fore 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 si gnificantly higher in pan and smelts 3 h after initiation of stress; in par r, plasma glucose returned to pre-stress levels within 8 h, but not until 4 8 h in smelts. Plasma chloride concentration in smelts decreased from 139 t o 124 mM 3 h after initiation of stress but returned to pre-stress levels w ithin 24 h; plasma chloride in parr was not altered by stress. Plasma thyro xine of parr and smelts peaked at 3 h after initiation of stress and return ed 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 demo nstrate that Atlantic salmon smelts are more responsive to stress than pan: and that developmental differences are more important than seasonal change s. (C) 1998 Elsevier Science B.V. All rights reserved.