Seasonal differences in plasma cortisol and gill corticosteroid receptors in upper and lower mode juvenile Atlantic salmon

Ž . Circulating plasma cortisol and gill corticosteroid receptors CR have been observed to change seasonally in conjunction with smolting in Atlantic salmon. To differentiate whether these changes Ž . are seasonal or ontogenic, juvenile Atlantic salmon parr were separated by size into upper UM Ž . and lower mode LM in September. At monthly intervals, the fish were sampled for plasma q q Ž . Ž . cortisol, gill Na K ATPase activity and CR abundance B and dissociation constant k . max D UM were significantly larger than LM, and showed the silver appearance characteristic of smolts in April and May. Gill NaKATPase activity of UM fish increased 6-fold during the spring; LM Ž . fish increased 1.5-fold. Plasma cortisol levels increased significantly 10-fold in UM fish in May, but not in LM fish. Gill CR B increased 5-fold over the duration of the study in both groups. max CR k was lowest in October and highest in May; a 1.8and 2-fold increase in LM and UM, D respectively. There were no significant differences in gill CR B and k between the two max D groups during the study, except in May, when k was significantly greater and B lower in UM D max than LM. Peak levels of gill NaKATPase activity occur coincident with an increase in plasma cortisol concentration. Seasonal increases in CR B and k are similar in UM and LM fish and max D occur independent of smolting in juvenile Atlantic salmon. In UM fish, plasma cortisol increases in spring are concurrent with increased smolt characteristics. q 1998 Elsevier Science B.V. All rights reserved.