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

Circulating plasma cortisol and gill corticosteroid receptors (CR) have bee n observed to change seasonally in conjunction with smelting in Atlantic sa lmon. To differentiate whether these changes are seasonal or ontogenic, juv enile Atlantic salmon parr were separated by size into upper (UM) and lower mode (LM) in September. At monthly intervals, the fish were sampled for pl asma cortisol, gill Na+K+ ATPase activity and CR abundance (B-max) and diss ociation constant (k(D)). UM were significantly larger than LM, and showed the silver appearance characteristic of smelts in April and May. Gill Na+K ATPase activity of UM fish increased 6-fold during the spring; LM fish inc reased 1.5-fold. Plasma cortisol levels increased significantly (10-fold) i n UM fish in May, but not in LM fish. Gill CR B-max increased 5-fold over t he duration of the study in both groups. CR k(D) was lowest in October and highest in May, a 1.8- and 2-fold increase in LM and UM, respectively. Ther e were no significant differences in gill CR B-max and k(D) between the two groups during the study, except in May, when k(D) was significantly greate r and B-max lower in UM than LM. Peak levels of gill Na+K+ ATPase activity occur coincident with an increase in plasma cortisol concentration. Seasona l increases in CR B-max and k(D) are similar in UM and LM fish and occur in dependent of smelting in juvenile Atlantic salmon. In UM fish, plasma corti sol increases in spring are concurrent with increased smelt characteristics . (C) 1998 Elsevier Science B.V. All rights reserved.