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 been observed to change seasonally in conjunction with smolting in Atlantic salmon. To differentiate whether these changes are seasonal or ontogenic, juvenile Atlantic salmon parr were separated by size into upper (UM) and lower mode (LM) in September. At monthly intervals, the fish were sampled for plasma cortisol, gill Na⁺K⁺ATPase activity and CR abundance (B_max) and dissociation constant (k_D). UM were significantly larger than LM, and showed the silver appearance characteristic of smolts in April and May. Gill Na⁺K⁺ATPase activity of UM fish increased 6-fold during the spring; LM 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_max increased 5-fold over the 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. There 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 greater 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. Seasonal increases in CR B_max and k_D are similar in UM and LM fish and occur independent of smolting in juvenile Atlantic salmon. In UM fish, plasma cortisol increases in spring are concurrent with increased smolt characteristics.