CHANGES IN CORTISOL DYNAMICS IN WILD AND HATCHERY-REARED JUVENILE COHO SALMON (ONCORHYNCHUS-KISUTCH) DURING SMOLTIFICATION

Cortisol dynamics were examined in hatchery and wild juvenile coho salmon (Oncorhynchus kisutch) over the spring of 1991 and 1992 when the fish were smelting. Wild fish were caught in the upper Quinsam River, British Columbia and at the downstream enumeration fence when they started to migrate. Two groups of hatchery fish were from the top and bottom ends of an earthen rearing channel. Plasma cortisol levels increased during the spring in all groups, but were significantly greater in wild than in hatchery smelts. The half-life of plasma cortisol decreased as fish smelted; however, there was no difference among the groups. Corticosteroid receptor (CR) concentration and affinity decreased during the spring. Wild fish consistently possessed the greatest gill concentration of CR. The change in dissociation constant (k(D)) was similar for hatchery and wild fish during spring 1991. In 1992, hatchery fish showed a similar gradual increase. In contrast, wild fish did not show an increase in k(D) until May. The changes in cortisol concentration in the plasma and the CR k(D) occurred synchronously with the increase in Na(+)K(+)ATPase activity in wild fish, which showed the greatest increase in k(D), plasma cortisol, and Na(+)K(+)ATPase activity.