Administration of cortisol to brown trout, Salmo trutta L., and its effects on the susceptibility to Saprolegnia infection and furunculosis

Oral administration and intraperitoneal implantation can be used to administer cortisol to the brown trout. Both techniques produce a dose-dependent increase in plasma cortisol concentration and the experimental procedures reduce the stress response in control fish to a minimum level. The plasma levels of the steroid are within the physiological range for the fish and the time of the response to acute oral administration is similar to that for such stresses as crowding, pollution and confinement. The evidence strongly indicates that elevated cortisol levels can significantly increase the susceptibility of salmonid fish to a wide range of infectious diseases including furunculosis and fungal infection. The present study has shown that this can occur at cortisol levels within the normal physiological range of the fish.