The effects of stress on plasma ACTH, α-MSH, and cortisol levels in salmonid fishes

Handling and confinement caused a steady increase in the plasma ACTH level in both coho salmon and rainbow trout. Whitin 2 min plasma ACTH levels had increased significantly, and by 30 min they were 5- to 8-fold higher than the basal ACTH level in unstressed fish. This type of stress also caused a pronounced elevation in plasma cortisol, which lagged behind the ACTH increase, although the degree of change was greater, the level rising between 20- and 50-fold. The plasma α-MSH level was unaffected by handling and confinement stress. A second series of experiments assessed the effects of a more severe stress, which consisted of 5 min out of water, during which the fish were restrained, followed by 25-min confinement in a small volume of water. This caused a very rapid, pronounced increase in the plasma ACTH level of sterile rainbow trout, the level reaching a peak at 5 min, and remaining elevated for the next 25 min. Plasma cortisol levels, which were low at the beginning of the experiment, remained so for the first 5 min, and rose thereafter. This type of stress also caused a rapid and pronounced elevation of the plasma α-MSH level. It rose in a very similar way, and at the same time, as the plasma ACTH level, but instead of remaining elevated it fell during the 25 min of confinement which followed the 5 min of restraint, to finish one-third of the peak value reached after 5 min. Sexually mature chinook salmon, which were infected with the fungus Saprolegnia, were also subjected to the severe stress. These fish had an initial cortisol level 20-fold higher than the basal level observed in all the other groups of salmonids. They did not respond to the stress of 5 min restraint out of water followed by confinement in as pronounced a manner as the sterile rainbow trout. The time course of the elevations of plasma ACTH and cortisol were similar in the two groups of fish, but the magnitude of response was reduced considerably in the chinook salmon.