Changes in numbers of leukocytes in immune organs of juvenile coho salmon after acute stress or cortisol treatment

We examined the effects of acute stress and cortisol treatment on the number of leukocytes (normalized for fish body weight) in the blood, thymus, spleen, and anterior kidney of juvenile coho salmon Oncorhynchus kisutch. In acutely stressed or cortisol-fed fish, the numbers of leukocytes increased significantly in the thymus and anterior kidney, and decreased significantly in blood and spleen within 1 d after treatment. Numbers of cells in the anterior kidney, blood, and spleen returned to control levels by 3 d after treatment, but cell numbers in the thymus remained significantly greater than control values until 3–7 d after acute stress. Although dietary cortisol resulted in increased plasma cortisol titers and caused the same changes in leukocyte distribution as those caused by acute stress, the magnitude or duration of elevated cortisol levels and leukocyte numbers were not correlated. These results suggest that, although increased plasma cortisol titers induced by stress may be involved in the change in number of cells in various immune organs, factors other than cortisol are involved as well.