Cortisol-induced lymphocytopenia in brown trout, Salmo trutta L.

The administration of cortisol in the food of brown trout produced peak plasma cortisol titres of about 140 ng ml-1 at 12 hr and markedly reduced the number of circulating lymphocytes, which reached their lowest count at 36 hr. Cortisol did not affect the erythrocyte, thrombocyte, or neutrophil count. Sexual maturation of the male fish was associated with an increase in erythrocytes and a decrease in lymphocytes in the blood. These findings are discussed in relation to the increased susceptibility of the brown trout to a variety of infections during sexual maturation and under conditions of stress.