Environmental stress and the survival of brown trout, Salmo trutta

SUMMARY. 1. The hypothalamic‐pituitary‐interrenal axis of the brown trout, Salmo trutta, is activated in response to most forms of environmental stress. This results in an elevation of blood cortisol levels. 2. Experimental elevation of blood cortisol levels in otherwise unstressed brown trout caused a dose‐dependent increase in mortality rate due to disease. In our studies., Saprolegnia‐infection, furunculosis and bacterial fin‐rot were the principal diseases. 3. Chronic cortisol elevation also suppressed several of the endocrine processes controlling sexual maturation, resulting in a significant reduction in the size of the gonad in both male and female fish. 4. It is argued that many of the deleterious effects of sublethal pollution (including acidification) on natural trout populations can be attributed to chronically‐elevated blood cortisol levels and that a knowledge of such physiological changes would allow an assessment of the impact of pollution events and act as an early warning of potential disease and recruitment problems.