Contrasting seasonal modulation of the stress response in male and female rainbow trout

Between June and September the magnitude of the plasma cortisol response of maturing male and female rainbow trout to confinement was indistinguishable. A progressive increase in confinement-induced cortisol levels in both sexes occurred during May to September, associated with the seasonal rise in water temperature. Between September and January a reduction of >50% in the magnitude of the cortisol response to confinement in male fish (but no decline in females) coincided with declining water temperature and significant increases in plasma 11-ketotestosterone and elevated plasma testosterone levels. Plasma oestradiol-17β levels were significantly greater in females than males throughout the study period and this difference was maximal between September and January. However, plasma testosterone was also elevated in females during this period and levels overall were higher than those in male fish. Previous studies have shown oestradiol-17β and testosterone to have diametrically opposed effects on stress responsiveness in trout, with the former enhancing, and the latter suppressing, the cortisol response to a stressor. The relative roles of androgens, estrogen and water temperature in modulating the stress responsiveness of rainbow trout are discussed.