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 indistinguishabl e. A progressive increase in confinement-induced cortisol levels in both se xes 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 dec line in females) coincided with declining water temperature and significant increases in plasma 11-kelotestosterone and elevated plasma testosterone l evels. Plasma oestradiol-17 beta levels were significantly greater in femal es than males throughout the study period and this difference was maximal b etween September and January. However, plasma testosterone was also elevate d in females during this period and levels overall were higher than those i n male fish. previous studies have shown oestradiol-17 beta and testosteron e to hair diametrically opposed effects on stress responsiveness in trout, with the former enhancing, and the latter suppressing, the cortisol respons e to a stressor. The relative roles of androgens, estrogen and water temper ature in modulating the stress responsiveness of rainbow trout are discusse d. (C) 2000 Fisheries Society of the British Isles.