Plasma cortisol levels of female rainbow trout (Salmo gairdneri) at the end of the reproductive cycle: relationship with oocyte stages.

The concentrations of plasma cortisol were determined in two populations of female rainbow trout at the end of their reproductive cycles. In females ending vitellogenesis or having oocytes with germinal vesicles in peripheral positions, mean cortisol levels were low (about 8 to 10 ng/ml). Cortisol concentrations remained low during oocyte maturation (about 8 and 11 ng/ml). In contrast, a 3-fold cortisol increase was observed in females sampled in the postovulation period (about 26 and 31 ng/ml). These results are discussed in relation to other studies on endocrinological changes during the spawning season with emphasis on the possible role of cortisol in reproduction in female teleosts.