Photoperiod effects on plasma cortisol levels in Gasterosteus aculeatus

No diel variations of plasma cortisol levels were found in Gasterosteus aculeatus acclimated to two different photoperiod conditions: 14L:10D and 9L:15D. However, under long photoperiods, G. aculeatus exhibited higher plasma cortisol levels than under short photoperiods, suggesting that elevated plasma cortisol levels are associated with the spring migration and/or reproductive activities. This decrease of plasma cortisol levels is particularly marked in male G. aculeatus. Under the 9L:15D photoperiod, males had lower plasma cortisol levels than females, in contrast to 14L:10D, where no significant sex differences were found.