Mechanisms underlying photoperiodic effects on age at sexual maturity in Atlantic salmon, Salmo salar

Atlantic salmon (Salmo salar L.), reared in sea cages for 18 months (age 36 months from hatching), were exposed to natural light (NL; control), or con tinuous light (LL) from January (ALJ) or March (ALM) until July. Oocyte dev elopment appeared to be arrested at the oil drop stage during winter and sp ring in a proportion of the females following exposure to LL. This was acco mpanied by a decrease in plasma oestradiol-17 beta and testosterone levels following the onset of LL. Somatic growth was affected by LL during winter and spring. However, the arrested sexual development in a proportion of the fish in the ALJ and ALM groups did not correlate with lower growth rate, b ody weight or condition factor compared with controls prior to the reductio n in plasma sex steroid levels. These results suggest a photoperiodic advan cement of the timing of an annual 'critical period' during which sexual mat uration can be arrested. (C) 1999 Elsevier Science B.V. All rights reserved .