Photoperiodic Determination of Preovulatory Gonadotropin Surge Onset Time in the Carp Cyprinus carpio

Abstract

Four experiments were conducted to clarify the effects of photoperiod on the onset time of the preovulatory GtH surge in the female carp Cyprinus carpio. After acclimation together with males under photoperiods of 12L-12D (Group A, lights-on 0600-1800 hours; Group B, 1800-0600 hours) in Experiment 1, 18L-6D (Group C, lights-on 0000-1800 hours; Group D, 0600-2400 hours) in Experiment 2, and 16L-8D (lights-on; 1600-0800 hours) in Experiments 3 (Group E and F) and 4 (Group G and H), water temperature was raised from 16 to 24°C beginning at 1730 hours to induce ovulation and spawning.
In Experiments 1 and 2, a preovulatory GtH surge initiated in the latter part of the light-phase in each photoperiod group after the water temperature was increased irrespective of the starting time of temperature elevation, and ovulation was observed mainly during the dark-phase. In Experiment 3, the light-phase period was changed in Group F from 1600-0800 to 0400-2000 hours after increasing the temperature so that the dark-phase was abruptly inserted after a 4-h light-phase. This change resulted in a delay of the preovulatory GtH surge. In Experiment 4, the GtH surge began at the same clock time as in Experiments 1 and 2 corresponding to the latter part of the light-phase of the acclimation photoperiod even though the photoperiod was changed to 24L and 24D in Group G and H, respectively, after increasing water temperature.
These results indicate that female carp have a reproductive circadian rhythm and the onset time of the preovulatory GtH surge is determined by a photoperiodic cue.