Gonadal effects of melatonin in the three-spined stickleback, Gasterosteus aculeatus L., during different seasons and photoperiods.

Males and females of the three-spined stickleback, Gasterosteus aculeatus L., were given melatonin injections (0.8 or 4 micrograms/day) at different seasons and under different photoperiods (8L: 16D and 16L: 8D). Long photoperiods stimulated reproduction in November and January, whereas maturation in both sexes in March/April and ovarian regression in July were only negligibly affected by light. The high dose of melatonin induced antigonadal effects in both sexes under 16L: 8D in November and January. The low melatonin dose had progonadal effects under long photoperiod in females that had been maintained for some months under winter conditions and in females during the 8L: 16D regime in July.