Effects of changes of photoperiod on gametogenesis in the rainbow trout (Salmo gairdneri)

Abstract

After their first reproductive period, adult male and female rainbow trout were put under three different photoperiodic regimes between January and June: (1) short daylength (6 h of light/day), (2) decreasing daylengths (16L : 8D leads to 10L : 14D), (3) skeleton photoperiod consisting of short day-length (6L : 18D) with a one-hour flash at various times during the dark cycle in a decreasing daylength fashion (between 16L and 10L), i.e. at the end of the daylight period of the preceding group. The rearing temperature was 15 degrees C. After the fish were killed in June, it was found that decreasing daylength (group 2) had stimulated vitellogenesis. In that group, spermatogenesis was also highly stimulated in all the males, and some were in spermiation. No significant stimulation of vitellogenesis was observed in any of the groups of females which showed a follicular diameter of less than 1 mm. Spermatogenesis had not begun in any males of group 1 and 3 but the testicular cycle was slightly advanced in group 3 in which residual spermatozoa had disappeared from the lobules, while they still subsisted in group 2. The hypothesis proposing the present pattern of photosensitive phase, shifting between 16L and 10L (group 3) during the gametogenetic cycle, cannot explain the stimulatory effect of the decreasing photoperiod (group 1).