Effects of acclimatization in long days at the end of the reproductive cycle on the timing of ovulation and on gonadotropin secretions in Artic charr(Salvelinus alpinus)

Arctic charr originated from Lake Geneva spawn at the end of autumn. In hat chery, it is advantageous to delay ovulation from several months because Ar ctic charr require temperatures below 6 degrees C for ovulation and it is e asier to supply cold water to broodstock in winter than in fall. The acclimatization of Arctic charr in long days during fall delayed ovulat ion time of several months. This method had the disadvantage to extend the ovulation time over more than three months. This problem was suppressed by transferring again the broodstock under short days during December. The eff iciency of the long day treatment to delay ovulation did not depend on its length when the treatment lasted longer than 1.5 months. In females acclimatized in long days during fall and winter, plasma gonadot ropin levels (GTH II) were always very low. After a transfer from long days to short days in January, plasma GTH II levels raised within three weeks. Plasma GTH II levels were significantly lower during the ovulation period i n females maintained under long days than in females transferred in short d ays. At this stage of reproductive cycle, plasma GTH I levels were always v ery low, whatever the photoperiod regime. A melatonin treatment given by in traperitoneal implants did not modify gonadotropin secretions in both long day or short day conditions. The pituitary responsiveness of females to a stimulation of gonadotropin se cretion by GnRH injections did not differ between fish reared in long days or in short days. Pimozide treatment did not modify the pituitary responsiv eness to GnRH in females acclimatized in long day photoperiod. As a whole, the results indicated that the inhibitory effect of long days on GTH II sec retion did not involve a blockade by dopamine contrary to the inhibitory ef fect of warm temperatures on the same phenomenon. The inhibition of GTH II secretion at final stages of the reproductive cycles probably involved chan ges in the hypothalamic control of GTH II secretion.