Seasonal changes in pineal melatonin content and hydroxyindole-O-methyltransferase activity in juvenile chinook salmon, Oncorhynchus tshawytscha.

Juvenile fall and spring chinook salmon were examined for seasonal changes in pineal hydroxyindole-O-methyltransferase (HIOMT) activity. Numerous small peaks in pineal HIOMT specific activity were found in fall chinook salmon and HIOMT activity per pineal gland increased linearly with age and/or size. In spring chinook salmon, cyclic changes in pineal HIOMT specific activity occurred in midsummer and midwinter, independent of the photoperiod under which the fish were reared. HIOMT activity per pineal gland showed a peak in midwinter. Activity increased with age and/or size of the fish during the following spring. Melatonin content of the pineal gland of spring chinook salmon did not change seasonally in fish reared under a normal photoperiod. Fish reared under constant illumination had about 50% higher pineal melatonin content than those under a normal photoperiod until November. At that time, pineal melatonin content decreased to the same level as that of fish reared under a normal photoperiod.