Environmental influences on melatonin secretion in Atlantic cod (Gadus morhua L.) and their relevance to commercial culture

This study investigated the effects of photoperiod and temperature on plasm a melatonin secretion in Atlantic cod (Gadus morhua L.). Initial work confi rmed the presence of a diel profile of melatonin synthesis, with elevated l evels during the dark phase. Unusually for fish, the peak in plasma melaton in occurred towards the end of the dark phase, which is indicative of a typ e 'A' melatonin profile. When exposed to 60 hours of continuous darkness a clear endogenous rhythm of melatonin synthesis was observed, which continue d for 4 cycles with a periodicity which, approximated to 24 h. When acclima ted to varying temperatures 14. 8, 12 or 16 degreesC) no variation in melat onin production was seen, however, body size appeared to be an important in fluence, with the smallest fish exhibiting significantly higher levels of d ark phase melatonin. Finally, the application of additional night-time illu mination to cod maintained in sea cages i.e. without blackout, did not sign ificantly reduce dark phase plasma melatonin, suggesting that cod are less sensitive to photoperiod manipulation in cages than salmonids.