The role of melatonin and the pineal gland on development and smoltification of Atlantic salmon (Salmo salar) parr

The present work used measurements of plasma melatonin throughout the year, pinealectomy, and/or intramuscular melatonin implants to investigate the role of melatonin and the pineal gland as intermediaries in the transfer of photic information on daily and calendar time in the control of the timing of the parr-smelt transformation in the Atlantic salmon. An initial experiment confirmed the presence of a seasonally changing light-dark rhythm of melatonin in potential S2 Atlantic salmon smelts, the profile of which accurately reflected the dark phase throughout the annual photoperiodic cycle. In the second experiment, potential S1 smelts were implanted and/or pinealectomised at the winter solstice. Although no variations in growth were observed between groups, smoltification in the pinealectomised plus implanted group was advanced by 3 weeks while the onset of smoltification in the pinealectomised fish was delayed by 3 weeks. In the final experiment, Atlantic salmon parr (3.5-7.0 g) which received melatonin-implants in June were significantly larger than the controls 1 month after implantation; this resulted in all the implanted fish entering the upper modal group by the time of smoltification whereas the control groups exhibited clear bimodal distributions. These results suggest that melatonin implantation and/or pinealectomy can alter the timing of seawater adaptation. Additionally, melatonin was shown to significantly alter the population structure of sibling Atlantic salmon parr resulting in 92% of individuals becoming S1 smelts. However, the mechanism(s) by which the pineal may influence development remains unclear. (C) 1998 Elsevier Science B.V. All rights reserved.