MELATONIN RHYTHMS IN ATLANTIC SALMON (SALMO-SALAR) MAINTAINED UNDER NATURAL AND OUT-OF-PHASE PHOTOPERIODS

Diel changes in circulating melatonin were measured in juvenile Atlant ic salmon, Sa[mo salar, maintained under natural and out-of-phase seas onal photocycles. Under natural day-lengths of autumn, winter, spring, and summer circulating melatonin levels were inversely related to lig ht intensity, with levels low during the day and high at night. The du ration of the nocturnal increase in circulating melatonin was related to the duration of darkness, i.e., longer in winter than in summer. Un der simulated seasonal photocycles circulating melatonin concentration s measured in August, October, and December were also elevated for the duration of darkness, irrespective of whether the photoperiods were s ynchronized or 6 months out-of-phase with the natural light and temper ature cycles. Circulating melatonin also provided an accurate represen tation of the prevailing photoperiod in fish initially maintained on s imulated natural photocycles, either synchronized or 6 months out-of-p hase with the natural light cycle, and then held for 3 months on dayle ngths approximating the summer and winter solstices. Well-defined mela tonin rhythms were always present, irrespective of time of year, photo period, and temperature. The amplitude of the nocturnal increase in ci rculating melatonin was similar in groups of fish maintained under sim ulated seasonal photoperiods 6 months out-of-phase with each other, bu t otherwise identical conditions, indicating that daylength per se did not influence the amplitude of the melatonin rhythm. The amplitude of the melatonin rhythm was slightly higher during the summer months, su ggesting that temperature may modify circulating melatonin levels. The se results demonstrate that circulating melatonin profiles always refl ect the prevailing daylength and hence have the potential to provide t he Atlantic salmon with accurate information on daily and calendar tim e, which could be utilised to time daily and seasonal events. (c) 1995 Academic Press, Inc.