The rhythmic life of the Arctic charr; how to keep track of time

Fish, along with other animals, use day-night changes in light (photoperiod) as an environmental cue to keep track of time, both on a daily basis and throughout the year. It is, however, still unclear how dial (day-night) and annual (seasonal) rhythms are generated and controlled on a genetic and physiological level in fish. In this respect, the role of the hormone melatonin, which in mammals and birds is a chemical messenger about day and night and serve as a key pacemaker, is less known in fish. Further, little is known about how the extreme light conditions at high latitudes, and overwintering under thick ice and snow, affects melatonin production and rhythms.

In salmonid fish species, like the Arctic charr, there is no endogenous control of melatonin production in the pineal gland; production is continuously high when the fish (or the pineal gland) is maintained in darkness and continuously repressed by light. We are using the anadromous Arctic charr to investigate how the melatonin production in the pineal gland is regulated, through both in vitro and in vivo studies. Since most of the knowledge on melatonin rhythms in fish is obtained in laboratory studies under controlled and well defined light conditions, field studies are conducted in order to gain more information about the influence of natural light conditions on melatonin rhythms. Experimental studies in which plasma melatonin levels are manipulated may reveal regulatory roles of melatonin on the timing of annual events and physiological mechanisms. Finally, studies on the tissue distribution and temporal changes in the gene expression of melatonin receptors are initiated in order to provide further clues about if, and how, melatonin may be involved in the regulation of physiological processes associated with seasonal adaptations such as smolting, reproduction etc.


  • Strand, J.E.T., Aarseth, J.J., Hanebrekke, T.L. and Jørgensen, E.H. 2008. Keeping track of time under ice and snow in a sub-arctic lake; plasma melatonin rhythms in Arctic charr overwintering under natural conditions. J. Pineal Res. 44: 227-233.
  • Aarseth, J.J., Frøiland, E. and Jørgensen, E.H. 2010. Melatonin implantation during spring and summer does not affect seasonal rhythms of feeding and maturation in anadromous Arctic charr (Salvelinus alpinus). J. Polar Biol. 33, 379-388.


Ansvarlig for prosjektet: Even Jørgensen