High-latitude fish species have developed adaptive mechanisms that enable them to cope with the strong seasonality of the arctic environment. Our research is aimed at how seasonal adaptations are regulated through internal clocks and environmental cues (e.g. photoperiod), and neuroendocrine mechanisms regulating seawater tolerance, migratory behavior and appetite.
Fish living in high-latitude environments have developed mechanisms that enable them to cope with the strong seasonality of high latitude environments. For example, many species synchronize their annual feeding activity with the seasonal changes in food availability; summer is the time for intense feeding and energy deposition whereas the winter is characterized by low feeding activity and energy saving. Moreover, some species have developed a migratory lifestrategy to cope with spatial differences in food availability. The anadromous (sea-migrating)
Arctic charr reside in freshwater throughout winter and perform annual migrations to the sea in summer to utilize the rich feeding opportunities there. This species, which is the northernmost distributed freshwater fish and well adapted to the harsh and changing environment in the Arctic, has become our most important model species for studying the biology of high-latitude fish. Our research encompasses studies in the field and experiments under controlled conditions in the laboratory and includes neuroendocrine, regulatory mechanisms.
In addition to a general improvement of our basic knowledge about how high-latitude fish work, our research is motivated by applied aspects such as their vulnerability toward environmental changes. It is also likely that extreme adaptations constitute mechanisms which may be of universal interest and a possible source of exploitation. My research encompasses the following main, integrated topics:
1) Temporal control; how do high-latitude fish keep track of time in order to prepare for forthcoming seasons?
2) Smolting; endocrine regulation of the preparatory changes preceding seaward migration.
3) Appetite regulation; how can the anadromous Arctic charr endure months of fast and emaciation without being hungry?
Collaboration:
Professor Jack Falcon, Universitè Pierre and Marie Curie, Laboratorie Arago, France.
Professor Matt M Vijayan, University of Waterloo, Canada
Professor Björn Th. Björnsson, University of Gothenburg, Sweden
Professor Malcolm Jobling, University of Tromsø, Norway
Professor Helge K Johnsen, University of Tromsø, Norway
Postdocs: Jo J Aarseth
PhD students: Lisa B Helgason, Iris Jæger, Damien Cazaméa-Catalan.
Master students: Mads Martinsen, Vidar Strøm

A silvery and lean charr smolt on its way to the sea in spring. The anadromous Arctic charr are able to live in the high Arctic through its adaptations to the harsh and changing environment. It utilizes the rich food resources in the se through annual seaward migrations. During the residence in the sea they deposit energy-reserves for the long overwintering in fresh water. A notable, and very interesting, feature of the anadromous Arctic charr is the fact that they don’t eat during the several month long residence in fresh water through winter. Photo: Even H Jørgensen.
Temporal control; how do high-latitude fish keep track of time in order to prepare for forthcoming seasons?
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.
Publications:

 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. http://onlinelibrary.wiley.com/doi/10.1111/j.1600-079X.2007.00511.x/pdf
 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. http://www.springerlink.com/content/537m677468040355/fulltext.pdf
Can fish living in fresh water keep track of time under thick ice and snow during the dark winter?
Only by measuring melatonin in fish held in their natural environment can answer that question and then you have to go out measuring it. Sometimes the weather can be a challenge. Photo:
Altaposten.
Smolting; endocrine regulation of the preparatory changes preceding seaward migration.
Most marine and freshwater fish species maintain the osmolality of their body fluids within a quite narrow range which is well below the osmolality of seawater in marine species and well above the osmolality of freshwater in freshwater fish. Since the outer surface of fish can be considered a semi-permeable membrane, water and ions fluxes over this surface have to be actively counteracted in order to maintain the osmolality of body fluids within tolerable limits.
The gill is a major organ for active ion transport; membrane transporters in mitochondria rich cells (“chloride cells”) secrete excess ions in fish residing in seawater and absorb ions from the surrounding water in fish residing in freshwater. The smolting process in anadromous salmonids implies, among other changes, a reversion of the transport machinery of the gill by upregulating transport molecules capable of ion extrusion.
The smolting process is preparatory, i.e. it is fulfilled before the fish enter seawater. This means that there must be cues that initiate smolting long before the fish migrate to the sea, and there must be regulatory mechanisms that orchestrate the changes. It is well known that increasing daylengths in the spring is the most important environmental cue that trigger smolting for most anadromous salmonids and that growth hormone (GH) and cortisol play key roles in regulating

the changes in ion transport mechanisms in the gill. Less is known about how water temperature affects the temporal progress of the smolting process and to what extent the timing of seawater entry is coordinated with the development of seawater tolerance.
Anadromous Arctic charr is found as far north as 82 o
N and a successful seawater residency at these latitudes probably require that all features of the smolting process have developed in a coordinated fashion. Our studies encompass the role of GH and cortisol on the regulation of gill ion transporters in anadromous charr, and how photoperiod and water temperature interact on the temporal progress of the changes taking place during smolting. Studies on the neuroendocrine mechanisms that regulate migratory behavior have now been initiated, with a particular emphasis on how the development of seawater tolerance (osmoregulation) and –preference (migration) is coordinated. Future studies will also focus on the change in climate taking place in the Arctic and possible consequences for the anadromous Arctic charr.
Publications:
 Jørgensen, E.H., Aas-Hansen, Ø. and Tau Strand, J.E. 2007. The parr-smolt transformation of
Arctic charr is comparable to that of Atlantic salmon. Aquaculture, 273: 227-234. http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6T4D-4PVPVK3-3-
1&_cdi=4972&_user=596705&_pii=S0044848607009386&_origin=search&_coverDate=12
%2F18%2F2007&_sk=997269997&view=c&wchp=dGLzVlzzSkzS&md5=4e538075fde7b39c41c50939ac952d8e&ie=/sdarticle.pdf
 Bottengård, L. and Jørgensen, E.H. 2008. Elevated spring temperature stimulates growth, but not smolt development, in anadromous Arctic charr ( Salvelinus alpinus ). Comp. Biochem
Physiol, 151: 596-601. http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6VNH-4T1FWJV-2-
F&_cdi=6179&_user=596705&_pii=S1095643308010738&_origin=search&_coverDate=12
%2F31%2F2008&_sk=998489995&view=c&wchp=dGLbVzzzSkWb&md5=ba80e2c1a735c39add6bb6f8dd00f3e0&ie=/sdarticle.pdf

Ojima, D., Pettersen, R.J., Wolkers, J., Johnsen, H.K. and Jørgensen, E.H. 2009. Growth hormone and cortisol treatment stimulate seawater tolerance in both anadromous and landlocked Arctic charr. Comp. Biochem. Physiol. 153, 378-385. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VNH-4VXTST1-
4&_user=596705&_coverDate=08%2F31%2F2009&_rdoc=1&_fmt=high&_orig=search&_o rigin=search&_sort=d&_docanchor=&view=c&_acct=C000030698&_version=1&_urlVersio n=0&_userid=596705&md5=e3ca7df6be6149321b112a480e5c9612&searchtype=a

Tau Strand, J.E., Davidsen JG, Jørgensen, E.H. and Rikardsen A.H. Descending Atlantic salmon smolts with low levels of gill Na+, K+ -ATPase activity; do they delay sea entry?
Environ. Biol. Fish. 90, 317–321. http://www.springerlink.com/content/664564w8x32h955t/fulltext.pdf

Post doc Jo is catching migrating charr. Photo: Rune Pedersen
Appetite regulation; how can the anadromous Arctic charr endure months of fast and emaciation without being hungry?
In mammals, appetite is regulated through a hypothalamic integration of stimulatory (orexigenic) and inhibitory (anorexigenic) factors. Although major gaps exist in our understanding of the control of food intake in fish, the regulation seems to involve the same signalling molecules
(hormones, neuropeptides) as in mammals. Food intake and energy balance are influenced by several long-term neuroendocrine signals, including such that reflect the size of energy stores. In common with mammals, body fatness seems to have an inhibitory effect on food intake in fish; feed intake is negatively related to adiposity, and feed intake after a period of feed restriction targets restoration of fat reserves. It is therefore likely that the “lipostatic model” for appetite regulation in mammals also relate to fish. Underpinning this mechanism, the hormone leptin in mammals provides the endocrine link between peripheral fat stores and the brain centers that control appetite and energy balance.
The anadromous life strategy of the Arctic charr implies that most of the annual growth and resource accumulation occurs during the short summer seawater residence. Hence this species possess an extreme feeding strategy, with months of fasting and emaciation during winter and intense feeding and energy deposition during a few summer weeks. The seasonal change in appetite and growth in this species seems to be a strictly regulated process; captive offspring of anadromous Arctic charr exhibit pronounced seasonal changes in appetite and growth when held at constant temperature and given food in excess. The anadromous Arctic charr is an excellent model for studying mechanisms underlying long-term regulation of appetite and energy homeostasis, in particular within a context of a lipostatic model, because of its extreme (and voluntary) seasonal cycles in body fattening and emaciation.
Within a broad context, our aim is to investigate the role of anorexigenic and orexigenic factors in the regulation of appetite in fish, and how leptin interact with photoperiod, melatonin and adiposity in the regulation of the somatotropic axis. In particular, we are interested in how the anadromous Arctic charr can maintain an anorexic state throughout winter despite loss of fat reserves and in a long-term view, reveal if there may be targets for the development of therapeutic agents for use in tackling problems related to disturbed energy homeostasis and human obesity.
References:

 Frøiland, E., Murashita, K., Jørgensen, E.H. and Kurokawa, T. 2010. Leptin and ghrelin in anadromous Arctic charr: cloning and change in expressions during a seasonal feeding cycle.
Gen. Comp. Endocrinol. 165, 136-143. http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6WG0-4WJBBS3-7-
F&_cdi=6808&_user=596705&_pii=S0016648009002421&_origin=search&_coverDate=01
%2F01%2F2010&_sk=998349998&view=c&wchp=dGLzVzbzSkWA&md5=71301e48567b99af189b6fd1be4ffc8d&ie=/sdarticle.pdf
 Frøiland,
E., Jobling, M., Björnsson, BT, Kling, P, Ravuri, C. and Jørgensen, E.H. LeptinA2 is not a signal of adiposity, but may be a metabolic regulator in Arctic charr ( Salvelinus alpinus ). Gen. Comp. Endocrinol. 178, 330-337. http://ac.elscdn.com/S001664801200264X/1-s2.0-S001664801200264Xmain.pdf?_tid=973f6a681004d06621e34521ebbbf1ba&acdnat=1343307708_1724890082529 e684b8ad4ad45c5ca39