Anadromy and smoltification in the world's northernmost fresh water fish species
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.
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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. 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.
Anadromous Arctic charr is found as far north as 82oN 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 theArctic 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.
- 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.
- 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.
- Tau Strand, J.E., DavidsenJG, 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. In press.