The Atlantic salmon ( Salmo salar ) life cycle involves a transformation called smoltification, where t he fish undergo a series of physiological changes that allow them to move from freshwater (FW) to seawater (SW). This process is primarily synchronized by increasing daylength; therefore, applying a 6-week period of reduced light (LD 12:12) to induce smoltification followed by a period of constant day light is a common practice in aquaculture. But knowing the exact time when the salmon have developed smolts characteristics, is still a concern for the aquaculture industry. G ills are considered the main osmoregulatory organ coping with osmotic challenges , but other organs like the intestine also play an important role. The Na+/K+-ATPase (NKA) is the main enzy me that allows the cell to cope with the osmotic gradient and the NKA activity in gills is commonly used as the main indicator of smoltification status. However, analyzing the expression of genes related with osmoregulation in the intestine may also help to assess smolt status.
This study aimed to characterize the development of smoltification in Atlantic salmon exposed to two photoperiods: constant light (LL) and a 6-week LD12:12 winter signal (LD). A total of 500 parr (33.23 ± 5.41 g) were randomly distributed in to 10 flow-through tanks (FW , 10°C ) at the Department of Biological Sciences (Bergen, Norway). After the winter signal, both groups were maintained in constant light . Fish were sampl ed every other week for a period of 8 weeks . Smoltification was assessed by measuring gill NKA activity , plasma cortisol and glucose, and gene expression of the isoforms α1a , α1b and α1c of the NKA, the Na+-K+-2Cl- cotransporter (nkcc ), the cystic fibrosis transmembrane conductance regulator isoforms cftr-i and cftr-ii, and the Na+/HCO3− cotransporter (nbc) in the gills and in the posterior midgut (PMG).
The fish in the LD group showed a coordinated osmoregulatory change, particularly at the 4th week after the winter signal . This included an increase in gill NKA activity, plasma glucose and cortisol levels, as well as a peak in transcription of gill cftr-i and PMG nkaα1c . In contrast, basically no change in these biomarkers were observed for the LL group over time. In conclusion , an increase in daylength induces and synchronizes smoltification in Atlantic salmon. Additionally, analysis of genes (like nkaα1c) in the PMG can be used as complementary biomarkers to the gill NKA activity.