The parr–smolt transformation of Arctic charr is comparable to that of Atlantic salmon

The present study was undertaken to investigate endocrine and osmoregulatory changes during the parr–smolt transformation of anadromous Arctic charr (Salvelinus alpinus). One year old, first generation hatchery-reared offspring of wild anadromous Arctic charr were kept at natural temperatures and a short daylength (10 h) during winter and continuous light from March until the end of the experiment in late June. At regular intervals, fish were sampled in freshwater for measuring gill Na+,K+-ATPase activity and the plasma levels of hormones known to be involved in smoltification, whereas other fish were exposed to 24-h seawater challenge tests for investigation of their hypoosmoregulatory ability. From mid-May onwards, there was an abrupt, and strong improvement in the fish's hypoosmoregulatory ability, concomitant with an increase in gill Na+,K+-ATPase activity. The development of hypoosmoregulatory ability was preceded by increased growth hormone (GH) levels from March to April, a continuous increase in insulin-like growth factor-I (IGF-I) levels from March to mid-June, and a transient increase in cortisol levels in parallel with the increase in gill Na+,K+-ATPase activity. These results support the roles of these hormones in stimulating the ion secreting machinery of the gills, and are in line with those considered typical for parr–smolt transforming Atlantic salmon (Salmo salar). A strong decrease in plasma thyroxine (T4) levels during the last part of the smoltification period deviates from the often reported increase in this parameter in parr–smolt transforming Atlantic salmon and supports the view that this hormone has a permissive or supportive role only in teleost salinity adaptation. To conclude, the parr–smolt transformation of the anadromous Arctic charr strain used in the present study resembles that of Atlantic salmon, both with regard to the development of hypoosmoregulatory ability and regulatory (endocrine) mechanisms.